Download A2 Module 2814: Chains, Rings and Spectroscopy

RPA4 14-15 acids & esters p.1
A2 Module F324: Rings, Polymers and Analysis
RPA 4: Carboxylic acids & esters
Carboxylic acids have the general formula RCOOH (or RCO2H). The structure of the carboxyl group
is given below:
O
O
OH
OH
ethanoic
acid
benzoic
acid
Carboxylic acids are polar molecules and can take part in hydrogen bonding, so they have higher
boiling points than hydrocarbons of similar molar mass, and are likely to be soluble in water.
Ethanoic acid is fully miscible with water (the pure solid melts at 17°C). Benzoic acid is a white
solid, which is only slightly soluble in cold water, but much more soluble in hot water.
Carboxylic acids dissociate to a limited extent in water, and are therefore weak acids:
RCOOH
RCOO– + H+
The reason that these compounds are much stronger acids than alcohols and phenols is that the
presence of the carbonyl group helps to stabilise the negative charge on the anion, by spreading it
evenly over both oxygens:
O
O
O
overall:
O
O
O
This moves the acid dissociation equilibrium further to the right, though these are still weak acids
by inorganic standards.
Reactions of carboxylic acids
With aqueous alkalis (e.g. NaOH)
Carboxylic acids will react with aqueous alkalis to form soluble salts:
CH3COOH + NaOH  CH3COONa (sodium ethanoate) + H2O
or ionically: CH3COOH +
OH –  CH3COO– + H2O
With alcohols
Carboxylic acids react with alcohols in a reversible reaction catalysed by concentrated sulfuric acid
to form esters: (heating under reflux produces the best yield)
CH3COOH + C2H5OH
CH3COOC2H5 + H2O
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The ester is insoluble, and can normally be recovered by shaking with sodium hydrogencarbonate
solution and using a separating funnel.
Esters
Esters are related to the parent acid by replacing the H in RCOOH with an alkyl group, R’, so the
ester is RCOOR’ (though, of course, this is not how they are made). They are named after the
parent acid: e.g. esters of formula CH3COOR are alkyl ethanoates, while C6H5COOR esters are alkyl
benzoates.
O
O
O
O
propyl ethanoate
ethyl benzoate
You should be able to recognise esters written either way round: for example, methyl propanoate
can be either C2H5COOCH3 or CH3OCOC2H5. It is important to realise that the second formula can
only have one meaning: i.e. CH3–O–(C=O)–C2H5. The C=O group can only be written CO (or COO,
or CO2): if OC is used, that means –O–C. Therefore, if you intend to write a structure which is CH3–
(C=O)–O–C2H5 (i.e. ethyl ethanoate) it can be either CH3COOC2H5 or C2H5OCOCH3.
Esters are obtained from reaction between carboxylic acids (or acid anhydrides – see below) and
alcohols, as described above. They are volatile liquids with sweet, fruity smells: many of the
common fruit smells like pineapple and pears are due to esters.
Synthesis
Although esters can be formed from alcohols with carboxylic acids, this reaction is not always
preferred synthetically because of the relatively slow rate and the reversibility which never allows
a 100% yield to be achieved.
An alternative method is to react an alcohol with the acid anhydride of the carboxylic acid. Acid
anhydrides are the product of two moles of carboxylic acid after the removal of one mole of
water, and have the following general structure:
O
O
O
O
e.g.
R
O
acid anhydride
R
H3C
O
ethanoic anhydride
CH3
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Their reaction with alcohols is fast and gives 100% yield:
O
O
O
O
+
H3C
O
ethanoic anhydride
(CH3CO)2O
+
HO
CH3
+
H3C
O
ethanol
ethyl ethanoate
CH3CH2OH
CH3COOCH2CH3
HO
CH3
ethanoic acid
+
CH3COOH
Uses
Esters are used in processed foods as flavourings and also as fast-evaporating solvents (e.g. for
glues like polystyrene cement), and in perfumes.
Hydrolysis
Esters are very slow to react with water but can be hydrolysed either with an acid catalyst or by
dilute alkali. Acid catalysed hydrolysis is an equilibrium reaction, which is the reverse of the
esterification process (warm ester with dilute HCl):
CH3COOC2H5 +
H 2O
CH3COOH + C2H5OH (H+ catalyst)
With phenyl benzoate the hydrolysis is effectively irreversible, since phenol does not react with
carboxylic acids:
C6H5COOC6H5 +
H2O  C6H5COOH + C6H5OH (H+ catalyst)
If an ester is warmed with dilute aqueous sodium hydroxide, the reaction is always irreversible,
since the salt of the acid is produced, and that cannot react with the alcohol:
CH3COOC2H5 + OH –  CH3COO– + C2H5OH
Phenyl benzoate produces the phenoxide ion as well as the benzoate ion:
C6H5COOC6H5 + 2OH –  C6H5COO– + C6H5O– + H2O
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Fatty Acids
Fatty acids are long-chained carboxylic acids that may contain one or several C=C bonds along
their length. ‘Fats’ in general are molecules with long alkyl chains in them – this can include fatty
acids and esters of fatty acids (see below).
Here are some examples:
O
octadecanoic acid
OH
O
octadec-9-enoic acid
OH
O
octadec-9,12-dienoic acid
OH
Of the examples above, the octadecanoic acid is a saturated fatty acid as it has only C-C single
bonds; the other two are unsaturated fatty acids because of the C=C bonds. You will notice that in
the unsaturated fatty acids cis/trans isomerism is possible about the C=C bond(s): in each case the
trans isomer has been drawn, but the acids could easily exist as either isomer.
Fatty acids can be named systematically, as shown above, or they can be named using a shorthand
formula.
For example: octadecanoic acid translates as 18,0
octadec-9-enoic acid translates as 18,1(9)
octadec-9,12-dienoic acid translates as 18,2(9,12)
How do these numbers work? Hopefully you can see that the first number is the number of
carbons in the fatty acid. Then there is a comma, followed by another number showing how
many C=C bonds there are. Finally there may be some numbers in brackets at the end – these
show where the C=C are (if there are any), numbering from the carboxylic acid functional group.
So, in our examples above:
octadecanoic acid has 18 carbons and 0 C=C bonds;
octadec-9-enoic acid has 18 carbons and 1 C=C bond which starts on carbon 9;
octadec-9,12-dienoic acid has 18 carbons and 2 C=C bonds which start on carbons 9 and 12.
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Health Risks
Studies have suggested that trans fats and fatty acids can be linked with increased risk of heart
disease, strokes and obesity because they increase the production of low-density (so-called ‘bad’)
cholesterol and decrease that of high-density (so-called ‘good’) cholesterol.
Triglycerides
A triglyceride is a tri-ester of glycerol (propane-1,2,3-triol) with fatty acids.
For example:
O
(CH2)8
OH
CH3
O
(CH2)10
O
OH
CH3
O
OH
O
(CH2)17
glycerol
O
triglyceride
CH3
Esters of fatty acids, such as triglycerides, which are present in vegetable oils, have seen increased
use as renewable contributions towards energy requirement, marketed as biodiesel.