Ch18: Aldol reaction of RCHO

Chapter 18: Enols and Enolates

The Aldol Reaction of Aldehydes Aldol reaction of an aldehdye

Reaction type : Nucleophilic addition

Summary

Reagents : commonly a base such as NaOH or KOH is added to the aldehyde.
The reaction involves an aldehyde enolate reacting with another molecule of the aldehyde.
Remember enolates are good nucleophiles and carbonyl C are good electrophiles.
Since the pK_a of an aldehyde is close to that of NaOH, both enolate and aldehyde are present simultaneously.
The products of these reactions are β-hydroxyaldehydes or aldehyde-alcohols = aldols.
The simplest aldol reaction is the condensation of ethanal. This is shown below in 2 different representations (the line diagrams are less cluttered).....


Try to identify the enolate portion and the carbonyl portion in the different representations

QUESTION Why isn't the simplest example of an aldol the condensation of methanal ? ANSWER

These Aldol products can often then undergo dehydration (loss of water) to give conjugated systems (an elimination reaction). This is then often called an Aldol Condensation.

STUDY TIPS:

Students often find this reaction difficult, probably because they are intimidated by it..... break it down into the component steps.
Think of it as a nucleophile adding to the carbonyl of the aldehyde as we saw in chapter 17 (review ?)
Try to recognise the two units that came together and identify the nucleophilic (enolate) portion and the electrophilic (carbonyl) portion.

The nucleophilic enolate will still contain the C=O unit
The electrophilic carbonyl will have become an alcohol C-OH (or dehydrated to C=C)

Recognise the products, either the β-hydroxyaldehydes = the aldol, or the conjugated product, the enal as being from an Aldol reaction.

Related Reactions

MECHANISM OF THE ALDOL CONDENSATION
1. MECHANISM OF THE ALDOL REACTION
Step 1: First, an acid-base reaction. Hydroxide functions as a base and removes the acidic α-hydrogen giving the reactive enolate.
Step 2: The nucleophilic enolate attacks the aldehyde at the electrophilic carbonyl C in a nucleophilic addition type process giving an intermediate alkoxide.
Step 3: An acid-base reaction. The alkoxide deprotonates a water molecule creating hydroxide and the β-hydroxyaldehydes or aldol product.

2. MECHANISM OF THE DEHYDRATION OF THE ALDOL PRODUCT
Step 1: First, an acid-base reaction. Hydroxide functions as a base and removes an acidic α-hydrogen giving the reactive enolate.
Step 2: The electrons associated with the negative charge of the enolate are used to form the C=C and displace the leaving group, regenerating hydroxide giving the conjugated aldehyde.