Esters
C.I. 13.5
What are they?
How are they made?
Formed when an alcohol reacts with a
carboxylic acid.
Very slow reaction, unless! an acid catalyst
used (usually sulphuric acid)
A condensation reaction
The condensation reaction between the
hydroxyl group and the carboxylic acid
known as esterification.
Reverse reaction = ester hydrolysis
Names of esters
end in –oate.
Naming
Named after alcohol & carboxylic acid from
which they are derived.
O
C
CH3
CH2
CH3
let’s name
some esters!
O
ethyl
this part comes from the
alcohol & is named after it
ethanoate
this part from the acid
and is named after it
Structural formulae
Although the previous structural formula are the clearest
way of showing esters, they can draw out in a
shortened form.
In this version
ethyl ethanoate
O
Either…
CH3 CH2
O
C
CH3
Or…
O
CH3 C
the acid part
has been
written first
O
CH2
CH3
 Identify the group attached to the C=O – this is from
the acid
 The group attached to the –O- is from the alcohol.
Wait! What’s that smell?
ester
ethyl methanoate
3-methylbutyl ethanoate
ethyl 2-methylbutanoate
phenylmethyl ethanoate
fragrance
raspberries
pears
apples
jasmine
• Esters have strong, sweet smells.
• Their bouquet is often floral or fruity.
• This means they are used in food flavourings &
perfumes.
• Also, very good at dissolving organic compounds so
often used as solvents.
Esters from phenols
• -OH group in phenol less reactive
to esterification, requires a more
vigourous reagent.
• Use of ethanoic acid in
esterification is also known as
ethanoylation.
• Ethanoic anhydride is a more
vigourous ethanoylating agent
OH
Esters from phenols
• Ethanoic anhydride used as it is reactive
but not too dangerous.
• Ethanoyl chloride is much more reactive,
but, also toxic and hazardous to use.
• Ethanoic anhydride & ethanoyl chloride
are described as acylating agents
using an acylating agent is the only way to esterify a
phenol
alcohols can be esterified using acylating agents or by
reacting with a carboxylic acid.
Ester hydrolysis
Breakdown of an ester by water.
Process sped up by catalysis
Can use an acid to catalyse (H2SO4)
Alkali catalysts (e.g. sodium hydroxide) can
also be used but instead of producing
carboxylic acid a carboxylate salt is
formed.
Alkaline hydrolysis goes to completion &
hence is usually preferred.
R
O
+
H
O
C
H
O
R’
⇋
O
C
R
R’
O
This is why it is a
condensation reaction
because water is produced!
+
H2O
Definition of a condensation reaction =
two molecules reacting to form a larger
molecule with the elimination of a
small molecule such as water
O
CH
CH3
CH2
ethyl
methanoate
O
O
C
CH3
CH2
CH3 methyl
propanoate
O
O
CH
CH3
O
methyl
methanoate
FORWARD REACTION = condensation reaction,
the esterification of an alcohol using
acid catalyst under reflux.
R
O
+
H
O
C
H
O
R’
⇌
O
C
R
R’
O
+
H2O
REVERSE REACTION = ester hydrolysis, same
catalyst works for both forward & reverse
reactions.
ethanol
ethyl ethanoate
O
CH3 CH2
O
C
+
OH-
CH3
→
CH3 CH2
+
O
-O
not a reaction that exists
in equilibrium (unlike
using acid catalyst)
OH
C
CH3
ethanoate ion