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Aldehydes and ketones
Part-7
(xv) Polymerization Reactions
When two or more molecules combine to form bigger molecule accompanied by the loss of
simple molecule like water, alcohol or ammonia, etc. the process is called condensation
polymerisation.
But if the number of molecules (of the same substance or of different substances) combine to
form larger molecule, the process is called polymerisation.
Condensation and polymerisation products of some carbonyl compounds are as follows Condensation and polymerisation product of formaldehyde
(a) Condensation with ammonia : Formaldehyde condenses with ammonia to form hexa
methylene tetramine (urotropine)
The reaction occurs in the following manner :
(b) Condensation with phenol - Lederer - Manasse’s reaction : Formaldehyde condenses with
phenol in alkaline medium to form o-and p-hydroxy benzyl alcohol which further condenses
with phenol to give polymer, Bakelite. This is called Lederer-Manasse reaction.
(c) Formation of penta erythritol : Formaldehyde and acetaldehyde combine to form
trihydroxy aldehyde (Claisen-Schmidt reaction) which reacts with alkali to give pentaerythritol
(Cannizaro reaction) :
(d) Formation of formose : On treating formaldehyde with dilute barium hydroxide (baryta
water), a mixture of several sugars (monosaccharides) is formed which is called formose.
Above reaction resembles aldol condensation although formaldehyde does not have
Polymers of formaldehyde : Formaldehyde forms three polymers :
(a) Trioxan or trioxymethylene : On distilling formaldehyde with a small quantity of dilute
sulphuric acid, trioxan is formed. On allowing formaldehyde to stand, it slowly changes to
trioxan.
(b) Paraformaldehyde : When a concentrated aqueous solution of formaldehyde is evaporated
off to dryness, a long linear polymer is obtained which is called paraformaldehyde.
On heating paraformaldehyde, formaldehyde is regenerated. Hence formaldehyde is
transported in the form of paraformaldehyde. when paraformaldehyde is heated in a sealed
tube at 115oC, trioxan is formed.
(c ) Polyoxymethylene : When dilute sulphuric acid is added into cold aqueous solution of
formaldehyde, a white insoluble solid is formed which is called polyoxymethylene. This is also
a linear polymer. In this case chain length is longer than in paraformaldehyde.
Condensation and polymerisation products of acetaldehyde
(a) Reaction with ammonia : Ammonia reacts with acetaldehyde to form simple nucleophilic
addition product- acetaldehyde ammonia which subsequently losses water to give
acetaldemine.
(b) Polymerisation:
Acetaldehyde when treated with hydrogen chloride gas at 0°C forms a solid tetramer called
metaldehyde. However, if temperature is not controlled, a trimer, called paraldehyde is
formed which is liquid.
Metaldehyde burns with a smokeless non-luminous flame and is used as a killer for snails and
slug and is marketed under the name ‘Snarol’. Paraldehyde is used as a hypnotic (sleepinducing drug).
Both the polymers of acetaldehyde get depolymerised on heating with dilute sulphuric acid and
thus they act as source of acetaldehyde reactions in acid medium.
The polymeric aldehydes do not show characteristic aldehyde reactions. Higher aldehydes and
ketones do not give definite isolable polymers.