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• Recognise and name aldehydes and ketones.

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The carbonyl functional group

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Formation of the C=O group π-bond

Naming aldehydes

• See p. 20 A2 text book.• In an aldehyde the carbon atom of the

carbonyl group is joined to at least 1 hydrogen atom.

• Aldehydes end in ‘al’.• The parent chain is the longest

unbranched carbon chain present in the molecule.

• The carbonyl C atom is always the 1 carbon in the chain.

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2-methylpentanal

Naming ketones

• See p. 21 A2 text book.• In ketones the carbonyl group is in the

middle of a chain with a C atom on either side.

• Ketones end in ‘one’.• The parent chain is the longest chain

containing the carbonyl group.• The carbonyl is numbered with the

smallest possible number.

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The structure of 2-methylpentan-3-one

Aromatic aldehydes and ketones

• These contain the carbonyl functional group and a ring.

• The simplest aromatic aldehyde is?• The simplest aromatic ketone is?• Aromatic carbonyls contribute to the

flavours and smells of many stone fruits, plums, peaches, cherries etc.

• Cinnamaldehyde is the characteristic smell and flavour of cinnamon.

• Phenylethanone resembles strawberries etc.

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Structures of benzaldehyde and phenylethanone

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Structure of cinnamaldehyde

Physical Properties of Carbonyl Compounds

• Aliphatic carbonyl compounds are generally less pleasant, particularly aldehydes which are described as lachrymatory.

• E.g. Heptan-2-one is responsible for the flavour of blue cheese.

• Lower members of both carbonyl series are soluble in water.

• Solubility decreases with increasing RMM.• Volatility decreases with increasing RMM.

Volatility

Compound BPoC

Ethanal 20

Propane -42

Ethanol 78

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• Describe the oxidation of primary alcohols to form aldehydes and carboxylic acids.

• Describe the oxidation of secondary alcohols to form ketones.

• Describe the oxidation of aldehydes to form carboxylic acids.

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Ethanol oxidised to ethanal, and finally to ethanoic acid

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Propan-2-ol can be oxidised to propanone

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Oxidation of an aldehyde to a carboxylic acid

Oxidation

• Oxidation is:• Gain of oxygen• Loss of hydrogen• Loss of electrons• In the oxidation of alcohols or aldehydes,

ion electron equations can be written which are more informative than the [O] symbol.

• See p. 23 in A2 text.

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• Describe the reduction of carbonyl compounds to form alcohols.

• Outline the mechanism for nucleophilic addition reactions of aldehydes and ketones with hydrides.

Reduction of carbonyl compounds

• Aldehydes and ketones are made by the OXIDATION of alcohols.

• It follows that with a suitable reducing agent carbonyl compounds should be reduced back to alcohols.

• NaBH4 is used with warming and water or ethanol as a solvent.

• The reaction can be considered as the addition of hydrogen across the double bond, though the actual reaction is nucleophilic addition using H- as the nucleophile.

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Reduction of an aldehyde produces a primary alcohol

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The primary alcohol propan-1-ol and the aldehyde propanal

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Reduction of a ketone produces a secondary alcohol

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NaBH4 readily generates hydride ions

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Reduction of an aldehyde by nucleophilic addition

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• Describe the use of 2,4-dinitrophenylhydrazine to detect a carbonyl group and to identify a carbonyl compound.

• Describe the use of Tollens’ reagent to detect the presence of an aldehyde group.

Test for the Presence of a Carbonyl Group

• 2,4-dinitrophenylhydrazine will give an orange precipitate with any carbonyl compound (not acids, esters etc).

• The reaction is an addition/elimination reaction or condensation reaction which involves addition across the double bond followed by loss of water.

• 2,4-DNP or Brady’s reagent is used because the big molecule gives better precipitates of the hydrazone derivative with distinct and diagnostic melting points.

Test for the Presence of a Carbonyl Group

• If the hydrazone derivative is recrystallised from hot ethanol the melting point determination can be used to identify an unknown carbonyl compound.

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Reaction of propanal with 2,4-dinitrophenylhydrazine

Aldehyde or Ketone?

• Aldehydes are oxidised to carboxylic acids with mild oxidising agents but ketones are unaffected.

• This can be used to distinguish between them.

• Tollen’s Reagent – ammoniacal silver nitrate, will oxidise an aldehyde when left to stand in mildly warm,not hot water to a silver mirror.

• The oxidation of the aldehyde is shown after this but the oxidising agent – Ag+ ions are reduced to metallic silver.

• Ag+(aq) + e- → Ag(s)

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Oxidation of an aldehyde using Tollens’ reagent