19.8 Nucleophilic Addition of Amines: Imine and Enamine Formation

Nucleophilic Addition of Amines to Aldehydes and Ketones

Primary and secondary amines act as nucleophiles that attack the carbonyl carbon of aldehydes and ketones. Depending on the type of amine, the product is either an imine (from primary amines) or an enamine (from secondary amines). Both reactions follow the same general pattern: nucleophilic addition, proton transfer to give a carbinolamine intermediate, then loss of water.

Formation of Imines and Enamines

Imines form when a primary amine ($RNH_2$) reacts with an aldehyde or ketone. The mechanism has three key stages:

1. The nitrogen lone pair attacks the electrophilic carbonyl carbon, forming a tetrahedral alkoxide intermediate.
2. An intramolecular proton transfer from nitrogen to oxygen gives a carbinolamine (an amino alcohol, $R_2C(OH)(NHR')$).
3. The carbinolamine loses water (dehydration) to produce the imine, which contains a $C=N$ double bond ($R_2C=NR'$).

For example, acetone reacting with ethylamine produces N-ethylideneethylamine after dehydration.

Enamines form when a secondary amine ($R_2NH$) reacts with an aldehyde or ketone. Steps 1 and 2 are the same as above, but step 3 differs: because the nitrogen has no N–H proton to lose, dehydration instead removes a proton from the alpha carbon. This produces a carbon-carbon double bond adjacent to nitrogen ($R_2C=CR–NR'_2$).

• The resulting enamine is stabilized by conjugation of the $C=C$ bond with the nitrogen lone pair.
• A classic example is cyclohexanone reacting with morpholine to give 1-morpholinocyclohexene.

pH Dependence of Imine Formation

Imine formation is acid-catalyzed, but there's a catch: too much acid or too little acid both slow the reaction. The optimal pH is mildly acidic, around pH 4–5.

• Mild acid helps because protonation of the carbinolamine hydroxyl group converts $–OH$ into $–OH_2^+$, a much better leaving group. This accelerates the dehydration step, which is the rate-determining step.
• Too much acid hurts because the amine nucleophile gets protonated ($RNH_3^+$), making it non-nucleophilic. If the amine can't attack the carbonyl, the reaction stalls at the very first step.
• Basic conditions disfavor the reaction because without acid catalysis, the dehydration step is slow. The equilibrium shifts back toward starting materials.

This is why imine-forming reactions are typically run with a mild acid catalyst (like $p$-toluenesulfonic acid) or in a buffered solution.

Crystalline Derivatives of Aldehydes and Ketones

Certain amine-type nucleophiles produce solid, crystalline products that are useful for identifying unknown carbonyl compounds by their melting points. The mechanism in each case is the same nucleophilic addition/dehydration sequence.

Oximes form from reaction with hydroxylamine ($NH_2OH$):

• Hydroxylamine attacks the carbonyl carbon, and after proton transfer and dehydration, the product is an oxime ($R_2C=NOH$).
• For example, acetophenone gives acetophenone oxime, a crystalline solid.
• Oximes are useful because they tend to be easy-to-handle solids with characteristic melting points.

2,4-Dinitrophenylhydrazones (2,4-DNP derivatives) form from reaction with 2,4-dinitrophenylhydrazine:

• 2,4-DNP ($ArNHNH_2$, where $Ar$ = 2,4-dinitrophenyl) attacks the carbonyl, and after dehydration the product is a hydrazone ($R_2C=NNHAr$).
• These derivatives are typically brightly colored (yellow to red-orange), highly crystalline solids with sharp melting points.
• They're especially useful for confirming the presence of an aldehyde or ketone and identifying it by comparing the melting point to literature values.

Reaction Mechanisms and Related Concepts

• Imine and enamine formation are condensation reactions: two molecules join together with the loss of a small molecule (water).
• The dehydration step is specifically an elimination reaction (loss of $H_2O$ from the carbinolamine).
• Imine-enamine tautomerism is possible. Just as keto-enol tautomerism involves a proton shift between carbon and oxygen, imine-enamine tautomerism involves a proton shift that moves the double bond between $C=N$ and $C=C$. In most cases, the imine tautomer is more stable, but the enamine form can be favored when conjugation or steric effects stabilize it.