CARBONYL COMPOUNDS – ALDEHYDES & KETONES
Introduction:
Aldehydes and ketones are known as carbonyl compounds because they
contain the carbonyl group. The carbon-oxygen bond is polar because of the
electronegativity of the oxygen.
C
O
Name and Structure:
Aldehydes:
 The name aldehyde is derived from alcohol dehydrogenated.
 The systematic name for aldehydes is alkanals.
 The carbonyl group is always on the end, and a hydrogen is attached to
the carbonyl carbon.
R
H
C
O


The name ends with the suffix –al. e.g. methanal, ethanal, propanal etc
Methanal is also known as formaldehyde and ethanal as acetaldehyde.
Ketones:
 Alkanone is the systematic name for ketones.
 The functional group is not at the end and its position must be indicated in
the name.
R'
R
C
O

The name ends in the suffix –one. Propanone (also known as acetone).
pentan-2-one etc.
Bonding, Properties & Uses
 Intermolecular bonding results from relatively strong dipole-dipole
interaction due to the polar nature of the carbonyl group in aldehyde and
ketone molecules.
 Melting and boiling points increase with increasing molecular mass. Most
ketones and aldehydes are volatile liquids with definite odours – aldehydes
tend to have unpleasant, pungent smells, while ketones tend to have
pleasant sweet smells.
 The oxygen of the carbonyl group is able to hydrogen bond with water
molecules hence C1-C3 aldehydes and C3 & C4 ketones are water soluble.
Aldehydes C4-C6 and ketones C5 are slightly soluble.
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



Both tend to be soluble in non-polar solvents.
Formalin is a solution about 40% methanal and 60% water, is used as a
preservative and disinfectant.
Acetone is a major industrial solvent (IPA). It is used in nail polish
remover.
Butanone is used in model aeroplane glue.
Preparation of carbonyl compounds.
 Aldehydes are made by the oxidation of primary alcohols. To stop the
product being further oxidised to carboxylic acids the aldehyde is removed
by fractional distillation.
CH3CH2OH

Cr2O72-- H +
CH3CHO
Cr2O72-- H +
CH3COOH
Ketones are prepared by oxidising secondary alcohols. It is a slow reaction
and to ensure all alcohol is oxidised it is carried out under reflux.
CH3CH2CH2CHOHCH3
Cr2O72-- H +
CH3CH2CH2COCH3
Distinguishing Aldehydes and Ketones:
Aldehydes are more reactive than ketones because:

The carbonyl group of the aldehyde is more polarised. In ketones, the
polarity of the carbonyl group is decreased by the presence of the second
alkyl group.

In ketones the large alkyl groups attached to the carbonyl group get in the
way of reactant species.

The carbon to hydrogen bond of the aldehyde carbonyl group can be
oxidised to a C-O-H bond.
The fact that aldehydes are easily oxidised and ketones are not is the basis of
the tests to distinguish these two carbonyl compounds.
Tests for Aldehydes:
Aldehydes are easily oxidised to carboxylic acids. As they do they reduce
the other reactant. This is the basis of tests for aldehydes.
Oxidising Agent
Potassium dichromate (acidified)
Potassium permanganate (acidified)
Tollens’ reagent – ammoniacal silver
nitrate
Fehling’s solution
Benedict’s solution
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Colour Change
Orange solution to green
Purple solution to colourless
Silver precipitate or silver mirror
Blue solution forms orange/red
precipitate
Blue solution forms orange/red
precipitate
Tollens’ Reagent:
Tollens’ reagent contains diamminesilver(I) complex ion [Ag(NH3)2]+, made up
by:



Putting 2 – 3 mL aqueous silver nitrate in a clean test tube.
Adding one drop of NaOH(aq) so that a precipitate of silver hydroxide
forms.
Adding just enough dilute ammonia NH3(aq) to redissolve the precipitate
forming [Ag(NH3)2]+
Aldehydes reduce the silver ions Ag+ to form metallic silver, which is seen as
a silver mirror on the inside of the test tube (or a black/grey precipitate)
Ketones do not react with Tollens’ reagent.
Benedict’s Test:
Benedict’s solution contains aqueous copper (II) ions Cu2+ which (this makes
to solution blue coloured), in alkaline conditions. To prevent the copper from
precipitating out as Cu(OH)2, the Cu2+ is complexed with citrate ions.
When Benedict’s solution is added to an aldehyde and warmed, a reddish,
brown precipitate of Cu (I) oxide, (Cu2O) is formed.
CH3CHO +
Ethanal
4OH- + 2Cu2+
Benedict’s soln.
CH3COOH + Cu2O
Fehling’s solution is another reagent with copper(II) ions in alkaline solution. It
behaves exactly the same as Benedict’s solution.
Ketones do not react with Benedict’s solution and Fehling’s solution because
the ketones cannot reduce metal ions.
Reaction Summary for Ethanal
Tollens’ reagent
Silver mirror/ppt
Ethanol
CH3CH2OH
H+/Cr2O72- ethanal
cold
CH3CHO
H+/Cr2O72
warm
ethanoic acid
CH3COOH
Benedict’s solution
Blue to orange
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