esters - wellswaysciences

advertisement
Week 4
•
Describe the esterification of carboxylic acids with alcohols in the presence
of an acid catalyst, and also of acid anhydrides with alcohols.
•
Describe the hydrolysis of esters.
•
State the uses of esters in perfumes and flavourings.
© Pearson Education Ltd 2009
This document may have been altered from the original
Esters
• Esters are characterised by the functional
group
They have no free OH groups so they cannot
hydrogen bond.
They are volatile compared to acids and
alcohols of similar mass and are not very
soluble in water.
Flavourings etc.
• Volatile esters have light, fruity smells and
the flavour and smells of many fruits and
flowers are due to mixtures of esters.
• Artificial flavourings are made by mixing
synthetic esters but they are only ever
approximate because it would be too
expensive to include all the flavour compounds
present in real fruit etc.
Flavour
Ester
apple
Ethyl-2methylbutanoate
3-methylbutyl
ethanoate
1-methylbutyl
ethanoate
Phenylmethyl
ethanoate
Butyl butanoate
pear
banana
Oil of jasmine
pineapple
Formation of Esters
• 1. By the reaction of an alcohol with a
carboxylic acid.
This is ESTERIFICATION. reflux the
mixture with conc. sulphuric acid catalyst.
It is an equilibrium reaction and rarely goes
to completion.
Week 4
An acid anhydride is formed by removal of a molecule of water from two
carboxylic acid molecules.
© Pearson Education Ltd 2009
This document may have been altered from the original
Week 4
Ethanoic anhydride reacts with methanol to make the ester methyl ethanoate
Carried out by gentle heating and since it is
not an equilibrium reaction it goes in far
better yield.
© Pearson Education Ltd 2009
This document may have been altered from the original
Naming Esters
• Remember!
• Esters are regarded as carboxylic acid
derivatives.
• Divide the formula into 2 portions by drawing
a line after the bridging oxygen of the COO
group.
• Name the portion which does NOT carry the
COO as the alkyl radical.
• Name the portion that carries the COO group
as the carboxylate radical.
• Combine the 2.
Examples
•
•
•
•
•
•
•
•
•
Draw displayed formulae and name:
C6H5COOCH3
Methyl benzoate
HCOOCH2CH3
Ethyl methanoate
CH3CH2COOCH3
Methyl propanoate
CH3CH2OOCCH2CH3
Ethyl propanoate
Naming Cont:
•
•
•
•
•
•
•
Draw displayed formulae for :
Heptyl decanoate
Phenyl benzoate
Butyl methanoate
Benzyl ethanoate
1-methylethylpropanoate
For each of the above state the acid and the
alcohol required to make them.
More Exercises
• Draw skeletal formulae for the isomers of
ethyl propanoate that are straight chain
esters and name them.
• Draw the structural formula of 1-methylethyl
propanoate.
• Name the carboxylic acid and alcohol that
would form the ester.
• Write a balanced equation for the reaction.
Hydrolysis of Esters
• Since the reaction of an acid with an alcohol
to form an ester and water is an equilibrium
reaction the back reaction of this is the
hydrolysis of an ester to reform the acid and
the alcohol.
• The reaction uses an aqueous (dilute) acid
catalyst and is refluxed as before.
• As before the reaction does not go to
completion so all reactants and products are
present in the reaction vessel.
Week 4
Acid hydrolysis of propyl ethanoate
© Pearson Education Ltd 2009
This document may have been altered from the original
Alkaline Hydrolysis
• The reaction goes faster and goes to
completion in the presence of an alkali.
• Why?
• When some of the acid molecules have been
formed in the reaction the hydroxide ions
present react with them to form a salt.
• This removes the acid from the reaction
mixture and so the reaction moves RIGHT.
• The base (or alkali) is used up in the reaction.
• This is not strictly catalysed by the alkali.
Why not?
Week 4
Alkaline hydrolysis of ethyl propanoate
© Pearson Education Ltd 2009
This document may have been altered from the original
Exercise
• Write a balanced equation for the base
hydrolysis of methyl benzoate.
• Name the products.
• Write a balanced equation for the acid
hydrolysis of methyl propanoate.
• Name the products.
Download