Alkanoic Acids and Esters
SUBSECTION 5Esterification is a naturally occurring process which can be performed in the laboratory
Methanoic acid (formic acid)
Ethanioic acid (acetic acid)
Acetyl salicylic acid (aspirin)
Functional group
C
O
O H
-COOH
The carboxyl acid group
IUPAC Systematic Naming• count no. carbons in straight chain • select prefix for parent alkane(carbon 1
from –COOH)• drop “e” and add ending –oic acid• formic acid and acetic acid are IUPAC
preferred names for methanoic and ethanoic acids
Ester functional group
CO
O
C
RC
O
O 1
RR = H or alkyl group
R1 = alkyl group
Naming Esters
• 1st part of name comes from the alkanol– e.g. methanol produces methyl-, ethanol
produces ethyl- etc...• 2nd part of name comes from alkanoic acid
– e.g. methanoic acid produces –methanoate, ethanoic acid produces –ethanoate etc...
CH O
CH
H HOH
HC C H
H
ethyl
ethyl ethanoate
ethanoate
Name this ester
C C C
O
O C C C C
H
H
H
H
HH
H H H H
H
HH H
Practice• Name the esters produced from...
– Butanol and pentanoic acid– Heptanoic acid and ethanol
• Name the alkanols and alkanoic acids used to produce...– Pentyl butanoate– Methyl octanoate
5.2.3• explain the difference in melting and
boiling point caused by straight-chained alkanoic acids and straight-chained primary alkanol structures
Reactions• alkanoic acids react with alkanols to
produce esters (alkyl alkanoates).
Producing Esters
• a condensation reaction where two molecules combine with the elimination of a small molecule like water.
Alkanol + Alkanoic acid D
Ester + Water
Example
CH
OCH
H H
OCH
OCH
HH
H
H
+ OH H+C
H OC
H
H HOH
HC C H
H
• The –OH is removed from the ACID• The –H is removed from the
ALCOHOL
CH3CH2CH2OH + HOOCCH2CH3 CH3CH2CH2OOCCH2CH3
• The C with H atoms attached is from the alcohol
• The C without H atoms attached is from the acid
Problem• Use structural formulae to illustrate the
reaction of:– Methanol with butanoic acid;– Propanoic acid with ethanol.
Properties of Esters• have pleasant fruity smells• colourless liquids (lower molecular
weight are more volatile)• polar molecules • short chain soluble in water
– solubility i with h in length of carbon chain
• good organic solvents• neutral to Litmus
• flavours & fragrances– contribute to the tastes and smells of fresh
fruits and flowers– used as fragrances in body care products
and cosmetics– methyl salicylate used in “Deep Heat” as an emollient to soothe sore muscles
Esters - Uses
• lubricants– used as jet engine lubricants for many
decades due to their low viscosity– polar molecules – stronger intermolecular
forces so lower rates of evaporation and higher flash points
Esters - Uses
• solvents– used in wood lacquers and thinners– esters of acetic acid used in personal care
products and cosmetics– ethyl acetate in nail polish remover
Esters - Uses
• plasticisers– phthalate esters are oily liquids used to
soften hard plastics like PVC– plasticised PVC used to make medical
tubing, gloves, shower curtains, toys
Esters - Uses
ASPIRIN (acetyl salicylic acid)an ester of acetic acid and salicylic acid
Triglycerides
Triesters of glycerol and fatty acids
The reverse reaction• The formation of esters is a reversible
reaction.
• This means that an alkanol and alkanoic acid can be produced from an ester.
Alkanol + Alkanoic acid
Ester + Water
This breakdown using water is called a “hydrolysis” reaction.
Hydrolysis of Esters
CH O
CH
H HOH
HC C H
H
COH
CHH O H
Ethanoic acid
CO
H
CHH
H
H H
Ethanol
CH3COOCH2CH2CH3
CH3COOHEthanoic acid
CH3CH2CH2OHPropan –1- ol
Esterification• slow reaction at room temperature that
reaches equilibrium• heating increases the reaction rate, but• the reactants and products with lower
boiling points will vaporise readily and be lost
ethanol bp 78oCethanoic acid 114ethyl ethanoate 77water 100
Esterification• slow reaction at room temperature that
reaches equilibrium• heating increases the reaction rate, but• the reactants and products with lower
boiling points will vaporise readily and be lost
• heat under reflux – use a cooling condenser vertically over reaction vessel so vapours condense and run back down into reaction flask
Esterification• add conc. H2SO4 as
– catalyst to improve reaction rate– dehydrating agent as it removes water
so not available for reverse reaction - drives the equilibrium to the right
• excess of alkanol shifts equilibrium to the right
Separation of Ester - Fractional DistillationComponent Boiling point (oC)methanol 64.7propanoic acid 140.8water 100methyl propanoate 79.9
Separation of Ester - Aqueous Separation
Mixture placed into separating funnel
Draining off the aqueous layer