Difficulty: Intermediate to Advanced | Prerequisites: Part 1 of these notes (structure, nomenclature, synthesis). Familiarity with nucleophiles, electrophiles, and acid-base catalysis.
This is the central mechanistic section of Chapter 17. The carbonyl group is electrophilic at carbon and nucleophilic at oxygen, and the dominant reaction type for aldehydes and ketones is nucleophilic addition. A nucleophile attacks the electrophilic carbonyl carbon, breaking the pi bond and pushing electron density onto oxygen. This single pattern, with variations in nucleophile strength, catalysis, and whether the reaction is reversible, accounts for hydration, cyanohydrin formation, Grignard addition, hydride reduction, imine and enamine formation, and the Wolff-Kishner reduction. Master the general mechanism and each specific case becomes a variation on the theme.
Nucleophiles attack the electrophilic carbonyl carbon of aldehydes and ketones, converting the sp2 carbon to sp3. Aldehydes are more reactive than ketones for steric and electronic reasons. The nucleophile can be charged (HO-, CN-, RMgX, H:-) or neutral (H2O, ROH, RNH2, R2NH), and the reaction may be reversible (hydration, imine formation) or irreversible (Grignard, hydride reduction).
Nucleophilic addition
The characteristic reaction of aldehydes and ketones: a nucleophile attacks the carbonyl carbon, the C=O pi bond breaks, and a tetrahedral alkoxide intermediate forms. In simple terms, something electron-rich smashes into the partially positive carbon and pushes the electrons onto oxygen.
Cyanohydrin
The product of HCN addition to an aldehyde or ketone: R(R')C(OH)(CN). The CN group can later be converted to a carboxylic acid (acid hydrolysis) or an amino alcohol (LiAlH4 reduction).
Grignard reagent (RMgX)
An organomagnesium halide that acts as a source of R:- (a carbanion equivalent). It adds irreversibly to a carbonyl, forming a new C–C bond. Think of it as a carbon nucleophile delivered in ether solvent.
Hydride reagents (NaBH4, LiAlH4)
Sources of H:- (hydride ion). NaBH4 is milder (works in EtOH or MeOH); LiAlH4 is more powerful (requires anhydrous ether, THF). Both reduce aldehydes and ketones to alcohols. The addition is irreversible.
Imine (Schiff base)
The product of a primary amine (RNH2) reacting with an aldehyde or ketone: R'R''C=NR. Formation involves nucleophilic addition followed by loss of water. In simple terms, the oxygen of the carbonyl is swapped for the nitrogen of the amine, with water leaving.
Enamine
The product of a secondary amine (R2NH) reacting with an aldehyde or ketone that has an alpha-hydrogen. The nitrogen ends up bonded to carbon and the double bond shifts to the C=C position (not C=N, because there is no N–H to lose). Think of it as the nitrogen analogue of an enol.
Wolff-Kishner reduction
Converts a carbonyl (C=O) all the way to a CH2 group by treatment with hydrazine (H2NNH2) and base (KOH), followed by acid workup. The mechanism goes through a hydrazone intermediate, then base removes protons while N2 gas is lost.
Oxime
The product of hydroxylamine (NH2OH) reacting with a ketone or aldehyde. Structure: R'R''C=N–OH. Formed by the same imine-type mechanism.
Semicarbazone
The product of semicarbazide (H2N–NH–CO–NH2) reacting with a carbonyl compound. Another imine-type derivative, historically used to identify unknown carbonyl compounds by their melting points.
Hydrazone
The product of hydrazine (H2NNH2) reacting with a carbonyl compound. Structure: R'R''C=N–NH2. This is the intermediate in the Wolff-Kishner reduction.
Hammond postulate
The principle that the transition state of a reaction resembles the species (reactant or product) closest to it in energy. Applied here: if the equilibrium favours addition (product is more stable), the transition state will also be lower in energy, so the reaction is also faster.
The carbonyl carbon is sp2 (trigonal planar, bond angles ~120 degrees). After nucleophilic attack, it becomes sp3 (tetrahedral, bond angles ~109 degrees).
Strong nucleophiles (HO-, CN-, R:-, H:-) attack the carbonyl directly without needing acid activation.
Weak nucleophiles (H2O, ROH, RNH2) often require acid catalysis: protonation of the carbonyl oxygen makes the carbon more electrophilic and lowers the activation energy for attack.
Base catalysis is also possible: the base generates the nucleophile (e.g., HO- from H2O, or CN- from HCN).
Aldehydes are more reactive toward nucleophilic addition than ketones, for four reasons:
Steric: only one R group flanks the carbonyl in an aldehyde, so the nucleophile can approach more easily.
The addition product is less sterically crowded for aldehydes.
The transition state is less crowded, resulting in a lower activation energy (lower delta-G-double-dagger).
Electronic: hydrogen does not donate electron density to the carbonyl carbon the way an alkyl group does, so the carbonyl carbon in an aldehyde is more delta-positive and more electrophilic.
Water adds reversibly to the carbonyl to form a gem-diol (hydrate): two OH groups on the same carbon.
The equilibrium constant (Keq) depends on the substituents. Aldehydes favour hydration more than ketones. Electron-withdrawing groups (e.g., Cl, CF3) on the carbonyl carbon strongly favour the hydrate because they destabilise the carbonyl form.
Formaldehyde (H2C=O) exists almost entirely as the hydrate in water. Acetone (CH3COCH3) has a very small Keq for hydration.
The equilibrium is established by either acid or base catalysis:
Base: HO- attacks the carbonyl, then the alkoxide picks up a proton from water.
Acid: protonation of the carbonyl oxygen, then water attacks the now highly electrophilic carbon, then deprotonation.
HCN is a weak acid (pKa ~9.1), so very little CN- is present at equilibrium.
The reaction is catalysed by base (to generate CN-) or by direct addition of a cyanide salt (NaCN or KCN).
The nucleophilic CN- attacks the carbonyl carbon, forming a cyanohydrin after protonation.
Cyanohydrins are synthetically useful: acid hydrolysis converts the CN to a carboxylic acid (alpha-hydroxy acid); LiAlH4 reduction converts it to a beta-amino alcohol.
A Grignard reagent (RMgX) acts as a source of R:-, a powerful carbon nucleophile.
It attacks the carbonyl carbon, forming a new C–C bond. The magnesium coordinates to the carbonyl oxygen (Lewis acid activation), making the carbon even more electrophilic.
The alkoxide intermediate is protonated during aqueous workup to give the alcohol.
This is irreversible (not an equilibrium), because the C–C bond formed is strong and stable.
Grignard + formaldehyde gives a primary alcohol. Grignard + other aldehyde gives a secondary alcohol. Grignard + ketone gives a tertiary alcohol.
NaBH4 (in EtOH) and LiAlH4 (in Et2O or THF) both deliver H:- to the carbonyl carbon.
The aldehyde or ketone is reduced to an alcohol.
NaBH4 is selective: it reduces aldehydes and ketones but does not reduce esters or carboxylic acids.
LiAlH4 is more powerful and reduces aldehydes, ketones, esters, carboxylic acids, and amides.
The reaction is irreversible.
A primary amine (RNH2) reacts with an aldehyde or ketone to form an imine (C=NR) plus water.
The mechanism has two stages: first, nucleophilic addition of the amine to the carbonyl to give a carbinolamine (tetrahedral intermediate with both OH and NHR); second, acid-catalysed dehydration to form the C=N double bond.
pH matters. The optimal pH is about 4.5. Too basic, and the protonation step needed for dehydration does not occur. Too acidic, and the amine is fully protonated (RNH3+), making it non-nucleophilic.
The equilibrium generally favours reactants (the carbonyl and free amine), so water must be removed (e.g., with molecular sieves or a Dean-Stark trap) to drive the reaction forward.
Imine derivatives include oximes (from NH2OH), semicarbazones (from semicarbazide), and 2,4-dinitrophenylhydrazones (from 2,4-dinitrophenylhydrazine). These derivatives are more thermodynamically stable than simple imines, and their formation can even be done in water.
A secondary amine (R2NH) reacts with an aldehyde or ketone that has an alpha-hydrogen.
The mechanism begins the same way as imine formation: nucleophilic addition, then protonation of OH to make it a leaving group (water leaves).
The key difference: after water leaves, the nitrogen has no hydrogen to lose (it has two R groups). Instead, a proton is lost from the alpha-carbon, forming a C=C double bond adjacent to nitrogen.
The product is an enamine: a vinyl amine with the double bond between the alpha and carbonyl carbons.
Enamines are important synthetic intermediates (covered in later chapters for alkylation and acylation of ketones).
Converts a C=O group completely to a CH2 group (deoxygenation).
Conditions: 1) H2NNH2 (hydrazine), KOH, heat; 2) H3O+.
The mechanism proceeds through a hydrazone intermediate (C=N–NH2). Under strongly basic conditions, a series of proton transfers and tautomerisations occur, culminating in loss of N2 gas and formation of the C–H bonds.
This is a complement to the Clemmensen reduction (Zn/Hg, HCl), which accomplishes the same transformation under acidic conditions. The choice between them depends on whether the substrate is sensitive to acid or base.
Hydration: R2C=O + H2O ⇌ R2C(OH)2 (acid or base catalysis)
Cyanohydrin: R2C=O + HCN (base or CN-) → R2C(OH)(CN)
Grignard: R2C=O + R'MgX / Et2O, then H2O → R2(R')COH
NaBH4 reduction: R2C=O + NaBH4 / EtOH, then H3O+ → R2CHOH
LiAlH4 reduction: R2C=O + LiAlH4 / Et2O, then H2O → R2CHOH
Imine: R2C=O + RNH2 ⇌ R2C=NR + H2O (pH ~4.5)
Enamine: R(CHR')C=O + R2NH ⇌ R(CR')=C–NR2 + H2O
Wolff-Kishner: R2C=O + H2NNH2, KOH, then H3O+ → R2CH2
Grignard reactions are workhorses in pharmaceutical synthesis for building complex carbon skeletons. Many drug molecules contain secondary or tertiary alcohol centres that were constructed by adding a Grignard reagent to a carbonyl.
Imine formation is central to biochemistry: the amino acid lysine forms imine (Schiff base) linkages with the cofactor pyridoxal phosphate (vitamin B6) in nearly all amino acid metabolism. The retinal chromophore in your eye is bonded to opsin through a Schiff base.
Students often assume all nucleophilic additions to carbonyls are irreversible. Many are equilibria (hydration, HCN addition, imine formation). The irreversible ones are Grignard addition and hydride reduction, both driven by the formation of very stable products.
A common error is forgetting the pH dependence of imine formation. At very low pH the amine is protonated and non-nucleophilic. At very high pH the dehydration step stalls.
Students sometimes confuse imines and enamines. The key distinction: primary amines give imines (C=N bond), secondary amines give enamines (C=C bond adjacent to nitrogen, because there is no N–H available to lose).
The Wolff-Kishner and Clemmensen reductions are frequently mixed up. Wolff-Kishner uses base (hydrazine, KOH). Clemmensen uses acid (Zn/Hg, HCl). Both produce the same CH2 product.
⚠️ The general nucleophilic addition mechanism is tested repeatedly. Be able to draw the full arrow-pushing mechanism for both acid-catalysed and base-catalysed pathways.
⚠️ Know the four reasons aldehydes are more reactive than ketones. This is a favourite short-answer or multiple-choice topic.
⚠️ Grignard addition problems appear constantly in synthesis questions. Be comfortable predicting the product and working retrosynthetically (given a target alcohol, identify the carbonyl and Grignard pieces).
⚠️ Imine vs enamine: which amine gives which product? Primary amine gives imine, secondary amine gives enamine. Expect a question that tests this distinction.
⚠️ Wolff-Kishner conditions (base) vs Clemmensen conditions (acid): know when to use each.
True or false: Ketones are more reactive toward nucleophilic addition than aldehydes.
Fill in the blank: The product of adding HCN to a ketone is called a ____.
True or false: A Grignard reagent added to formaldehyde gives a tertiary alcohol.
Fill in the blank: A primary amine reacting with a ketone forms a(n) ____; a secondary amine forms a(n) ____.
True or false: The Wolff-Kishner reduction uses acidic conditions.
Answers: 1) False (aldehydes are more reactive). 2) Cyanohydrin. 3) False (gives a primary alcohol). 4) Imine; enamine. 5) False (uses basic conditions: hydrazine + KOH).
Q: Draw the mechanism for base-catalysed hydration of an aldehyde.
A: HO- attacks the electrophilic carbonyl carbon, breaking the pi bond and forming a tetrahedral alkoxide. The alkoxide then abstracts a proton from a water molecule, regenerating HO- and producing the gem-diol (hydrate).
Q: Why does the addition of electron-withdrawing groups increase the Keq for hydration?
A: Electron-withdrawing groups (e.g., Cl, CF3) destabilise the carbonyl form by withdrawing electron density from the already electron-poor carbonyl carbon. This makes the reactant higher in energy relative to the hydrate product, shifting the equilibrium toward the hydrate.
Q: A ketone is treated with CH3MgBr in Et2O, then with aqueous acid. What type of alcohol is the product?
A: A tertiary alcohol. The methyl group from the Grignard adds to one side of the ketone carbonyl, and the two R groups of the original ketone remain, giving three carbon substituents on the hydroxyl-bearing carbon.
Q: Cyclohexanone is treated with methylamine (CH3NH2) at pH 4.5. What is the product?
A: An imine: N-methylcyclohexanimine (the C=O is replaced by C=N–CH3, with loss of water).
Q: What is the product of treating an aryl ketone (PhCOCH3) with H2NNH2, KOH, then H3O+?
A: Ethylbenzene (PhCH2CH3). The Wolff-Kishner reduction removes the carbonyl oxygen entirely, replacing C=O with CH2.
Nucleophilic addition to carbonyls builds directly on the concepts of nucleophilicity and electrophilicity from substitution and elimination chemistry (Chapters 7 and 11). Grignard reactions tie back to organometallic chemistry. Imine and enamine formation set the stage for alpha-carbon chemistry and the aldol reaction in Chapters 18–19. The Wolff-Kishner reduction is often used in tandem with Friedel-Crafts acylation to install straight-chain alkyl groups on aromatic rings (an alternative to the Clemmensen approach).
nucleophilic addition, carbonyl electrophile, sp2 to sp3, hydration gem-diol, cyanohydrin, HCN addition, Grignard reagent, RMgX, organomagnesium, hydride reduction, NaBH4, LiAlH4, sodium borohydride, lithium aluminium hydride, imine, Schiff base, enamine, primary amine, secondary amine, carbinolamine, Wolff-Kishner reduction, hydrazone, oxime, semicarbazone, 2,4-DNP, Hammond postulate, acid catalysis, base catalysis, organic chemistry chapter 17