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Esters
• When alcohols are heated with carboxylic
acids in the presence of concentrated
sulfuric acid, they produce sweet smelling
compounds called esters.
• Because of their aroma and taste, esters are
often incorporated into artificial perfumes
and flavours.
• They are also used as solvents and
plasticisers.
Carboxylic acids and Esters
 Carboxylic acids contain the carboxyl group
on carbon.
 In ester the hydrogen in the carboxyl group
replaced by an alkyl or aryl group.
O
H3C
O
O
H
H3C
O
R
Esters
 Esters
are
sweet
smelling
organic
compounds.
 Esters give their pleasant fragrances to fruits
and flavors.
Ethyl formate
 Raspberries contain ethyl formate;
HCOOCH2CH3
 You can make this compound from formic acid
and ethanol.
Ethyl butyrate
 Pineapples contain ethyl butyrate; CH3CH2CH2
COOCH2CH3
 You can make this compound from butric acid
and ethanol.
Propyl acetate
 Pears contain propyl acetate;
CH3COOCH2CH2CH3
 You can make this compound from acetic acid
and propanol.
Esterification Reactions
A simple reaction of this type is that of
ethanol with ethanoic acid to form ethyl
ethanoate:
Intermolecular Esterification
 Formed from two different compounds by
removing a mole of water in presence of an acid
catalyst.
 It is also called Fisher Esterification (a classical
transformation).
 Condensation process.
 It is a reversible reaction.
Esterification
H+ (catalyst)
O
R
R
OH
R
R
OH
Carboxylic Acid
O
Alcohol
(a reversible
condenstation
reaction)
O
Ester
H2O
Esterification Reactions
• In these reactions, known as esterification
reactions, the small amount of sulfuric acid
has two functions.
• Firstly,and most importantly, the hydrogen
ions act as a catalyst to increase the rate of
the reaction and
• Secondly it reacts with the water formed to
shift the position of the equilibriumm to the
right hand side (Le Chatelier’s principle)
ensuring a good yield of product.
• Note that unlike the acid and alcohol, an
ester does not contain an –OH group and so
is much more limited in its ability to
hydrogen bond to water molecules, hence
esters tend to be insoluble in water.
Naming of Esters
• It can be seen that the naming of esters is rather
different named as from that of other organic
compounds.
• They are if they were salts of the alcohol and the acid;
the alcohol provides the first half of the name (alkyl) and
the organic acid provides the second half of the name
(alkanoate).
• In naming an ester it is important to remember that the –
CO–group is part of the carboxylic acid.
• The molecule below is therefore methyl propanoate (not
propyl methanoate),because it can be considered as
being formed from methanol and propanoic acid.
Amides
• Ammonia and primary amines initially react with
carboxylic acids to form a salt of the acid, but if this is
heated it dehydrates to form an amide.
• If ethanoic acid is reacted with methylamine, and the
initially formed methylammonium ethanoate is heated,
then N-methylethanamide is formed:
Polyesters
• The two reactions discussed above are made use of in polymers
known as condensation polymers.
• In these polymers, two different functional groups are required and
for each new bond between the monomer units (shown coloured
below), a small molecule (often water) is produced.
• Each monomer must also have two functional groups.
• This can involve two different functional groups on the same
monomer or more frequently, as in the examples below, two
different monomers which have two identical groups on them.
• One group of condensation polymers are the polyesters, so called
because the bonding depends on the reaction of an alcohol with a
carboxylic acid to form an ester.
• The best known example of this polymer is Terylene, formed by the
reaction of benzene–1.4–dicarboxylic acid with ethane–1.2–diol. as
shown in Figure 1042(a)
• The repeating unit in Terylene is therefore
Polyesters