RepetoIn conjugate 1,4-addition to an enone, where does the nucleophile form its new bond?
At the β carbon.
Organic chemistry: reaction mechanisms
360 questions in English on electron flow, substitution, elimination, addition and carbonyl reactions, with stereochemical outcomes.
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What two π systems participate in a conventional Diels–Alder reaction?
A conjugated diene and a dienophile, usually an alkene or alkyne.
What unsaturated aldehyde forms by dehydration of 3-hydroxybutanal?
But-2-enal.
What usually happens first when a carboxylic acid is simply mixed with an amine at room temperature?
Acid–base salt formation, rather than immediate efficient amide formation.
Under carefully controlled low-temperature partial-reduction conditions, what carbonyl class can DIBAL-H produce from an ester after workup?
An aldehyde.
Can a correctly written elementary mechanism step create net electric charge without an omitted charged species?
No. Total charge must be conserved across the step.
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Question Answer
In conjugate 1,4-addition to an enone, where does the nucleophile form its new bond? At the β carbon.
What two π systems participate in a conventional Diels–Alder reaction? A conjugated diene and a dienophile, usually an alkene or alkyne.
What unsaturated aldehyde forms by dehydration of 3-hydroxybutanal? But-2-enal.
What usually happens first when a carboxylic acid is simply mixed with an amine at room temperature? Acid–base salt formation, rather than immediate efficient amide formation.
Under carefully controlled low-temperature partial-reduction conditions, what carbonyl class can DIBAL-H produce from an ester after workup? An aldehyde.
Can a correctly written elementary mechanism step create net electric charge without an omitted charged species? No. Total charge must be conserved across the step.
What does acidic workup do to the enolate present at the end of a typical Claisen reaction? It protonates the product enolate to give the neutral β-dicarbonyl compound.
Why is about one equivalent of base consumed in a typical Claisen condensation rather than acting only catalytically? The acidic β-dicarbonyl product is deprotonated, which drives the equilibrium forward.
Why is an α hydrogen of a ketone more acidic than a typical alkane hydrogen? Its removal gives a resonance-stabilized enolate rather than an unstabilized alkyl carbanion.
Can the familiar peroxide-driven anti-Markovnikov addition of HBr be assumed for HCl and HI as well? No. Their propagation energetics do not support the same useful chain in the usual way.
What alcohol results predominantly from 1-hexene treated with BH3 then H2O2/OH⁻? Hexan-1-ol.
Would a carbocation rearrangement be expected during a clean SN2 reaction? No. The concerted pathway does not form a free carbocation intermediate.
What happens to the original carbonyl oxygen during cyanohydrin formation and protonation? It becomes the product's OH group.
What step is shared by typical SN1 and E1 pathways from the same alkyl halide? Leaving-group ionization to a carbocation.
Why does bromide open a bromonium ion from the opposite face? The bridging bromine blocks same-face approach, favoring backside opening.
In a polar mechanism, where must the tail of a curved two-electron arrow begin? At an electron pair, such as a lone pair or a bond.
Why does base-promoted ester hydrolysis tend to proceed effectively toward products? Deprotonation traps the carboxylic product as a carboxylate, making reversal unfavorable under those conditions.
What net change does Wolff–Kishner reduction make to an aldehyde or ketone carbonyl? The C=O group is deoxygenated: R2C=O becomes R2CH2 (and RCHO becomes RCH3), using hydrazine under strongly basic conditions.
Why does the donor in a conventional reversible alkoxide-promoted self-Claisen reaction usually need at least two α hydrogens? One is removed to make the reacting enolate, and a remaining one permits the product-deprotonation driving step.
With a strong anionic nucleophile under basic conditions, which carbon of a simple unsymmetrical epoxide is usually attacked? The less substituted, less hindered carbon.
A cyclohexene reaction gives a cis vicinal diol. Is this consistent with OsO4 addition or with epoxidation followed by aqueous acid opening? OsO4 addition, because it gives syn dihydroxylation.
Why can the more substituted alkene fail to form in a cyclohexane E2 reaction even with a small base? The required anti axial β hydrogen may be absent in the reactive chair.
What neutral product is obtained after self-Claisen condensation of ethyl acetate and acidic workup? Ethyl 3-oxobutanoate, also called ethyl acetoacetate.
Why can mixing two different enolizable aldehydes under base give several aldol products? Either can serve as donor or acceptor, allowing multiple self- and crossed combinations.
What stereochemical change follows when a suitable chiral alkyl tosylate then undergoes clean SN2 displacement by iodide? Inversion at the carbon where iodide replaces the tosylate group.
After alkylation, ester hydrolysis and decarboxylation, what class of product does a typical malonic ester synthesis give? A substituted acetic acid, R–CH2–CO2H after one alkylation.
Where should a nitro group lie relative to an aryl leaving group to strongly assist ordinary addition–elimination SNAr? Ortho or para, allowing stabilization of the anionic addition intermediate.
Why can protonating a conjugated diene lead to both 1,2- and 1,4-addition products? The allylic cation intermediate has delocalized electrophilic character at more than one carbon.
After protonation and tautomerization, does ordinary conjugate addition retain a carbonyl group? Yes. The alkene is consumed while the carbonyl is regenerated.
Why can epoxides undergo nucleophilic ring opening more readily than ordinary unstrained ethers? Opening releases substantial three-membered-ring strain.
Which atom forms a new C–C bond when a terminal acetylide attacks a primary alkyl halide by SN2? The negatively charged terminal carbon.
What two carbonyl compounds form from 2-methylbut-2-ene after reductive ozonolysis? Propanone and ethanal.
Which two carbon–carbon bond-forming reactions make up a standard Robinson annulation? A Michael addition followed by an intramolecular aldol condensation.
In 1,2-addition to an enone, where does the nucleophile form its new bond? At the carbonyl carbon.
In a typical radical-chain propagation step, what happens to the radical carrier? One radical is consumed and another is formed, allowing the chain to continue.
How can strong hydrogen bonding to an anionic nucleophile affect its SN2 reactivity? It can reduce its availability for attack and slow substitution.
With an accessible acyclic substrate and a small strong base, which alkene is often favored in E2 elimination? The more substituted alkene, subject to stereoelectronic and substrate constraints.
Can ordinary achiral reagents distinguish two enantiomeric substrates solely because one is R and the other S? They have equivalent reactivity in a completely achiral environment.
What class of product results when an imine C=N bond is reduced during reductive amination? An amine.
Which atom of the second carbonyl compound is attacked during aldol addition? The carbonyl carbon.
After workup, what happens to an ester's acyl portion during LiAlH4 reduction? It becomes a primary alcohol; the alkoxy leaving portion also gives an alcohol.
When PBr3 converts a suitable chiral secondary alcohol to an alkyl bromide by the usual displacement pathway, what happens at the reacting carbon? Inversion of configuration.
What is the usual relative stereochemistry of Br and OH added across an alkene using Br2/H2O? Anti.
Why does an unprotected carboxylic acid interfere with a Grignard addition planned elsewhere in the molecule? Its acidic proton quenches the organomagnesium reagent.
What relative addition pattern results when an alkene is epoxidized and the epoxide is then opened with aqueous acid? Overall anti dihydroxylation.
In acid-catalyzed esterification, what neutral molecule is expelled from the tetrahedral intermediate? Water, after the appropriate proton transfers.
Why can a secondary carbocation rearrange to a tertiary carbocation before nucleophile capture? The rearranged cation can have lower energy through greater stabilization.
In an ester's double Grignard addition, which carbon becomes the alcohol-bearing carbon? The original ester carbonyl carbon.
Why might a ketone be converted to an acetal before a Grignard reaction intended at another group? The acetal prevents unwanted nucleophilic addition to that ketone.
With a tertiary alkyl halide in aqueous alcohol and no strong base, which two pathways commonly compete? SN1 substitution and E1 elimination.
Why does a tertiary alcohol resist ordinary oxidation to a carbonyl without skeletal cleavage? Its OH-bearing carbon has no hydrogen available for the usual carbonyl-forming oxidation.
For a tertiary alkyl bromide treated with a strong alkoxide base, which common pathway is favored over SN2? E2 elimination.
Why are simple aldehydes generally more reactive than comparable ketones toward nucleophilic addition? They are less sterically hindered and have less electron donation from carbon substituents.
During a 1,2-hydride shift next to a carbocation, what moves with the hydrogen? The electron pair of its original C–H bond.
Which species attacks the ester carbonyl in ordinary base-promoted saponification? Hydroxide ion.
Which fragment leaves when an amine attacks one carbonyl of an acid anhydride by acyl substitution? A carboxylate group, later protonated depending on conditions.
Which new bond forms when cyanide adds to a ketone during cyanohydrin formation? A bond from the carbonyl carbon to the carbon of CN⁻.
What aryl product results when a benzenediazonium salt undergoes a Sandmeyer reaction with CuCl? Chlorobenzene, with loss of N2.
Which ring sizes commonly form most readily in ordinary Dieckmann condensations? Five- and six-membered rings, when the tether permits them.
Why does nitration before bromination favor meta-bromonitrobenzene under suitable EAS conditions? The nitro group directs the later bromination mainly meta.
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Why can an achiral nucleophile attack either face of a simple planar carbocation? The empty p orbital is accessible from both faces of the trigonal center.
Why can NaBH4 reduction of an achiral unsymmetrical ketone give a racemic alcohol when a new stereocenter forms? Hydride can attack either enantiotopic face in an achiral environment.
Why is benzaldehyde useful as the acceptor in a directed crossed aldol design? It has no α hydrogen, so it cannot form the competing ordinary aldehyde enolate.
For a strongly endergonic elementary step, which neighboring state does its transition state tend to resemble more? The higher-energy product of that step.
Which carbon of an enolate forms the new bond in a typical aldol addition? Its α carbon.
For tert-butanol reacting with HBr under ordinary acidic substitution conditions, what pathway commonly forms tert-butyl bromide? Ionization of protonated alcohol followed by bromide capture, an SN1 pathway.
What does stereoselective mean when one substrate can form several stereoisomeric products? One stereoisomer forms preferentially; exclusive formation is not required.
What organic product forms when ethylmagnesium bromide is quenched with water? Ethane.
During ordinary saponification of R–C(=O)–18O–CH3, which product retains the labeled alkoxy oxygen? Methanol after proton transfer; the acyl–oxygen bond is cleaved.
Why is benzyne substitution called elimination–addition? Base-induced loss of HX first forms benzyne, then a nucleophile adds to it.
Does the usual hydroboration step pass through a free carbocation? No. B–C and H–C bonds form in a concerted addition.
What alkene is favored when hex-2-yne is reduced with sodium in liquid ammonia? (E)-Hex-2-ene.
Why can acid-catalyzed hydration of a branched alkene give a rearranged alcohol? A carbocation can rearrange before water attacks.
What geometric intermediate commonly forms first when a nucleophile attacks an acid derivative's carbonyl carbon? A tetrahedral intermediate.
Why might an acid-sensitive substrate favor Wolff–Kishner rather than Clemmensen carbonyl reduction, if otherwise compatible? Wolff–Kishner uses basic conditions, whereas Clemmensen uses strongly acidic conditions.
Why does tert-butyl bromide ionize more readily than an ordinary primary bromide under comparable SN1 conditions? It forms a more stabilized tertiary carbocation.
What two oxygen substituents share one carbon in a hemiacetal? One OH group and one OR group.
Does heterogeneous alkene hydrogenation normally use a freely solvated carbocation intermediate? No. Adsorption and hydrogen delivery occur at the catalyst surface.
Why does ordinary amide hydrolysis often require stronger conditions than ester hydrolysis? Amide resonance stabilization and the poor nitrogen leaving group make acyl substitution more difficult.
Why does ordinary conversion of a hemiacetal to an acetal require acid catalysis? Protonation converts the OH group into a water leaving group.
After oxymercuration–demercuration of a simple unsymmetrical alkene, where is OH normally installed? On the more substituted alkene carbon: Markovnikov hydration.
Why might an alcohol be protected before using a strongly basic organometallic reagent elsewhere in the molecule? Its unprotected O–H proton would quench the reagent by acid–base reaction.
Does SN2 inversion guarantee that an R substrate becomes an S product? No. Spatial inversion occurs, but the R/S label also depends on the product's priority ordering.
What products form when Cl• abstracts H from methane? HCl and the methyl radical, CH3•.
Why can an acid chloride undergo nucleophilic acyl substitution whereas a simple ketone usually undergoes addition? The acid chloride has a group that can leave from the tetrahedral intermediate.
Which step is commonly rate-limiting in electrophilic substitution of benzene? Electrophilic attack that forms the nonaromatic arenium ion.
Why does amide nitrogen donation reduce carbonyl electrophilicity? Its lone pair delocalizes toward the carbonyl, reducing positive character at the acyl carbon.
Why should a mechanism card specify reagents and solvent instead of asking only what a secondary bromide does? The same substrate can follow different pathways under different conditions.
Why does a nitro group meta to an aryl halide usually provide less resonance assistance for classical SNAr than a para nitro group? It cannot delocalize the relevant σ-complex negative charge into the nitro group in the same way.
For HBr addition to buta-1,3-diene, what condensed structure describes the 1,4-addition product? CH3–CH=CH–CH2Br.
What functional-group change occurs when cyclohexene is hydrogenated with H2/Pd-C? The C=C bond becomes a C–C bond, giving cyclohexane.
Why does ionic protonation of propene preferentially place the positive charge on its middle carbon? That route forms a secondary rather than primary carbocation.
What carbonyl arrangement is formed in the self-Claisen condensation of a simple ester with α hydrogens? A β-keto ester.
Why does iodide not normally cleave anisole by ordinary SN2 attack at the phenyl carbon? That carbon is sp² and does not allow the usual backside SN2 pathway.
Does a catalyst change the equilibrium constant for a reaction at fixed temperature? No. It changes the route and rates, not the reactant–product free-energy difference.
For an ordinary bimolecular SN2 step involving RX and Nu⁻, what rate law is expected? Rate = k[RX][Nu⁻].
Why is acetic acid more acidic than ethanol under comparable conditions? Its conjugate base delocalizes negative charge over two oxygens, whereas ethoxide lacks that stabilization.
Is nitrobenzene a suitable substrate for ordinary Friedel–Crafts alkylation? No. Its strongly deactivated ring reacts poorly under the usual conditions.
Why does a balanced overall reaction equation not establish the reaction mechanism? It gives the net atom balance, not the sequence of elementary steps.
For HBr addition to buta-1,3-diene, what condensed structure describes the 1,2-addition product? CH3–CHBr–CH=CH2.
Which atoms form the new C–C bond during a Claisen condensation? An ester enolate's α carbon and another ester molecule's carbonyl carbon.
Why check whether a proposed acetylide alkylation substrate contains a proton appreciably more acidic than the parent terminal alkyne? Proton transfer can consume the acetylide before the intended C–C bond formation.
What oxygen-containing intermediate forms when an anionic nucleophile adds to an unprotonated aldehyde? An alkoxide.
Why can aldol dehydration be favorable even when hydroxide is a poor leaving group? The pathway forms a conjugated α,β-unsaturated carbonyl product, often with additional thermodynamic driving forces.
Is endo addition a common kinetic preference or an exceptionless rule for Diels–Alder reactions? A common kinetic preference in suitable systems, not an exceptionless rule.
Why is ordinary tub-shaped cyclooctatetraene classified as nonaromatic rather than as a planar antiaromatic ring? Its nonplanarity prevents continuous planar p-orbital overlap around the ring.
What distinguishes homolytic bond cleavage from heterolytic cleavage in electron allocation? Homolysis gives one bonding electron to each fragment; heterolysis gives both to one fragment.
What type of dihalide is commonly obtained by adding excess HBr ionically to a terminal alkyne? A geminal dibromide, with both bromines on the same carbon.
In the usual SN2 reaction of an alkyl halide with ionic cyanide such as NaCN, which cyanide atom bonds to the alkyl carbon? Carbon, giving a nitrile rather than an isocyanide.
Why is planar, fully conjugated cyclobutadiene antiaromatic rather than aromatic? Its four π electrons fit 4n, not 4n + 2.
Over which two atom types is the negative charge of a simple enolate delocalized? The oxygen and the α carbon.
What class of amine commonly gives an imine when condensed with an aldehyde or ketone? A primary amine, RNH2.
Using comparable pKa values, what is the approximate equilibrium constant for HA + B⁻ ⇌ A⁻ + HB? K ≈ 10 raised to [pKa(HB) − pKa(HA)].
What formal charge does a neutral alcohol oxygen acquire when it uses a lone pair to bond to H⁺? A +1 formal charge, producing an oxonium ion.
What carbonyl product results from hex-1-yne after a suitable bulky borane followed by H2O2/OH⁻? Hexanal after tautomerization.
What conditions regenerate a carbonyl compound from an ordinary acetal? Aqueous acid.
How are two cis substituents on an alkene dienophile related in the usual concerted Diels–Alder product? They remain cis to each other.
Why does acid protonation make an alcohol OH group a better leaving group? The protonated group can leave as neutral water rather than hydroxide.
What happens when a growing carbon radical adds to another vinyl monomer's double bond? A new C–C bond forms and the radical moves to the newly added unit.
In cleavage of anisole by HI, which C–O bond is normally broken by iodide attack? The methyl–oxygen bond, yielding iodomethane and phenol.
Can an internal alkyne be converted to a terminal acetylide simply by removing an sp C–H proton? No. It has no hydrogen attached to either alkyne carbon.
What alcohol is favored when propene undergoes acid-catalyzed hydration? Propan-2-ol.
When drawing resonance contributors, may a hydrogen atom move from one atom to another? No. Only electron placement changes; moving hydrogen would describe a chemical transformation.
Why do basicity and nucleophilicity not mean the same thing? Basicity describes proton-transfer equilibrium; nucleophilicity describes the rate of attack at an electrophile.
Why do electron-withdrawing groups often increase an alkene's reactivity in a normal-electron-demand Diels–Alder reaction? They lower the dienophile's accepting orbital energy and improve interaction with the diene.
During one E2 step, what happens to the β C–H bond, the C–leaving-group bond and the intervening C–C bond? C–H and C–leaving-group bonds break while the C=C π bond forms.
Why can an α,β-unsaturated ketone be attacked at its β carbon as well as its carbonyl carbon? Conjugation delocalizes electrophilic character across the C=C–C=O system.
What product class commonly results from strong aqueous chromium(VI) oxidation of a primary alcohol? A carboxylic acid.
What does anti addition to an alkene specify about the two newly added groups? They are delivered to opposite faces of the original double bond.
What bond forms during carbon alkylation of an enolate with a primary alkyl halide? A C–C bond between the enolate α carbon and the carbon bearing the leaving group.
Which electron movement restores aromaticity when an arenium ion loses the proton at the attacked carbon? The C–H bond electrons form the missing ring π bond.
What hydrocarbon forms from hex-2-yne with excess H2 and an ordinary active Pd catalyst? Hexane.
In a typical organic oxidation, how does carbon's bonding to oxygen or hydrogen change? It gains bonds to oxygen or loses bonds to hydrogen.
In ionic addition of HBr to an alkene, which electrons initially attack the proton? The alkene π electrons.
What rate law describes the simplest E2 reaction of an alkyl halide with a base? Rate = k[alkyl halide][base].
What product class forms when a ketone with an α hydrogen condenses with a secondary amine? An enamine.
In SN2 substitution, where does the nucleophile approach relative to the leaving-group bond? From the opposite side of the electrophilic carbon, along the backside direction.
Why is benzyl bromide much more suitable for ordinary SN2 than bromobenzene? Its C–Br bond is at an accessible sp³ benzylic carbon rather than an sp² aryl carbon.
Which relative diol is produced by syn dihydroxylation of cyclohexene? Cis-cyclohexane-1,2-diol.
What organic product forms when hydroxide substitutes for bromide in CH3Br by SN2? Methanol, CH3OH.
Where does a nitro group direct further electrophilic aromatic substitution, and how does it affect rate? Predominantly meta; it deactivates the ring.
Without additives that alter selectivity, what addition mode do ordinary Grignard reagents often favor with enones? Direct 1,2-addition at the carbonyl carbon.
Why can an acetal protect a carbonyl during many basic nucleophilic reactions? Ordinary acetals resist base and lack the electrophilic carbonyl group.
Why is an unprotonated alcohol OH group usually a poor leaving group in substitution? It would depart as the strongly basic hydroxide ion.
What carbonyl class forms by ordinary oxidation of a secondary alcohol without C–C cleavage? A ketone.
What is the dominant initial reaction when a Grignard reagent encounters an unprotected alcohol O–H group? Proton transfer destroys the organomagnesium carbon nucleophile, producing its hydrocarbon.
After carbocation formation, what produces the alkene in an E1 pathway? A base removes a β proton while the C=C bond forms.
What carboxylic product forms when propanoyl chloride reacts with water? Propanoic acid.
For acyclic 2-bromobutane undergoing ordinary E2 elimination with ethoxide, which but-2-ene geometry is usually favored? (E)-But-2-ene, although other alkene products can also form.
What substitution mechanism forms an ether from an alkoxide and an unhindered primary alkyl halide? SN2.
Why is substantial loss of stereochemical purity consistent with an SN1 pathway at a stereocenter? A planar carbocation permits nucleophile approach from more than one face.
With HBr and no radical initiator, what is the major addition product from propene? 2-Bromopropane.
Which occupies a local minimum rather than a maximum along a reaction-coordinate diagram? An intermediate occupies a local minimum.
For an unhindered primary alcohol reacting with HBr, what pathway commonly displaces the protonated OH group? Bromide attacks by SN2 while water leaves.
What preferred dihedral relationship do the breaking C–H and C–leaving-group bonds have in ordinary E2 elimination? They are anti-periplanar, with an approximately 180° dihedral angle.
What does a negative overall ΔG imply about a reaction under the stated conditions? The forward change is thermodynamically favorable; it need not be fast.
What ether forms when phenoxide reacts by SN2 with iodomethane? Anisole, or methoxybenzene.
What happens to configuration at the carbon attacked in an SN2-like epoxide opening? It inverts.
When substrate ionization is rate-limiting in an SN1 reaction, what is the usual rate law? Rate = k[alkyl halide].
Why is NBS useful for allylic bromination under radical conditions? It maintains a low Br2 concentration, supporting allylic substitution while limiting direct alkene addition.
What structural feature makes a carboxylic acid derivative a lactone? It is a cyclic ester.
What are the usual regio- and stereochemical descriptors for hydroboration–oxidation of a simple alkene? Anti-Markovnikov hydration with overall syn addition of H and OH.
Does converting a chiral alcohol to its tosylate itself invert the carbon stereocenter? No. The carbon's spatial configuration is retained because its bond to oxygen remains intact.
Which neutral nucleophilic intermediate attacks Br2 during acid-catalyzed α bromination of a ketone? The enol.
What carbon skeleton results from SN2 reaction of 1-iodopropane with NaCN in a suitable polar aprotic solvent? Butanenitrile; the nitrile carbon adds one carbon to the original three-carbon chain.
When designing bromonitrobenzene by successive EAS steps, why does bromination before nitration favor ortho/para rather than meta placement? Bromine directs subsequent nitration to ortho and para positions.
What product can PCC in anhydrous conditions give from an ordinary primary alcohol? An aldehyde.
When hydroxide removes the O–H proton of an acid, where does the arrow from that acid's O–H bond point? Back to the acid's oxygen, giving its conjugate base.
When the C–Br bond of an alkyl bromide breaks heterolytically to Br⁻, where does the curved arrow end? On bromine, which receives both bonding electrons.
Why can dehydration of a tertiary alcohol under strong aqueous acid involve an E1 pathway? Protonated OH can leave as water to form a relatively stable tertiary carbocation.
What alkene results from propanone reacting with the methylene ylide Ph3P=CH2? 2-Methylpropene.
Before tautomerization, what oxygen functional group is formed by addition of water across an alkyne? An enol, with OH attached to an alkene carbon.
When an alcohol is converted to its tosylate with TsCl and a base, is its original carbon–oxygen bond broken? No. Modification occurs at oxygen, retaining the carbon–oxygen bond.
What type of carbon nucleophile is commonly used in a Michael addition? A resonance-stabilized enolate, often derived from a β-dicarbonyl compound.
How does removing water affect acid-catalyzed acetal formation from a carbonyl compound and alcohol? It shifts the equilibrium toward the acetal.
Which step regenerates the acid catalyst after water attacks the hydration carbocation? Deprotonation of the oxonium intermediate.
Why can strongly acidic conditions slow imine formation despite the need for acid catalysis? They protonate the amine, reducing its nucleophilicity.
For an unhindered primary bromide with NaN3 in a polar aprotic solvent, which pathway is commonly favored? SN2 substitution; azide is a useful nucleophile and a relatively weak base.
Which addition mode do suitable organocuprates commonly favor with α,β-unsaturated ketones? Conjugate 1,4-addition.
Besides an appropriate π-electron count, what structural features are required for ordinary Hückel aromaticity? A cyclic, approximately planar, continuously conjugated p-orbital system.
Does bromobenzene normally undergo ordinary backside SN2 substitution at its aryl carbon? No. The sp² aryl carbon does not support the usual alkyl-halide SN2 pathway.
What arrangement enables ordinary E2 elimination in a cyclohexane chair? An axial leaving group and an anti axial β hydrogen: a trans-diaxial arrangement.
What distinguishes a Dieckmann condensation from an intermolecular Claisen condensation? It is an intramolecular ester condensation that forms a ring.
What feature separates two transition-state peaks in a two-step mechanism diagram? A reaction intermediate in a local energy minimum.
Can an E1 reaction give a rearranged alkene carbon skeleton? Yes. A carbocation may rearrange before losing a β proton.
Which pairing is better for ethyl tert-butyl ether: tert-butoxide with bromoethane, or ethoxide with tert-butyl bromide? tert-Butoxide with bromoethane; the tertiary halide would strongly favor elimination over SN2.
What does a curved arrow from an oxygen lone pair to a carbon atom represent? Formation of an O–C bond using that electron pair.
Under reversible enolate-forming conditions with time to equilibrate, what controls the major enolate? Relative enolate stability, often favoring the more substituted enolate.
What extra transformation distinguishes aldol condensation from aldol addition? Dehydration of the β-hydroxy carbonyl product to an α,β-unsaturated carbonyl compound.
What bond arrangement makes a diene conjugated? Two C=C bonds separated by one single bond, allowing adjacent p-orbital overlap.
For an unsymmetrical ketone with two enolizable sides, what enolate often forms with bulky LDA at low temperature under irreversible conditions? The less hindered kinetic enolate, when that site's proton is removed faster.
Why is combination of two carbon radicals a chain-termination event? It consumes radical carriers without generating a new radical.
What role does the nitrogen lone pair play in pyrrole's aromaticity? It contributes two electrons to the six-electron aromatic π system.
Why is the cyclopentadienyl anion aromatic? The carbanion lone pair joins the cyclic π system, giving six π electrons.
Why is LDA often chosen to form an enolate before adding an alkyl halide? It is a strong, sterically hindered base that favors deprotonation over nucleophilic carbonyl addition.
What is the principal role of FeBr3 in bromination of benzene with Br2? It activates Br2, making bromine more electrophilic toward the aromatic ring.
How many π electrons occupy benzene's aromatic system? Six.
If ionization controls a simple E1 reaction's rate, what concentration appears in its rate law? The substrate concentration; rate = k[substrate].
Which simple diol can convert an aldehyde into a five-membered cyclic acetal under acid catalysis with water removal? Ethane-1,2-diol, also called ethylene glycol.
What two bond changes occur together in an elementary SN2 step? The nucleophile–carbon bond forms while the carbon–leaving-group bond breaks.
What alkyne forms when the acetylide from propyne reacts by SN2 with bromoethane? Pent-2-yne.
When a nucleophile attacks an ordinary carbonyl carbon, why must the C=O π electrons move to oxygen? Otherwise the second-row carbon would exceed an octet.
Does dissolving every ketone in water convert it almost completely to a hydrate? No. Hydrate abundance depends strongly on the carbonyl substituents.
In Br2/H2O addition to propene, on which carbon does OH predominantly appear? The middle, more substituted carbon.
What competing pathway can remove a β proton from the carbocation formed in an SN1 reaction? E1 elimination, producing an alkene.
Under conditions where substitution and elimination compete, why is heating often used to favor elimination? Elimination can become relatively more favorable at higher temperature, though the outcome remains condition-dependent.
Why does a methoxy group usually activate ortho and para positions toward electrophilic aromatic substitution? Oxygen can donate a lone pair into the ring and stabilize the corresponding arenium ions.
Why is a tertiary alkyl halide unsuitable for routine enolate C-alkylation by SN2? Steric hindrance blocks substitution and the enolate can promote E2 elimination.
After acidic workup, what alcohol class results from adding a Grignard reagent to formaldehyde? A primary alcohol.
What alkene is favored when hex-2-yne is partially hydrogenated with H2 and Lindlar catalyst? (Z)-Hex-2-ene.
Which common electrophilic aromatic substitution can be reversed by heating with dilute aqueous acid or steam? Sulfonation; the sulfonic acid group can be removed.
What functional-group arrangement characterizes an ordinary aldol addition product? A β-hydroxy aldehyde or β-hydroxy ketone.
Which is generally more reactive toward nucleophilic acyl substitution: an acid chloride or an ordinary amide? The acid chloride.
Does E1 require the original substrate's C–H and C–leaving-group bonds to be anti-periplanar at the instant of ionization? No. Those bonds break in separate steps rather than one concerted E2 event.
Why can NaNH2 deprotonate a terminal alkyne effectively in liquid ammonia? The resulting NH3 is a much weaker acid than the terminal alkyne.
Does forming two stereocenters necessarily make a product chiral? No. Internal symmetry can make the product meso and achiral.
How many π electrons make the tropylium cation aromatic? Six, delocalized over the seven-membered ring.
What relative stereochemistry is expected for Br2 addition to cyclohexene in a nonnucleophilic solvent? Trans-1,2-dibromocyclohexane from anti addition.
What carbonyl product forms from but-2-ene after O3 followed by dimethyl sulfide? Two equivalents of ethanal.
What product class results from LiAlH4 reduction of a carboxylic acid followed by workup? A primary alcohol.
What does syn addition to an alkene specify about the two newly added groups? They are delivered to the same face of the original double bond.
Where does a methyl group generally direct further electrophilic substitution on a benzene ring? To ortho and para positions.
With all else fixed in a simple SN2 reaction, what happens to the rate when only nucleophile concentration doubles? The rate doubles.
In methanol solvolysis of tert-butyl bromide, what substitution product forms after deprotonation? Methyl tert-butyl ether.
Which original oxygen becomes the β-OH group in an aldol addition product? The oxygen of the electrophilic carbonyl acceptor.
Why is excess amine or an added base used in many acid-chloride amidations? It neutralizes the acid formed and keeps a useful amount of amine unprotonated.
Which simple elimination pathway normally has base concentration in its elementary rate law, E1 or E2? E2.
Under ordinary mild conditions, which reacts readily with NaBH4: a ketone or an unactivated ester? The ketone.
What structural feature makes methyl vinyl ketone a Michael acceptor? Its alkene is conjugated with an electron-withdrawing carbonyl group.
What small molecule is lost when a β-keto acid undergoes thermal decarboxylation? CO2.
What intermediate containing both OH and amino groups on one carbon precedes dehydration in imine formation? A carbinolamine, also called a hemiaminal.
How does carbonyl carbon geometry change during ordinary nucleophilic addition? From approximately trigonal planar to tetrahedral.
In an ordinary Brønsted acid–base step, what does the base accept? A proton.
Why are a ketone and its enol tautomers rather than resonance contributors? A hydrogen changes attachment and a σ bond changes, not merely electron placement.
What can a substantial rate decrease after replacing a reacting C–H with C–D support? That C–H bond cleavage contributes to the rate-controlling transition state.
What broad conditions favor conversion of an alcohol to an alkene by acid-catalyzed dehydration? Acid and heat, with conditions that favor loss of water.
Why can an unsymmetrical benzyne give two positional substitution products? The nucleophile can attack either carbon of its strained formal triple bond.
Which electrophile attacks benzene during nitration with nitric and sulfuric acids? The nitronium ion, NO2⁺.
If a cis-disubstituted alkene undergoes concerted peracid epoxidation, how are its substituents related in the epoxide? They remain cis.
Why can a tertiary alkyl halide react readily by E2 even though ordinary SN2 is strongly hindered? A base can remove a β hydrogen without attacking the crowded carbon bearing the leaving group.
Why does electrophilic substitution preserve a useful advantage over permanent addition to benzene? Substitution restores the aromatic π system after the initial attack.
During hydroboration of a simple terminal alkene with BH3, which alkene carbon preferentially bonds to boron? The less substituted terminal carbon.
Why can POCl3/pyridine dehydration avoid carbocation rearrangements in a suitable alcohol? It activates oxygen and promotes an E2-type elimination rather than a free-carbocation pathway.
What relative stereochemistry results from alkene dihydroxylation with OsO4 followed by a suitable reductive workup? Syn addition of two OH groups.
What happens to the oxygen formal charge when RO⁻ forms an O–H bond to become ROH? It changes from −1 to zero.
What is the conjugate acid of methylamine, CH3NH2? Methylammonium, CH3NH3⁺.
Why can acid dehydration of an alcohol produce a rearranged alkene? The intermediate carbocation may rearrange before β-proton loss.
How many electrons does a single-headed fishhook arrow represent in a radical mechanism? One electron.
Does acid-assisted epoxide opening normally give same-face attack at the carbon being opened? No. Backside opening gives inversion at that carbon and an anti relationship to the retained oxygen bond.
What ester forms when propanoyl chloride reacts with methanol in the presence of a suitable acid-scavenging base? Methyl propanoate.
In the simple ionization-limited SN1 model, does doubling the trapping nucleophile concentration double the rate? No. The rate-limiting ionization does not include the nucleophile.
Why may a cyclohexyl halide with an equatorial leaving group need a chair flip before E2 elimination? The leaving group must usually become axial to align with an anti axial β hydrogen.
For HA + B⁻ ⇌ A⁻ + HB in the same solvent, which side is favored if pKa(HB) is much larger than pKa(HA)? The right side, containing the weaker acid HB.
After writing a neutral nucleophile's bond-forming attack, what bookkeeping step should be checked before calling the product neutral? Whether deprotonation or another charge-balancing step is still required.
What is the stereochemical product of ideal anti bromination of (E)-but-2-ene? Meso-2,3-dibromobutane.
Does an experimentally first-order rate law by itself prove a unique SN1 mechanism? No. Several mechanisms or limiting conditions can produce the same observed rate law.
What activates an alkene during acid-catalyzed hydration? Protonation of the π bond forms a carbocation.
Why must a product-prediction question specify reductive versus oxidative ozonolysis workup? Aldehydes retained by reductive workup can be further oxidized under oxidative workup.
What two changes occur when a tetrahedral acyl intermediate collapses in substitution? The C=O bond reforms and a leaving group is expelled.
What ring size is created by the usual [4+2] Diels–Alder cycloaddition? A six-membered ring.
When a malonate enolate adds to methyl vinyl ketone, to which alkene carbon does the donor carbon bond? The terminal β carbon, farthest from the carbonyl.
What is the first acid–base event when an ordinary ether is cleaved by concentrated HI? Protonation of the ether oxygen.
What alcohol forms from propanone and ethylmagnesium bromide followed by aqueous acid? 2-Methylbutan-2-ol.
What aldol addition product results when one ethanal enolate adds to another ethanal and is protonated? 3-Hydroxybutanal.
Why is aqueous acid normally added after organomagnesium addition rather than before it? Acid would protonate and destroy the carbon nucleophile before C–C bond formation.
Before acidic workup, what form does the carboxyl-derived product take during saponification? A carboxylate anion.
What type of intermediate forms when an anionic nucleophile adds to a neutral activated haloarene in a stepwise SNAr pathway, before halide departure? A nonaromatic anionic σ complex, often called a Meisenheimer intermediate.
Why is tert-butyl bromide a poor electrophile for extending an acetylide chain by SN2? Backside substitution is blocked, and the strongly basic acetylide can instead promote elimination.
What functional group forms when water adds across a carbonyl to give two OH groups on one carbon? A geminal diol, also called a carbonyl hydrate.
How does protonating a carbonyl oxygen assist attack by a weak neutral nucleophile? It increases the electrophilic character of the carbonyl carbon.
Why can methyl bromide not undergo an ordinary β-elimination E2 reaction? It has no β carbon or β hydrogen.
Which sequence can install an unbranched alkyl side chain on benzene while avoiding rearrangement of an alkyl carbocation? Friedel–Crafts acylation followed by reduction of the carbonyl to a methylene.
Why can ordinary ionic HX addition to an alkene produce a rearranged skeleton? Its carbocation intermediate may undergo a hydride or alkyl shift before halide capture.
How many transition states appear in the simplest energy diagram for one concerted elementary step? One.
Is the cyclohexadienyl cation formed during electrophilic aromatic substitution fully aromatic? No. One ring carbon becomes sp³, interrupting the continuous aromatic π system.
Why can an allylic carbon radical be more stabilized than an ordinary primary alkyl radical? Its unpaired electron is delocalized over the neighboring π system.
What functional group results from LiAlH4 reduction of an amide? An amine; the carbonyl oxygen is removed while the C–N connection is retained.
Why can tert-butoxide favor a less substituted alkene in E2 elimination? Its bulk can make removal of a less hindered β hydrogen faster.
What intermediate forms immediately after hydride delivery to the carbonyl carbon of a ketone? A metal-associated alkoxide, protonated during workup to give an alcohol.
What bridged intermediate forms during ordinary Br2 addition to an alkene in a nonnucleophilic solvent? A bromonium ion.
What amide forms when propanoyl chloride reacts with excess methylamine? N-Methylpropanamide.
To make 1-methoxybutane by Williamson synthesis, which methyl electrophile can pair with butan-1-olate? Iodomethane; alkoxide attack gives the ether by SN2.
What products form when CH3• reacts with Cl2? Chloromethane and a chlorine radical.
What major bromide forms when propene reacts with HBr under peroxide-initiated radical conditions? 1-Bromopropane.
What bond-cleavage mode generates radicals when Cl2 absorbs suitable light? Homolytic cleavage of the Cl–Cl bond.
Which carbon is called α to a carbonyl group? A carbon directly adjacent to the carbonyl carbon.
Which conformation must an ordinary acyclic diene access for Diels–Alder reaction? An s-cis conformation.
Why can hydration of an unsymmetrical internal alkyne give more than one ketone? Water can add in either regiochemical orientation before tautomerization.
In base-promoted E1cB dehydration of a β-hydroxy carbonyl compound, which event precedes OH departure? Removal of an α proton to form a resonance-stabilized anion.
After prolonged acidic hydrolysis of N-methylpropanamide, in what form is the nitrogen-containing product present? Primarily methylammonium ion under the acidic conditions.
What protonation event increases electrophilicity at the start of the standard Fischer esterification mechanism? Protonation of the carbonyl oxygen.
How do halogens combine deactivation with ortho/para direction in electrophilic aromatic substitution? Inductive withdrawal slows reaction, while lone-pair donation stabilizes ortho/para attack intermediates relative to meta.
Why does oxymercuration usually avoid the skeletal rearrangements possible in acid hydration? Its bridged mercurinium pathway avoids a freely rearranging carbocation.
Under kinetic control, which competing product is favored? The product formed faster through the lower effective activation barrier.
Why should pKa values from different solvents not be compared as though they shared one scale? Solvation changes acid–base equilibria, so the numerical values are solvent-dependent.
Why need an SN1 reaction at one stereocenter not give an exactly 50:50 enantiomer mixture? Ion pairing and the departing group can make the two faces unequally accessible.
What alcohol results from CH3MgBr reacting with ethylene oxide, followed by acidic workup? Propan-1-ol; epoxide opening extends the methyl fragment by two carbons.
What step restores aromaticity after nucleophilic addition to an activated haloarene? Loss of the leaving group with re-formation of the ring π system.
What carbonyl product forms from hex-1-yne with HgSO4, aqueous H2SO4 and water? Hexan-2-one after enol–ketone tautomerization.
How can excess alcohol favor an ester in Fischer esterification? It shifts the reversible equilibrium toward ester formation.
Why are carbocation rearrangements generally absent in bromonium-mediated alkene bromination? The pathway uses a bridged halonium ion rather than a freely rearranging carbocation.
Can a rule that 1,2-addition is kinetic and 1,4-addition is thermodynamic be applied to every substituted diene without checking structures? No. Relative barriers and product stabilities depend on the particular diene and conditions.
Under reversible conditions allowing equilibration, which product is favored thermodynamically? The product with lower free energy under those conditions.
When an alkene reacts with Br2 in water, which abundant neutral nucleophile commonly opens the bromonium ion? Water.
Which hydrogen can be removed to make an alkene by ordinary E2 elimination? A β hydrogen on a carbon adjacent to the carbon bearing the leaving group.
Immediately after water bonds to a carbocation in SN1 hydrolysis, what oxygen-containing intermediate is formed? A protonated alcohol, which must lose a proton to give the neutral alcohol.
In a Diels–Alder reaction of a diene with an alkene, where does the remaining double bond form in the new ring? Between the two internal carbons of the original diene.
Which partner retains its carbonyl in a simple aldol addition product: enolate donor or carbonyl acceptor? The enolate donor.
What makes an SN1 substitution a solvolysis? The solvent itself acts as the nucleophile that traps the carbocation.
Why is calling a substrate secondary insufficient to choose between SN2 and E2? Nucleophile/basicity, steric bulk, solvent and temperature also affect the competing rates.
Why can DMSO accelerate SN2 reactions of many anionic nucleophiles compared with a protic solvent? It solvates cations well without hydrogen-bonding as strongly to the anionic nucleophile.
What π-electron count fits the Hückel rule for a simple monocyclic aromatic system? 4n + 2 π electrons, where n is a nonnegative integer.
Why is ordinary SN2 substitution at a tertiary alkyl carbon strongly disfavored? The substituents obstruct backside approach to the carbon.
What substitution product forms when 1-fluoro-4-nitrobenzene reacts with methoxide by SNAr? 1-Methoxy-4-nitrobenzene.
Why is a neutral carbon bearing five ordinary two-electron bonds invalid in an introductory organic mechanism? It would give second-row carbon ten valence electrons rather than an allowed octet.
After a neighboring C–H bond shifts into a carbocation center, where is the new positive charge located? On the carbon that lost the hydrogen and its bonding electron pair.
Why is deprotonation needed after water opens a bromonium ion? The newly bonded water oxygen initially bears a positive charge.
What relative stereochemistry characterizes hydrogen addition on an ordinary metal-catalyst surface? Syn addition of the two hydrogen atoms.
What is the conjugate base formed when acetic acid loses its O–H proton? Acetate, CH3CO2⁻.
What is the relative face relationship of H and B added in hydroboration? Syn.
Why are dry ether solvents commonly used to prepare and react Grignard reagents? They coordinate magnesium without supplying an acidic O–H proton that would quench the reagent.
After workup, what alcohol class usually results from a Grignard reagent adding to an aldehyde other than formaldehyde? A secondary alcohol.
What electrophilic species is associated with Friedel–Crafts acylation using an acid chloride and AlCl3? An acylium ion or closely associated acylium-like electrophile.
What functional group forms when a Grignard reagent reacts with CO2 and then undergoes acidic workup? A carboxylic acid with one additional carbon.
How does weaker leaving-group basicity generally affect acyl-substitution reactivity, other factors comparable? It makes expulsion easier and tends to increase reactivity.
Does the nitrogen lone pair of pyridine supply two electrons to the aromatic π sextet? No. It lies in an sp² orbital outside that π system.
What functional group replaces the carbonyl C=O bond in a Wittig reaction? A C=C bond connecting the carbonyl carbon to the ylide carbon.
Which end of a C–Mg bond in a Grignard reagent behaves nucleophilically? The carbon end.
What makes a reaction stereospecific with respect to its starting stereoisomers? Different starting stereoisomers lead to correspondingly different stereoisomeric products through the mechanism.
In acid-catalyzed enolization of a ketone, what is protonated before α deprotonation? The carbonyl oxygen.
What adjacent atom is typically required to generate benzyne from a haloarene by strong-base elimination? An ortho hydrogen next to the leaving group.
Why can fluoride be an effective leaving group in addition-limited SNAr even though alkyl fluorides are poor SN2 substrates? Its strong inductive effect can accelerate the initial nucleophilic addition; the controlling step differs from alkyl SN2.
What structural feature is needed for ordinary vigorous oxidation of an alkylbenzene side chain to benzoic acid? At least one benzylic hydrogen.
Why does Br• preferentially add to the terminal carbon of propene in the radical HBr chain? That orientation leaves the more stabilized secondary carbon radical.
Why does Friedel–Crafts acylation normally avoid the carbon-skeleton rearrangements possible in alkylation? The resonance-stabilized acylium electrophile does not undergo ordinary alkyl-carbocation rearrangements.
Are hydride shifts normally expected during a concerted E2 step? No. There is no carbocation intermediate to rearrange.
What product class can an acid chloride give with an appropriate lithium diorganocuprate under controlled conditions? A ketone, rather than the usual double-addition alcohol obtained with excess Grignard reagent.
Why can Friedel–Crafts alkylation give repeated alkylation of a benzene ring? The first alkyl group activates the product toward further electrophilic substitution.
What does base remove to initiate ordinary ketone enolate formation? An α proton.
Which atom of an ordinary aldehyde C=O is attacked by a carbon nucleophile? The carbonyl carbon, which is partially positive.
Why is ethoxide commonly paired with an ethyl ester in a Claisen condensation? Using the matching alkoxide limits unwanted ester exchange.
What elementary event initiates a typical SN1 solvolysis of a tertiary alkyl halide? Ionization of the carbon–halogen bond to form a carbocation and halide.
Why does an ordinary ester often consume two equivalents of a Grignard reagent to give a tertiary alcohol after workup? The first acyl substitution forms a ketone, which undergoes a second addition.
What additional step follows nucleophilic addition in a Claisen condensation because the acceptor is an ester? Collapse of the tetrahedral intermediate with alkoxide expulsion.
What portion of an ester is exchanged when methyl propanoate undergoes acid-catalyzed transesterification with excess ethanol? The methoxy group is replaced by ethoxy, giving ethyl propanoate.
After workup, what alcohol class results from a Grignard reagent adding to a ketone? A tertiary alcohol.
What three-membered functional group forms when an alkene reacts with a peroxycarboxylic acid such as mCPBA? An epoxide.
In hydroboration followed by H2O2/OH⁻, what replaces the carbon–boron bond at carbon? A carbon–oxygen bond, with retention of configuration at that carbon.
To make hex-2-yne by alkylating a propyne-derived acetylide, which unbranched primary bromide supplies the remaining carbon fragment? 1-Bromopropane, supplying three carbons.
Which bond is severed when an alkene undergoes ozonolysis followed by a reductive workup? The original C=C bond is cleaved, giving carbonyl compounds at its two carbons.
Which free-energy difference controls the rate of an elementary step at a specified temperature? The activation free energy between its reactant state and transition state.
How does an acetal differ from a hemiacetal at the former carbonyl carbon? It has two OR groups and no OH group at that carbon.
What organic functional-group products form from an ordinary ester with aqueous acid at equilibrium? A carboxylic acid and an alcohol.
Which two functional-group classes combine in Fischer esterification? A carboxylic acid and an alcohol under acid catalysis.
Why is radical bromination of an alkane generally more selective than chlorination for different C–H sites? Its hydrogen-abstraction transition state is more sensitive to the stability of the carbon radical formed.
What resonance-stabilized anion is commonly formed after nucleophilic 1,4-addition to an enone? An enolate.
What common ring motif is formed in a classical Robinson annulation after dehydration? A six-membered α,β-unsaturated ketone ring.
For an epoxide with one tertiary and one primary carbon, where does a weak neutral nucleophile preferentially attack after protonation? The tertiary carbon, which bears greater carbocation-like character in the activated epoxide.
Why does replacing a hydrogen of acetic acid with chlorine increase its acidity? Chlorine withdraws electron density inductively and stabilizes the conjugate base.
Are resonance contributors separate molecules rapidly interconverting in equilibrium? No. They are representations contributing to one electronic structure.
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