Carbonyl Compounds

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Cambridge International AS & A Level Chemistry (9701)

Carbonyl Compounds

The big idea: The C=O bond is polar, so its carbon is δ+ and attracts nucleophiles: hydride ions reduce it, and cyanide ions add to it. Aldehydes still have an H on the carbonyl carbon, so they can be oxidised further, and that is how every test tells them apart from ketones.

Summary — What This Chapter Covers

Aldehydes from primary alcohols, ketones from secondary alcohols (acidified K₂Cr₂O₇ or KMnO₄, distil).
Reduction with NaBH₄ or LiAlH₄ back to alcohols.
HCN with KCN catalyst gives hydroxynitriles by nucleophilic addition.
2,4-DNPH detects any carbonyl; Fehling’s, Tollens’ and dichromate pick out aldehydes.
Alkaline iodine detects a CH₃CO– group.

1. Aldehydes and Ketones

17.1

Making carbonyl compounds

Primary alcohol + acidified K₂Cr₂O₇ (or KMnO₄), distil as it forms → aldehyde: CH₃CH₂OH + [O] → CH₃CHO + H₂O
Secondary alcohol + acidified K₂Cr₂O₇ (or KMnO₄), distil → ketone: CH₃CH(OH)CH₃ + [O] → CH₃COCH₃ + H₂O

Reduction

NaBH₄ (in water or ethanol) or LiAlH₄ (in dry ether) reduces aldehydes to primary alcohols and ketones to secondary alcohols.

Reduction CH₃CHO + 2[H] → CH₃CH₂OH CH₃COCH₃ + 2[H] → CH₃CH(OH)CH₃

Nucleophilic addition of HCN

Reagents: HCN with a KCN catalyst, heat. The product is a hydroxynitrile, with one more carbon than the carbonyl compound.

CH₃CHO + HCN → CH₃CH(OH)CN, 2-hydroxypropanenitrile
CH₃COCH₃ + HCN → (CH₃)₂C(OH)CN, 2-hydroxy-2-methylpropanenitrile

The mechanism

The lone pair on C of CN⁻ (from KCN) attacks the δ+ carbonyl carbon. At the same time a curly arrow moves the C=O π bond pair onto the oxygen, giving an intermediate with O⁻.
A lone pair on O⁻ takes H⁺ from HCN (curly arrow from O⁻ to the H, and from the H–C bond onto C). This forms the –OH group and regenerates CN⁻, which is why KCN is a catalyst.

Worked Example

Name the product when butanone, CH₃COCH₂CH₃, reacts with HCN and KCN, and give its structural formula.

17.1

Tests

Test	Aldehyde	Ketone
2,4-DNPH (2,4-dinitrophenylhydrazine)	orange precipitate	orange precipitate
Fehling’s solution, warm	blue solution → brick-red precipitate (Cu₂O)	stays blue
Tollens’ reagent, warm	silver mirror (Ag)	no change
acidified K₂Cr₂O₇, warm	orange → green	stays orange

2,4-DNPH confirms a C=O group in an aldehyde or ketone (it does not react with carboxylic acids or esters). The other three rely on aldehydes being easily oxidised (to carboxylic acids or carboxylate ions) while ketones are not.

Tri-iodomethane test

₂ yellow precipitate of CHI₃ CH₃CO– ₃ ₃ ₂ ⁻

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Carbonyl Compounds