Aldol Addition Reaction

In this video let's discuss aldol addition reaction of carbonyl compounds.

The name aldol is derived from aldehyde (ald-) and alcohol (-ol), but compounds which contain ketone and alcohol are also considered as aldols.

This reaction is not in A Level Chemistry Syllabus but is quite common in JC Prelim questions so it's a good idea to have an idea what this reaction is about.

aldol addition reaction equation

Using a strong acid or base catalyst, 2 carbonyl compounds can react to form a beta-hydroxy carbonyl compound.

The alcohol functional group is 2 carbons away from the carbonyl functional group.

Let's consider the mechanism of aldol addition reaction, using a base catalyst as an example, to understand this better.

Mechanism (base catalysed)

1. The base can extract a proton from alpha-alkyl carbon to form a negatively charged alpha carbanion.

aldol addition reaction mechanism step 1

2. The carbonanion acts as nucleophile and attack carbonyl carbon.

aldol addition reaction mechanism step 2

Notice this is very similar to the nucleophilic addition reaction that is in A Level Chemistry syllabus.

3. The negative oxygen extracts a proton from water molecule to form the product and regenerates OH- catalyst

aldol addition reaction mechanism step 3

Predicting Products for Aldol Addition Reaction

With the understanding of the mechanism we can now predict the products that each of these carbonyl compounds can form when they undergo aldol addition reactions.

1. ethanal

aldol addition reaction reactant ethanal

2. propanone

aldol addition reaction reactant propanone

3. butanone

aldol addition reaction reactant butanone

Notice when the ketone is asymmetrical we can have a mixture of products.

In fact we can have 2 different carbonyl compounds reacting together via aldol addition to give many different products.

To figure out the products, watch the step-by-step discussion in this video!

Topic: Carbonyl Compounds, Organic Chemistry, A Level Chemistry, Singapore

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