2. STRUCTURE OF ESTERS
Carboxylic acid derivatives in which the hydroxy
group (-OH) is replaced by an alkoxy group (-OR).
General formula:
O
R
C O R' or RCOOR' or RCO2R'
Esters have pleasant odors such as smell like fruits
(apples or bananas) and flower, especially those
with low molecular weight.
4. NOMENCLATURE
According to the IUPAC system the alcohol part of the
ester (R΄) is named first.
ethyl
This is followed by the name of the acid where the
–ic ending of the acid has been changed to –ate.
ethanoate
7. FROM CARBOXYLIC ACIDS
• Esterification:
- The reaction between an alcohol and a carboxylic acid to form
an ester and H2O.
O
R
C
O
H+
O
H
H O
carboxylic acid
ethanol
O
CH3
C
methanol
O
R'
H2O
O
H+
O
H
ethanoic acid
C OH
benzoic acid
C
ester
O
CH3-O-H
R
alcohol
EXAMPLES
CH3CH2-O-H
R'
CH3
C
OCH2CH3
H2O
ethyl ethanoate
H+
O
C OCH3
H2O
methyl benzoate
H+ = catalyst
8. FROM ACYL CLORIDES
Acyl chloride react with an alcohol and phenols to produce an ester.
Does not require catalyst even through alcohol is a weak nucleophile
because acyl chloride is highly reactive.
The reaction is irreversible unlike the reaction of carboxylic acid with
alcohol.
Pyridine is added to prevent side reaction between HCl and ROH.
O
O
R
C
Cl
R-OH
pyridine
R
C
OR
HCl
Example:
O
H3C C Cl
O
pyridine
CH3CH2
O H
CH3
C OCH2CH3
HCl
9. FROM ACID ANHYDRIDES
Acid anhydrides react with alcohol to produce esters and carboxylic acids.
Does not required catalyst, but still requires heating.
The reaction is slower than the reaction with acyl chlorides
O
O
R
C O C
O
R
R
R' OH
O
C OR'
ester
R
C OH
carboxylic acid
EXAMPLE
O
H3C
O
C O C
O
CH3
ethanoic anhydride
O
H3C
O
C O C
CH3CH2 OH
ethanol
H3C
C OCH2CH3
ethyl ethanoate
ethanoic anhydride
H3C
phenol
H3C
C O
phenyl ethanoate
C OH
ethanoic acid
O
O
OH
CH3
O
H3C
C OH
ethanoic acid
11. HYDROLYSIS
Ester molecules undergoes hydrolysis to form carboxylic acids or
carboxylate salts and alcohols.
Hydrolysis occurs in either acidic or basic condition.
The reaction with pure water is so slow.
The reaction is catalyzed by dilute acid such as HCl or H2SO4 and
heated under reflux.
O
R
C O R
O
heat
H OH
R
ester
C OH
carboxylic acid
ROH
alcohol
O
O
R
C O R
H+
R
C OH
ROH
carboxylic acid
H OH
O
OH
R
C O
carboxylate salts
ROH
12. AMMONOLYSIS
Esters reacts with ammonia, primary and secondary amines to
produce amides and alcohols.
O
R
C O R
ester
H N H
R
ammonia
primary amide
O
R
C O R
ester
C O R
ester
C NH2
R OH
O R
H
R
H N R
C NH
primary amine
secondary amide
R
R OH
O R
O
R
O
H
R
H N R
secondary amine
C N R
R OH
tertiary amide
EXAMPLE
O
H3C
C O CH2CH3
H
H N H
O
O
0-5 C
H3C
C NH2
CH3CH2 OH
13. REACTION WITH GRIGNARD REAGENT
Esters undergo nucleophilic acyl substitution then nucleophilic
addition with two equivalents moles of Grignard reagent, RMgX to
yield two different alcohols which one of them is tertiary.
O
R
H3O+
2RMgX
C O R
Grignard reagent ether
ester
O
C O CH3
ester
ROH
R
tertiary alcohol
EXAMPLE
CH3CH2
R
R C OH
2CH3MgCl
H3O+
ether
CH3CH2
CH3
C OH
CH3
CH3OH
14. MECHANISM
The initial product is unstable and it loses a magnesium alkoxide to
form a ketone.
This ketone is more reactive towards Grignard reagent and will react
with a second molecule of the Grignard reagent.
O MgX
O
R
C
OR
R'
MgX
R
C
O
OR
R
C
R'
R'
O
R
C
O MgX
R'
R'
MgX
R
C
R'
R'
OH
H3O+
R
C
R'
The reaction is followed by hydrolysis to give tertiary alcohol with two
alkyl group (R’) coming from Grignard reagent.
R'
15. TRANSESTERIFICATION
Substitution of one alkoxy group for another in an ester.
Alcoholysis of ester to prepare more complex esters from simple esters.
Can take place under either acidic (in HCl or H2SO4) or basic conditions.
O
R
O
+
C O R'
ester
R'' OH
large excess alcohol
H
R
heat
C O R''
R'-OH
ester
EXAMPLE
O
CH3CH2CH2
C OCH3
OH
O
H+
CH3CH2CH2
CH3CH2CHCH3
heat
CH3
C OC CH2CH3
H
CH3 OH
