Aldehydes, Ketones and Carboxylic Acids

The C=O bond is polarised ($\text{C}^{\delta+}\text{=O}^{\delta-}$), making the carbon electrophilic. Nucleophiles attack the δ+ carbon.

Reaction with HCN (Hydrogen Cyanide)

$\text{RCHO} + \text{HCN} \rightarrow \text{RCH(OH)CN}$

Product: hydroxynitrile (also called cyanohydrin)

Conditions: KCN with dilute H₂SO₄ (or NaCN/HCl) — generates HCN in situ

Mechanism:

1. CN⁻ (nucleophile) attacks the δ+ carbonyl carbon
2. C=O π bond breaks; oxygen gains electrons → alkoxide intermediate
3. H⁺ from HCN protonates the O⁻

This is nucleophilic addition.

Importance: Extends the carbon chain by 1 carbon. The CN group can be hydrolysed to COOH or reduced to CH₂NH₂.

2,4-DNPH Test (for any carbonyl)

Add 2,4-dinitrophenylhydrazine → yellow/orange precipitate confirms C=O present. Does NOT distinguish aldehyde from ketone.

The precipitate can be recrystallised and its melting point measured to identify the specific carbonyl compound.

Tollens' Reagent (for aldehydes only)

Tollens' reagent = $\text{Ag(NH}_3\text{)}_2^+$ (silver nitrate + ammonia)

• Aldehyde: silver mirror forms on the test tube wall
• Ketone: no reaction

$\text{RCHO} + 2\text{Ag}^+ + 2\text{OH}^- \rightarrow \text{RCOO}^- + 2\text{Ag} + \text{H}_2\text{O}$

Fehling's Solution (for aldehydes only)

Fehling's = Cu²⁺ in alkaline solution (blue)

• Aldehyde: blue solution → red precipitate of Cu₂O
• Ketone: no reaction

Properties

• Weak acids: partially dissociate in water
• Contain the −COOH group
• Form hydrogen bonds → relatively high boiling points

Reactions

With bases: $\text{CH}_3\text{COOH} + \text{NaOH} \rightarrow \text{CH}_3\text{COONa} + \text{H}_2\text{O}$

With carbonates: $2\text{CH}_3\text{COOH} + \text{Na}_2\text{CO}_3 \rightarrow 2\text{CH}_3\text{COONa} + \text{H}_2\text{O} + \text{CO}_2$

Esterification: $\text{RCOOH} + \text{R'OH} \xrightleftharpoons{\text{H}^+, \text{reflux}} \text{RCOOR'} + \text{H}_2\text{O}$

Reduction: $\text{RCOOH} \xrightarrow{\text{LiAlH}_4} \text{RCH}_2\text{OH}$

Esters have the general structure RCOOR' and have sweet, fruity smells.

Naming: alcohol part first (as alkyl group), then acid part (with "-oate" ending).

Ethyl ethanoate: CH₃COOC₂H₅ (from ethanoic acid + ethanol)

Hydrolysis: Esters can be hydrolysed back to acid + alcohol

• Acid hydrolysis: $\text{RCOOR'} + \text{H}_2\text{O} \xrightarrow{\text{H}^+} \text{RCOOH} + \text{R'OH}$
• Base hydrolysis (saponification): $\text{RCOOR'} + \text{NaOH} \rightarrow \text{RCOONa} + \text{R'OH}$

• Aldehydes and ketones contain C=O; aldehydes are terminal, ketones are internal
• Nucleophilic addition (e.g. HCN) → carbon chain extended
• 2,4-DNPH tests for C=O (both); Tollens'/Fehling's tests for aldehydes only
• Carboxylic acids: weak acids, form esters with alcohols
• Esters: sweet smells, hydrolysed by acid or base