Why Benzaldehyde Cannot Undergo an Aldol Reaction
Understand the mechanism of nucleophilic addition to aldehydes and ketones and why aldehydes react faster, then learn their condensation, oxidation and reduction reactions, including the aldol and Cannizzaro reactions.
Why is the carbonyl group such a reactive site?
The carbon-oxygen double bond of an aldehyde or ketone is strongly polar: oxygen pulls electrons towards itself, leaving carbon slightly positive and open to attack. That single feature explains how carbonyl compounds add hydrogen cyanide, join together into larger molecules and switch between alcohols and acids.
This lesson covers the mechanism of nucleophilic addition, and the condensation, oxidation and reduction reactions of aldehydes and ketones, including the aldol and Cannizzaro reactions.
This lesson covers the mechanism of nucleophilic addition, and the condensation, oxidation and reduction reactions of aldehydes and ketones, including the aldol and Cannizzaro reactions.
How does nucleophilic addition to aldehydes and ketones work?
In nucleophilic addition, a nucleophile attacks the partly positive carbonyl carbon from above or below the flat C=O group, forming a tetrahedral alkoxide that then picks up a proton; aldehydes react faster than ketones because they are less crowded and their carbonyl carbon is more positive.
Mechanism:
- The carbonyl carbon is and planar, with a partial positive charge
- Step 1 — the nucleophile bonds to carbon and the pi electrons move onto oxygen, giving a tetrahedral alkoxide ion
- Step 2 — the alkoxide takes a proton from the medium, giving the neutral product
Why aldehydes react faster than ketones:
- Electronic — a ketone's two alkyl groups push electrons towards the carbonyl carbon, reducing its positive charge
- Steric — two alkyl groups crowd the approaching nucleophile
- Order: methanal > ethanal > propanone; benzaldehyde is less reactive than ethanal, because resonance with the ring lowers the carbon's positive charge
Important additions:
- Hydrogen cyanide, with a base to supply , gives cyanohydrins:
- Sodium hydrogensulphite gives crystalline adducts, used to separate aldehydes from mixtures
- Alcohols with dry HCl turn aldehydes into hemiacetals and then acetals
- Ammonia derivatives add and then lose water, giving C=N compounds: hydroxylamine gives oximes, hydrazine gives hydrazones, and 2,4-DNP gives coloured hydrazones
Worked example. Propanone with HCN gives ; hydrolysing its cyano group then gives 2-hydroxy-2-methylpropanoic acid — a way to add one carbon to a molecule.
An everyday example. The starch in rice and wheat holds its glucose units together through acetal-type links, formed when an -OH group adds to a carbonyl carbon in exactly this way.
The substance. The nucleophile can attack either face of the flat carbonyl group — so adding HCN to ethanal gives a racemic cyanohydrin.
Mechanism:
- The carbonyl carbon is and planar, with a partial positive charge
- Step 1 — the nucleophile bonds to carbon and the pi electrons move onto oxygen, giving a tetrahedral alkoxide ion
- Step 2 — the alkoxide takes a proton from the medium, giving the neutral product
Why aldehydes react faster than ketones:
- Electronic — a ketone's two alkyl groups push electrons towards the carbonyl carbon, reducing its positive charge
- Steric — two alkyl groups crowd the approaching nucleophile
- Order: methanal > ethanal > propanone; benzaldehyde is less reactive than ethanal, because resonance with the ring lowers the carbon's positive charge
Important additions:
- Hydrogen cyanide, with a base to supply , gives cyanohydrins:
- Sodium hydrogensulphite gives crystalline adducts, used to separate aldehydes from mixtures
- Alcohols with dry HCl turn aldehydes into hemiacetals and then acetals
- Ammonia derivatives add and then lose water, giving C=N compounds: hydroxylamine gives oximes, hydrazine gives hydrazones, and 2,4-DNP gives coloured hydrazones
Worked example. Propanone with HCN gives ; hydrolysing its cyano group then gives 2-hydroxy-2-methylpropanoic acid — a way to add one carbon to a molecule.
An everyday example. The starch in rice and wheat holds its glucose units together through acetal-type links, formed when an -OH group adds to a carbonyl carbon in exactly this way.
The substance. The nucleophile can attack either face of the flat carbonyl group — so adding HCN to ethanal gives a racemic cyanohydrin.
What are the aldol, Cannizzaro, oxidation and reduction reactions of carbonyl compounds?
Aldehydes and ketones with alpha hydrogens undergo aldol condensation in dilute alkali, aldehydes without alpha hydrogens undergo the Cannizzaro reaction in concentrated alkali, aldehydes are easily oxidised to acids, and both classes can be reduced to alcohols or all the way to hydrocarbons.
Alpha hydrogens. A hydrogen on the carbon next to C=O is weakly acidic, because the carbanion left behind is stabilised by resonance with the carbonyl group.
Aldol condensation:
- Two molecules with alpha hydrogens combine in dilute NaOH to give a beta-hydroxy aldehyde or ketone, the aldol, which loses water on heating:
- Cross aldol reactions between two different carbonyl compounds give mixtures, unless one partner has no alpha hydrogen
Cannizzaro reaction. Aldehydes with no alpha hydrogen disproportionate in concentrated alkali: one molecule is oxidised to a carboxylate and another reduced to an alcohol.
Oxidation:
- Aldehydes are oxidised to carboxylic acids even by mild reagents such as Tollens' and Fehling's
- Ketones resist mild oxidation, but methyl ketones give the haloform reaction:
Reduction:
- , or catalytic hydrogen give primary alcohols from aldehydes and secondary alcohols from ketones
- Clemmensen reduction, with zinc amalgam and concentrated HCl, and Wolff-Kishner reduction, with hydrazine and KOH, turn C=O into
Worked example. Ethanal in dilute NaOH gives the aldol 3-hydroxybutanal, but benzaldehyde in concentrated NaOH gives sodium benzoate and benzyl alcohol, because it has no alpha hydrogen.
An everyday example. Cinnamaldehyde, the flavour of cinnamon used in many Indian sweets and perfumes, can be made by a cross aldol reaction between benzaldehyde and ethanal.
The substance. The Cannizzaro reaction needs an aldehyde with no alpha hydrogen — when alpha hydrogens are present, the faster aldol reaction takes over.
Alpha hydrogens. A hydrogen on the carbon next to C=O is weakly acidic, because the carbanion left behind is stabilised by resonance with the carbonyl group.
Aldol condensation:
- Two molecules with alpha hydrogens combine in dilute NaOH to give a beta-hydroxy aldehyde or ketone, the aldol, which loses water on heating:
- Cross aldol reactions between two different carbonyl compounds give mixtures, unless one partner has no alpha hydrogen
Cannizzaro reaction. Aldehydes with no alpha hydrogen disproportionate in concentrated alkali: one molecule is oxidised to a carboxylate and another reduced to an alcohol.
Oxidation:
- Aldehydes are oxidised to carboxylic acids even by mild reagents such as Tollens' and Fehling's
- Ketones resist mild oxidation, but methyl ketones give the haloform reaction:
Reduction:
- , or catalytic hydrogen give primary alcohols from aldehydes and secondary alcohols from ketones
- Clemmensen reduction, with zinc amalgam and concentrated HCl, and Wolff-Kishner reduction, with hydrazine and KOH, turn C=O into
Worked example. Ethanal in dilute NaOH gives the aldol 3-hydroxybutanal, but benzaldehyde in concentrated NaOH gives sodium benzoate and benzyl alcohol, because it has no alpha hydrogen.
An everyday example. Cinnamaldehyde, the flavour of cinnamon used in many Indian sweets and perfumes, can be made by a cross aldol reaction between benzaldehyde and ethanal.
The substance. The Cannizzaro reaction needs an aldehyde with no alpha hydrogen — when alpha hydrogens are present, the faster aldol reaction takes over.
Exam tip
What earns full marks on reactions of aldehydes and ketones?
Before predicting any base-catalysed product, count the alpha hydrogens — if they are present, expect aldol; if an aldehyde has none, expect Cannizzaro.
- Nucleophilic addition: attack on carbon, then protonation of oxygen; aldehydes react faster than ketones
- HCN gives cyanohydrins; gives crystalline adducts; ammonia derivatives give C=N compounds
- Aldol: dilute alkali, a beta-hydroxy carbonyl compound, then dehydration on heating
- Cannizzaro: concentrated alkali, giving a carboxylate and an alcohol
- Clemmensen and Wolff-Kishner reduce C=O to
The trap. Writing a Cannizzaro reaction for ethanal. Ethanal has alpha hydrogens, so in alkali it undergoes aldol condensation instead.
- Nucleophilic addition: attack on carbon, then protonation of oxygen; aldehydes react faster than ketones
- HCN gives cyanohydrins; gives crystalline adducts; ammonia derivatives give C=N compounds
- Aldol: dilute alkali, a beta-hydroxy carbonyl compound, then dehydration on heating
- Cannizzaro: concentrated alkali, giving a carboxylate and an alcohol
- Clemmensen and Wolff-Kishner reduce C=O to
The trap. Writing a Cannizzaro reaction for ethanal. Ethanal has alpha hydrogens, so in alkali it undergoes aldol condensation instead.
Did you know
How does your body run an aldol reaction in reverse?
When you digest a chapati, your cells break down its glucose through a chain of reactions, and one key step is an aldol reaction running backwards.
An enzyme called aldolase splits a six-carbon sugar, fructose 1,6-bisphosphate, into two three-carbon fragments by breaking exactly the kind of carbon-carbon bond that an aldol condensation forms.
The chemistry you draw in a notebook with dilute alkali happens continuously in your muscles and brain, guided by enzymes instead of sodium hydroxide.
An enzyme called aldolase splits a six-carbon sugar, fructose 1,6-bisphosphate, into two three-carbon fragments by breaking exactly the kind of carbon-carbon bond that an aldol condensation forms.
The chemistry you draw in a notebook with dilute alkali happens continuously in your muscles and brain, guided by enzymes instead of sodium hydroxide.
Exam relevance
How do JEE Main and NEET test nucleophilic addition, aldol and Cannizzaro reactions?
Aldehydes, Ketones and Carboxylic Acids is a recurring chapter in both JEE Main and NEET, and carbonyl reactions are central to organic reaction-sequence questions.
What gets asked. The mechanism and relative reactivity of nucleophilic addition, products of aldol and cross aldol reactions, Cannizzaro products, Clemmensen and Wolff-Kishner reductions, and the haloform reaction.
Question types. Mostly single-correct and multi-step conversion questions, with JEE Advanced setting longer reaction sequences.
Why it matters later. Carbonyl chemistry underlies Biomolecules, where sugars form hemiacetals, and the chemistry of carboxylic acids in the next part of this chapter.
The trap that costs marks. Forgetting the dehydration that follows aldol addition on heating — the final product is often the unsaturated aldehyde or ketone, not the aldol itself.
What gets asked. The mechanism and relative reactivity of nucleophilic addition, products of aldol and cross aldol reactions, Cannizzaro products, Clemmensen and Wolff-Kishner reductions, and the haloform reaction.
Question types. Mostly single-correct and multi-step conversion questions, with JEE Advanced setting longer reaction sequences.
Why it matters later. Carbonyl chemistry underlies Biomolecules, where sugars form hemiacetals, and the chemistry of carboxylic acids in the next part of this chapter.
The trap that costs marks. Forgetting the dehydration that follows aldol addition on heating — the final product is often the unsaturated aldehyde or ketone, not the aldol itself.
Key takeaways
What must you be able to do from this lesson?
- Nucleophilic addition: attack on the carbonyl carbon, faster for aldehydes than ketones, giving cyanohydrins, hydrogensulphite adducts, acetals and C=N derivatives
- Aldol and Cannizzaro: aldol with alpha hydrogens in dilute alkali, and Cannizzaro without them in concentrated alkali
- Oxidation and reduction: aldehydes oxidise easily, and both classes reduce to alcohols or, by Clemmensen or Wolff-Kishner, to hydrocarbons
What product forms when propanal is warmed with dilute sodium hydroxide, and what happens if the mixture is heated further?
- Aldol and Cannizzaro: aldol with alpha hydrogens in dilute alkali, and Cannizzaro without them in concentrated alkali
- Oxidation and reduction: aldehydes oxidise easily, and both classes reduce to alcohols or, by Clemmensen or Wolff-Kishner, to hydrocarbons
What product forms when propanal is warmed with dilute sodium hydroxide, and what happens if the mixture is heated further?