ESTERS
Esters are derived from carboxylic acids in a process by which the HYDROXYL (-OH) group of the carboxylic acid
is replaced with an ALKOXY group (-O-R) from the alcohol.
O
||
The generic formula of an ester is: R’—C - O—R
where R represents an alkyl group from an alcohol and R’ represents an alkyl group (or a hydrogen atom) from a
carboxylic acid. The condensed formula is R’ – COO – R”.
NAMING OF ESTERS
In naming esters, we obtain the first part of the name from the name of the alkyl group attached to the oxygen;
the second part is obtained by changing the “-ic acid” ending of the acid to -ate.
EXAMPLES
O
O
||
||
CH3 – C – O – CH3
CH3 – CH2 – CH2 – C – O – CH – CH3
methyl ethanoate
2 – propylbutanoate
|
CH3
PHYSICAL AND CHEMICAL PROPERTIES OF ESTERS
1. Esters cannot form hydrogen bonds between their molecules, thus they have lower boiling points than isomeric
carboxylic acids.
O
||
2. Due to the polar nature of the carbonyl group (- C -), they can form hydrogen bonds with water. Thus, simple
esters (3 carbons) are miscible with water.
3. Esters have characteristic odours.
4. Esters make excellent solvents.
5. Esters are responsible for many of the characteristic tastes and odours of variety of foods. The table below
includes examples of some esters and their characteristic aromas:
IUPAC NAME
AROMA
3-methylbutyl ethanoate
Bananas
Octyl ethanoate
Orange
Propyl ethanoate
Pear
2-methylpropyl propanoate
Rum
Methyl salicylate
Ethyl benzoate
3-methylbutyl methanoate
Oil of wintergreen
Cherry
Raspberry
PREPARATION OF ESTERS
The method for preparing esters is called ESTERIFICATION. It is a reaction in which a carboxylic acid reacts
with an alcohol in the presence of a dehydrating agent, such as sulfuric acid.
H2SO4
Carboxylic acid
+ Alcohol
----
Ester
O
O
||
||
CH3-C - OH
HO-CH2-CH2-CH3
ethanoic acid
1-propanol
+
Water
+ H2O
CH3-C-O – CH2 – CH2 – CH3
propyl ethanoate
The hydrolysis of esters will reverse the above reaction. If an ester is placed in a basic solution it will split into an
alcohol and the sodium salt of the carboxylic acid.
O
CH3
O
CH3
||
|
||
|
CH3-CH2 - C – O - CH2-CH-CH3 + NaOH -
CH3-CH2 - C-O- Na+ +
2-methylpropyl propanoate
sodium propanoate
HO – CH2 – CH – CH3
2-methyl – 1 - propanol
QUESTIONS
I. Give the IUPAC name for:
a)
O
b)
||
H—C-O-CH2 – CH2 – CH3
O
CH2 – CH3
||
|
CH3 – CH2 - C – O – CH – CH2 –CH3
2. There are two esters with the formula C3H6O2
a) Draw their condensed formulas.
b) Give their IUPAC names
c) Identify the alcohol and the carboxylic acid used in the preparation of each ester.
d) Name and draw the isomeric carboxylic acid.
e) Compare the BP of either ester to (d).
3. Write the esterification reaction between:
a) ethanoic acid and ethanol.
b) methanoic acid and 1-butanol.
c) propanoic acid and 2-methyl-2-propanol.
4. Write a balanced equation to show how each ester below is prepared
a) CH3 – CH2 – COO – CH2 – CH2 – CH2 – CH2 - CH2 – CH3
b) H – COO – CH2 – CH2 –CH2 – CH3