10.4 Hydroxy Compounds
Alcohols
Learning Outcomes
(a) recall the chemistry of alcohols, exemplified by ethanol:
(i) combustion
(ii) substitution to give halogenoalkanes
(iii) reaction with sodium
(iv) oxidation to carbonyl compounds and carboxylic acids
(v) dehydration to alkenes
(vi) ester formation
(b) (i) classify hydroxy compounds into primary, secondary
and tertiary alcohols
(ii) suggest characteristic distinguishing reactions e.g. mild oxidation
Alcohols
Primary alcohols
Secondary alcohols
Tertiary alcohols
(i) Combustion
C2H5OH(l) + 3O2(g)
2CO2(g) + 3H2O(l)
(ii) substitution to give halogenoalkanes
Alcohols react with hydrogen halides (HCl, HBr, HI) to give the
halogenoalkane
2KBr(s) + H2SO4(l)
C2H5OH(l) + HBr(g)
K2SO4(s) + 2HBr(g)
acid catalyst
C2H5Br(l) + H2O(l)
With potassium iodide, phosphoric(V) acid is used, as conc. sulphuric
reacts with KI to produce iodine.
Mechanism of substitution
SN2 mechanism - nucleophilic substitution: Primary Alcohols
H H
H
H C C O
H H
fast
H H
H H
H H
H C C O
H H
X-
+
+
H
H H
slow
H
H
H C C O
+ H+
H C C X
H
+ H2O
H H
SN1 mechanism - nucleophilic substitution: Tertiary Alcohols
CH3
H3C C O
+
CH3
H
H
slow
-H2O
CH3
H3 C C + XCH3
carbocation
fast
CH3
H3C C X
CH3
A tertiary alcohol will react with concentrated acids, e.g. shake with conc. HCl
(iii) Reaction with sodium
2C2H5OH(l) + 2Na(s)
C2H5O-Na+(s) + H2(g)
sodium ethoxide
Alkoxides are strongly basic and will remove a proton from water to form a
hydroxide ion (pH 14), so they are not used in aqueous solution!
C2H5O-Na+(s) + H20(l)
C2H5OH (l) + Na(OH)(aq)
Alkoxides are also good nucleophiles (S2N); ether formation:
Ester formation (esterification)
Esters are fragrant smelling liquids used in perfumes and fruit flavourings
(often larger molecular masses) formed when an alcohol reacts with a
carboxylic acid in the presence of an acid catalyst (small amount of conc.
H2SO4). It is an equilibrium reaction.
acid
+ alcohol
ester
ethyl ethanoate
Esters are named with the alcohol side first (ethyl from ethanol)
then the carboxylic acid (ethanoate from ethanoic acid). It is not necessary to
understand the mechanism of their formation (catalytic addition to C=O/elimination
of H2O).
(iv) Oxidation to carbonyl compounds and carboxylic acids
Primary alcohols can be oxidised to either aldehydes or carboxylic acids
depending on the reaction conditions. In the case of the formation of carboxylic
acids, the alcohol is first oxidised to an aldehyde which is then oxidised further
to the acid. Note that the orange chromate(VI) is reduced to green Cr3+(aq).
The Police use this procedure to detect alcohol levels in the breath of drunk drivers.
(a) Partial oxidation to the aldehyde
You get an aldehyde if you excess of the alcohol and distill off the aldehyde
as soon as it forms.
(b) Full oxidation to the carboxylic acid
You need to use excess of the oxidizing agent and make sure the
aldehyde formed half-way stays in the reaction mixture. Again we see a colour
change in the chromate.
Secondary alcohols
Secondary alcohols are oxidized to ketones; there is no further oxidation
to the carboxylic acid
For example, if you heat the secondary alcohol propan-2-ol
with sodium or potassium dichromate(VI) solution acidified
with dilute sulphuric acid, you get propanone formed.
Tertiary alcohols can’t be oxidized in this way. We can used selective oxidation
as distinguishing reactions between primary, secondary and tertiary alcohols.
In summary:
Dehydration to alkenes
With longer chain or branched alcohols, a mixture of alkenes may be produced
including cis and trans isomers, at around 600 K.
Aluminium oxide is often used (or a ceramic like pumice). Concentrated sulphuric
and phosphoric (V) acids are alternative dehydrating agents when boiled.