Carbonyl Compounds and Carboxylic Acids

This lesson covers aldehydes, ketones, and carboxylic acids. Aldehydes and ketones both contain the carbonyl group (C=O) but differ in their position within the molecule and their reactivity. Carboxylic acids contain the –COOH group and are weak acids. Understanding the nucleophilic addition mechanism and distinguishing tests is essential for A-Level.

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Aldehydes vs Ketones

Feature	Aldehyde	Ketone
Functional group	–CHO (C=O at end of chain)	–CO– (C=O within chain)
General formula	CₙH₂ₙO	CₙH₂ₙO
Suffix	-al	-one
Example	Ethanal (CH₃CHO)	Propanone (CH₃COCH₃)
Oxidation	Can be further oxidised to carboxylic acid	Resists further oxidation

Key structural difference: In an aldehyde, the carbonyl carbon is bonded to at least one hydrogen atom. In a ketone, the carbonyl carbon is bonded to two carbon-containing groups (no H on the C=O carbon).

Key Definition: The carbonyl group is C=O. The carbon is sp² hybridised, giving a trigonal planar arrangement (bond angle ~120°). The C=O bond is polar: carbon is δ+ (electrophilic) and oxygen is δ− (due to oxygen's higher electronegativity).

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Nucleophilic Addition — The Key Mechanism

The C=O bond is polar (Cδ+=Oδ−), making the carbon susceptible to attack by nucleophiles. The mechanism for carbonyl reactions is nucleophilic addition.

Reaction 1: Addition of HCN (Hydrogen Cyanide)

This reaction extends the carbon chain by one carbon and introduces a hydroxyl group — the product is a hydroxynitrile (cyanohydrin).

Equation:

CH₃CHO + HCN → CH₃CH(OH)CN (2-hydroxypropanenitrile)

CH₃COCH₃ + HCN → (CH₃)₂C(OH)CN (2-hydroxy-2-methylpropanenitrile)

Reagents: HCN (generated in situ from KCN + dilute H₂SO₄ for safety — HCN is extremely toxic).

Mechanism — Nucleophilic Addition:

1. The CN⁻ ion is the nucleophile. A curly arrow goes from the lone pair on the carbon of CN⁻ to the δ+ carbon of the C=O group.
2. Simultaneously, a curly arrow goes from the C=O π bond to the oxygen atom. The π bond breaks and both electrons go to the oxygen, giving it a negative charge (alkoxide intermediate: R₂C(CN)O⁻).
3. The negatively charged oxygen is then protonated: a curly arrow from an H⁺ (from HCN or the acid) to the lone pair on O⁻, forming the –OH group.

Product: A 2-hydroxynitrile (cyanohydrin).

Exam Tip: In the mechanism, the nucleophile must be CN⁻ (not HCN). State that CN⁻ is generated from the dissociation of HCN or from KCN. HCN itself is a poor nucleophile because the lone pair on C is tied up in bonding. Also state that HCN is used in practice (not pure CN⁻) because a source of H⁺ is needed for the final protonation step.

Why is this reaction useful?

The hydroxynitrile product contains both –OH and –CN groups:

• The –CN can be hydrolysed to –COOH (carboxylic acid) using dilute acid/reflux.
• The –CN can be reduced to –CH₂NH₂ (primary amine) using LiAlH₄ or H₂/Ni.
• The reaction extends the carbon chain by one carbon, which is invaluable in organic synthesis.

Chirality in Nucleophilic Addition

When HCN adds to an aldehyde (other than methanal), the product contains a chiral centre. The CN⁻ can attack the planar carbonyl equally from above or below, so a racemic mixture of enantiomers forms.

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Reduction of Carbonyls with NaBH₄

Aldehydes and ketones can be reduced to alcohols using the reducing agent sodium borohydride (NaBH₄).

Equations:

CH₃CHO + 2[H] → CH₃CH₂OH (ethanal → ethanol, a primary alcohol)

CH₃COCH₃ + 2[H] → CH₃CH(OH)CH₃ (propanone → propan-2-ol, a secondary alcohol)

Reagent: NaBH₄ in aqueous or methanolic solution.

Mechanism: NaBH₄ provides the H⁻ (hydride) ion, which is the nucleophile. The H⁻ attacks the δ+ carbonyl carbon (nucleophilic addition), then the intermediate is protonated by the solvent.