CI 13.4 The –OH Group
The –OH group is one of the more common functional groups in organic chemistry. It can
be found in:
H
H
R’
 Alcohols:
R-OH
R-C-OH
R-C-R’
R-C-R’’
H
1ry

Phenols:

Carboxylic Acids:
OH
2
PhOH [Ph = Phenyl = C6H5]
O
R-C
OH
3ry
OH
ry
RCOOH
RCO2H
H
Oxidation of the OH Group:
The -OH group can be oxidised using a strong oxidising agent such as acidified dichromate
solutions (Cr2O72-(aq)) . The product is a CARBONYL compound: an aldehyde or a ketone.
However, there must be a hydrogen on the carbon to which the –OH group is bonded.
Therefore only 1ry and 2ry alcohols can be oxidised.
3ry alcohols, phenols and carboxylic acids cannot be oxidised
1ry alcohols
aldehydes
H
R-C-OH
H
2ry alcohols
R-C
further oxidation
carboxylic acids
O
H
R-C
O
OH
Can be
extracted if
careful
ketones
H
O
R-C-R’
R-C-R’
OH
O
It is difficult to oxidise ketone
further then this because it would
involve breaking of a C-C bond
In an aldehyde the carbonyl group
is at the end of an alkane chain, so the
-Cfunctional group is RCOH. Aldehydes are named using the suffix –AL: ethanAL.
In a ketone the carbonyl group is inside an alkane chain so the functional group is RCOR’.
Ketones are named using the suffix –ONE: propanONE.
Dehydration of alcohols:
Many alcohols (but not PhOH or RCOOH) can lose H2O to form an alkene e.g
OH
+ H 2O
This is an example of an elimination reaction
Esters are formed in reactions between alcohols and carboxylic acids:
Esters: R-OH + R’COOH
OH
RCOOR’ [an ester] + H2O
O
+
HO
O
O
Aspirin [2-ethanoylhydroxybenzoic acid] is an example of an ester!
+ H2O