Chemistry, Third edition

18 · COMMON CLASSES OF ORGANIC COMPOUNDS

Carbonyl compounds

Carbonyl compounds contain the carbonyl group, C=O. The carbonyl group is

part of the structural formulae of a number of organic families. The name of the

family is determined by the other atoms bonded to the carbonyl carbon. If one of

the atoms bonded is hydrogen, the compound belongs to the class of organic

compounds known as aldehydes. However, if two carbon atoms are bonded to the

carbonyl carbon, the compound is a ketone. To summarize,

aldehydes contain the R—C==O group (sometimes written R—CHO)

|

H

ketones contain the R—C==O group (where R and Rare alkyl groups)

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R

The name of an aldehyde is derived from the number of carbon atoms present

and ends in ‘al’. For example:

HCHO methanal (old name formaldehyde)

CH 3 CHO ethanal (old name acetaldehyde)

CH 3 CH 2 CHO propanal

Ketones are named by changing the end of the name of the parent alkane (the

one with the same number of carbon atoms) to ‘one’. Sometimes it is necessary to

number the position of the carbonyl group to avoid confusion:

CH 3 COCH 3 propanone (commonly called acetone)

CH 3 COCH 2 CH 3 butanone

CH 3 COCH 2 CH 2 CH 3 pentan-2-one

Aldehydes and ketones can be isomeric with one another. For example,

CH 3 CH 2 CHO and CH 3 COCH 3 both have the molecular formula C 3 H 6 O. These

structural isomershave some different chemical properties as well as different

physical properties.

Physical properties of aldehydes and ketones

The carbonyl group is polar and has unshared electron pairs on the oxygen atom:

The group can therefore take part in hydrogen bonding with water molecules:

Aldehydes and ketones with a low molecular mass are therefore soluble in water.

Propanone (acetone) is widely used as a solvent. It is soluble in water and dissolves many

polar and non-polar molecules. Examples include paint, nail varnish and plastics.

A 37% solution of methanal (common name formaldehyde) dissolved in water, with

some methanol added, is called formalinand is used for preserving biological specimens.

18.3

334

Aldehydes and

ketones 1

Classify each of the

following compounds as an

aldehyde or a ketone:

(i) C 6 H 5 CHO

(ii)CH 3 CH 2 COCH 2 CH 3

(iii)CH 3 CH 2 CH 2 CHO

(iv)CH 3 CH 2 CCH 3

O

(v) CCH 3

O

Exercise 18E