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Aldehydes and ketones

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Polymerization Reactions

(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 115
^o
C, 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 (sleep-inducing 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.

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