Carbonyl compounds

Carbonyl compounds have the functional group C=O and form an homologous series with a general

formula CnH2nO

There are two types; 1) Aldehydes and 2) Ketones

Aldehydes vs Ketones

Aldehydes have a carbonyl group C=O, at one end.

Ketones also have a carbonyl group, but in the interior of the molecule.

Naming Carbonyl compounds

• Aldehydes and ketones are named using the usual IUPAC rules with the addition of an appropriate suffix to the number of carbons.

For aldehydes -al For ketones -one

Eg; CH3(CH2)3CHO is pentanal

Eg; CH3(CH2)3COCH3 is hexanone

Eg; 3C molecule with the carbonyl group at one end. So

this is propanal.

3C molecule with the carbonyl group in the interior. So this is

propanone.

• Name the following compounds;

• CH3(CH2)8CHO

• Decanal

• C6H5CHO

• Benzaldehyde • (aka benzenecarbaldehyde)

• CH3CO(CH2)3CH3

• Hexan 2 one

Testing for carbonyl compounds.

Both aldehydes and ketones give an orange/red precipitate with 2,4 dinitro phenyl hydrazine.

Identifying organic compounds

2,4 DNPH precipitates have very sharp, easily distinguishable melting points.

So organic compounds were once identified by conversion to carbonyls and precipitation of the 2,4 dinitrophenylhydrazones.

These were filtered, recrystalised and their MPs determined and compared with text book values.

Eg; An unknown carbonyl compound has

a MP of 123OC.

• It must therefore have been Butanal.

MP (OC)

Methanal 167

Ethanal 164

Propanal 156

Butanal 123

Benzaldehyde 237

Distinguishing between aldehydes and ketones.

Aldehydes are oxidised as

orange chromium (vi) is

reduced togreen chromium (iii) when heated

with acid.

Only one organic product is possible; a Carboxylic Acids.

Dichromate redution Cr2O7 2- + 14H+ + 6e → 2Cr3+ + 7H20

Aldehyde oxidation; CH3CHO + H2O→ CH3CO(OH) + 2H++ 2e-

Ethanal Ethanoic acid

•NB Instead of H+ [H] or ‘H’ can be used in writing half equations.

Half equations

Distinguishing between aldehydes and Ketones

Ketones cannot be oxidised by acidified

potassium dichromate.

To oxidise them a much stronger oxidising agent is needed that can break C/C bonds.

Reaction with Fehling’s Solution

Aldehydes change the colour of

Fehling’s Reagent upon heating from

blue to red.

Blue copper (ii) ions are reduced to red

copper (i).

Cu 2+ + e- → Cu+

Ketones do not react with Fehling’s

Solution as they are not oxidised by mild

oxidising agents.

Reaction with Tollen’s Reagent

Aldehydes react with Tollen’s Reagent (ammonical silver solution [Ag(NH3)2]+

(aq), ), depositing a silver mirror on the side of the tube.

Silver ions are reduced to metallic silver.

Ag+(aq) + e- → Ag(s)

But Ketones do not react with Tollen’s reagent as they are not oxidised by mild oxidising agents.

Sugars and carbonyl groups.

Sugars form unbranched

chains in which each

carbon atom, bar one, has an hydroxyl

group.

The exception has a carbonyl group.

If this is an aldehyde the sugar is an

aldose.

If in the ketone form

the sugar is a ketose.

Eg; Glyceraldehyde is an aldose

Dihydroxyacetone is a ketose

Aldoses can be oxidised like any other aldehyde.

In the process another chemical is reduced.

So aldoses are referred to as reducing sugars.

Ketoses, like ketones,

cannot be easily

oxidised.

So they are known as

non-reducing sugars.

Reaction with Benedict’s Reagent

Benedict’s Reagent is similar to Fehlings. So Aldoses

change its colour of Fehling’s blue to red.

Blue copper (ii) ions are reduced by the reducing sugar to red copper (i).

Cu 2+ + e- → Cu+

Ketones do not react with Benedict’s

Reagent as they are not oxidised by mild

oxidising agents.