Part 6.
The Chemistry of Carboxylic Acids and their Derivatives
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Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study
Notes for UK KS5 A/AS GCE IB advanced level organic chemistry students US
K12 grade 11 grade 12 organic chemistry
6.8 Esters - preparation,
physical and chemical properties and uses including
transesterification (interesterification)
Sub-index for this page
6.8.1
Two methods for
preparing of esters - esterification
6.8.2
Physical properties of
esters
6.8.3
Hydrolysis of esters
with acid or alkali
6.8.4
Transesterification of
esters (swapping the acid or alcohol component)
6.8.5
Uses of esters
INDEX of all carboxylic acids
and derivatives notes
All Advanced A Level Organic
Chemistry Notes
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6.8.1 Methods of esters
In principle the reaction equations for
the two methods described here are:
(a) alcohol/phenol +
carboxylic acid ===> ester + water
(b) alcohol/phenol + acid chloride
===> ester + hydrogen chloride
The first part of the ester name is derived from the
alcohol or phenol e.g. methyl, ethyl, phenyl.
The second part of the name is derived from
the carboxylic acid - examples in table below in
6.8.2.
The reverse is 'usually' true when writing out the
abbreviated formula, so take care!
See
Structure & naming of
carboxylic acids and derivatives
Brief descriptions of the methods
with equations
(a) Esters from carboxylic acid plus alcohol using an acid
catalyst
Concentrated
sulphuric acid acts as a catalyst in this reaction.
General word equation for esterification:
carboxylic acid + alcohol == acid catalyst ==> ester + water
e.g.
ethanoic acid + ethanol
ethyl ethanoate + water
+
+ H2O
sometimes more simply written as
CH3COOH + CH3CH2OH
CH3COOCH2CH3 + H2O
The reaction is reversible and
the mixture reaches equilibrium, and only about 2/3rds
of the carboxylic acid and alcohol have been converted to the ester.
Without a strong acid catalyst
e.g. conc. sulfuric acid, the reaction is very slow and the mixture is
heated to further increase the rate of reaction (see details of the
method below).
See another page for a full description of making an
ester like ethyl ethanoate
Its
easy to do a simple laboratory demonstration
Mix equal volumes of small
quantities of butanoic acid (pungent rancid small) and methanol with an even smaller volume
of concentrated sulfuric acid.
The mixture gently warmed in beaker of warm
water for 5-10 minutes.
butanoic acid +
methanol
methyl butanoate + water
CH3CH2CH2COOH
+ CH3OH
CH3CH2CH2COOCH3
+ H2O
The mixture is then poured into a beaker of
sodium hydrogencarbonate solution.
The sodium hydrogencarbonate neutralises the acid
catalyst and any unreacted 'smelly' butanoic acid. The excess methanol
dissolves in the water too.
You should get some drops of methyl butanoate ester left on
the surface which can be carefully smelled to appreciate the aroma of the
ester, which in this case should smell of apples of pineapple!
You can do this is as a nice class
experiment with various alcohols and ethanoic acid - far less rancid than
butanoic acid, as long as you don't mind a super vinegar smell !!!
Notes what students
think the esters smells like (likely to be 'fruity') alongside appreciating
its molecular structure too!
If you repeat the experiment with ethanol you
get ethyl butanoate which also smells of pineapples and it is used as a
flavour enhancer in processed orange juices.
CH3CH2CH2COOH
+ CH3CH2OH
CH3CH2CH2COOCH2CH3
+ H2O
(b)
Reaction between an acid chloride and an alcohol
The acyl chloride
and alcohol usually readily react at room temperature, especially if
both are aliphatic.
Examples of
nucleophilic addition of an alcohol to acid/acyl chlorides,
followed by elimination to give the ester and
hydrogen chloride.
(i) ethanoyl chloride
+ ethanol ===> ethyl ethanoate + hydrogen chloride
+ CH3-CH2-OH ===>
+ HCl
(ii) ethanoyl chloride + phenol
===> phenyl ethanoate + hydrogen chloride
+ ===>
+ HCl
(iii) pentanoyl chloride +
propan-1-ol ===> propyl pentanoate + hydrogen chloride
+
===>
+ HCl
Unlike the acid catalysed
esterification described in method (a) with ~67% yield, this reaction gives
a very high yield >90%.
For more details see
6.7
Preparation & reactions of acid chlorides
6.8.2
Physical properties of esters
The lower members of the aliphatic acid
esters are colourless liquids at room temperature with relatively low
boiling points. A few of the lower
members are quite soluble in water, but most are ~insoluble or sparingly
soluble. Table of abbreviated
structural formula and boiling point in oC, plus comments on
water solubility and isomerism.
Table of names, abbreviated
structural formula and boiling point in oC. |
Abbreviated
formula |
Name |
Boiling point |
Comments |
1. HCOOCH3 |
methyl
methanoate |
31 |
Very soluble in
water. |
2. CH3COOCH3 |
methyl
ethanoate |
57 |
Moderately
soluble in water. |
3. HCOOCH2CH3 |
ethyl
methanoate |
54 |
Moderately
soluble in water, 2. & 3. are isomers. |
4. CH3COOCH2CH3 |
ethyl ethanoate |
77 |
Slightly
soluble in water. |
5. HCOOCH2CH2CH3 |
propyl
methanoate |
81 |
Slightly
soluble in water, 4. and 5. are isomers. |
6. CH3CH2CH2COOCH2CH2CH3 |
propyl
butanoate |
143 |
Very slightly
soluble in water. |
7. CH3CH2COOCH2CH2CH2CH3 |
butyl
propanoate |
146 |
Very slightly
soluble in water, 6. and 7. are isomers |
8. C6H5COOC6H5 |
phenyl benzoate |
314 |
Insoluble in
water, melting point 71oC. |
Notes and further comments on the
information table above.
(i) The isomers tend to have similar
boiling points and solubility and have similar polarity.
With equal numbers of electrons
in the molecule, they have similar intermolecular forces - mainly
instantaneous dipole - induced dipole plus permanent dipole -
permanent dipole attractive forces.
(ii) Despite being polar molecules,
they generally have a low solubility in water because they cannot hydrogen bond
with water.
The first few lower members are
quite or very soluble in water, but, with increasing length of the molecule, too many hydrogen bonds between water molecules
are disrupted without compensation intermolecular attractions, so
the solubility rapidly decreases.
(iii) Alternative names
propyl
butanoate can also be called 1-propyl butanoate, to distinguish it
from structural isomer CH3CH2CH2COOCH(CH3)2,
which can called 2-propyl butanoate (1-methylethyl butanoate)
Odour
Esters tend to have pleasant odours
and are used in flavourings and perfumes and they occur naturally in
many plants.
6.8.3 Hydrolysis with acid or alkali - the
opposite of esterification
Esters can be hydrolysed by refluxing
with strong mineral acids or alkali.
The reaction is the opposite of
esterification.
(a)
Hydrolysis of an ester with dilute
hydrochloric or sulfuric acid
RCOOR' + H2O
RCOOH + R'OH
This reaction cannot go to
completion because it is a reversible reaction, despite the use of a
catalyst and heating the mixture under reflux.
You can go to a high % yield to the
right using an aqueous solution with lots of excess water.
e.g.
butyl propanoate +
water
propanoic acid + butan-1-ol
CH3CH2COOCH2CH2CH2CH3
+ H2O
CH3CH2COOH + CH2CH2CH2CH3OH
(b)
Hydrolysis of an ester with aqueous
sodium hydroxide
Hydrolysing an ester with strong alkali e.g.
aqueous or ethanolic sodium/potassium hydroxide is called saponification,
i.e. its a specific name for a particular type of hydrolysis reaction.
RCOOR' + NaOH
===> RCOONa + R'OH
RCOOR' + OH-
===> RCOO- + R'OH
e.g. using the same ester as in (a)
for this non-reversible reaction ...
e.g.
butyl propanoate +
sodium hydroxide
===> sodium propanoate + butan-1-ol
CH3CH2COOCH2CH2CH2CH3
+ NaOH ===> CH3CH2COONa +
CH2CH2CH2CH3OH
CH3CH2COOCH2CH2CH2CH3
+ OH- ===> CH3CH2COO-
+ CH2CH2CH2CH3OH
This reaction is not reversible and
goes to 100% completion forming the sodium salt of the acid.
The carboxylic acid is freed by
adding excess of a strong hydrochloric or sulfuric mineral acid.
e.g.
RCOONa + HCl ===> RCOOH + NaCl
or
RCOO- + H+ ===> RCOOH
CH3CH2COONa
+ HCl ===> CH3CH2COOH
+ NaCl
CH3CH2COONa
+ H+ ===> CH3CH2COOH
Note on hydrolysis and
SOAP making
What is it? How is it made? and it is another use of
esters!
'Traditional'
soap is a product of
the hydrolysis of fats
from animals and vegetable oils from plants.
You reflux the fat or vegetable
oil with aqueous sodium hydroxide.
A
saponification reaction
'Soapy' soaps (not modern 'soapless' detergents) are
the sodium salts of long chain fatty acids formed by heating fatty oils
with concentrated alkalis like sodium hydroxide or potassium hydroxide to hydrolyse them.
This is known as a saponification reaction and a typical equation is
illustrated
above and the general word equation quoted below.
vegetable oil/animal
fat + sodium hydroxide ==> soap molecule + glycerol
In the example illustrated above,
the saponification (hydrolysis) product is the soap sodium palmitate.
The original ester is a saturated fat
found in olive oil, palm oil, and body lipids.
This reaction breaks the fat molecule
down into one glycerol molecule (a triol alcohol) and three sodium salts of the long
chain carboxylic fatty acids that formed part of the original oil/fat ester.
Sodium stearate is another soap
molecule - it is actually a salt of a long chain fatty acid like stearic
acid shown below.
6.8.4 Transesterification of esters (swapping the acid
or alcohol component - interesterification)
Since the acid catalysed esterification
reaction is reversible, you can actually swap the alcohol or carboxylic acid
derived component in the ester molecule.
You gently reflux the ester with another
alcohol or another carboxylic acid to effect the transesterification.
So, there are two possibilities e.g.
(a)
Swapping the alcohol
derived component of the ester
RCOOR' + R"OH
RCOOR" + R'OH
ester 1 + alcohol 1
ester 2 + alcohol 2
So this is effectively exchanging the
alcohol reactant in the esterification.
Examples
(i)
ethyl ethanoate +
propan-1-ol
propyl ethanoate + ethanol
+
+

(ii)
butyl pentanoate +
ethanol
ethyl pentanoate + butan-1-ol
+
+

The first ester can be named 1-butyl
pentanoate.
(b) Swapping the
carboxylic derived component of the ester
RCOOR' +
R"COOH
R"COOR' + RCOOH
ester 1 + carboxylic
acid 1
ester 2 + carboxylic acid 2
So this is effectively exchanging the
carboxylic acid reactant in the esterification.
Examples
(i)
propyl pentanoate +
butanoic acid
propyl butanoate + pentanoic acid
+
+

(ii)
methyl ethanoate +
benzoic acid
methyl benzoate + ethanoic acid
+
+

A transesterification
reaction is used in the production of margarine and biodiesel
See
6.9
Natural esters - triglyceride fats and oils,
manufacture of margarine and biodiesel
6.8.5
The uses of esters
Esters are used in perfumes and flavourings.
Perfumes are complex mixtures of esters and ketones -
blended to a give a particular volatility and aroma.
Many esters are used as flavour enhancers in fruit
drink.
Solvents may contain esters e.g. nail varnish remover
includes some ethyl ethanoate.
Biofuels are made from the
production of methyl esters of fatty acids using transesterification.

Esters crop up in all sorts of pharmaceutical products!
Aspirin (right) is an ester of ethanoic acid - but
the 'alcohol' is actually a phenol called 2-hydroxybenzoic acid.
Some anaesthetics like benzocaine and procaine are
esters.
Benzocaine (left) is the ethyl ester of
4-aminobenzoic acid.
Many anaesthetics ('painkillers') are derived from natural
products. Synthetic codeine is made from natural morphine, but neither is an
ester. However, the related molecule, natural heroin, is an ester, I
think you can you spot the two ester linkages (and an ether link and
tertiary amine group too!!!).
I've already written a lot on the
uses of esters
on another webpage
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INDEX of all carboxylic acids
and derivatives notes
All Advanced Organic
Chemistry Notes
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