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6.1.3 Carboxylic acids and esters
Module no.
6
AS
level
Assessed in exam paper
N/A
A level
2016 - 17
Assessed in exam paper
02 & 03
Traffic light the following statements:
Before
After
teaching teaching
After
revision
(a) explanation of the water solubility of carboxylic
acids in terms of hydrogen bonding
(b) reactions in aqueous conditions of carboxylic
acids with metals and bases (including carbonates,
metal oxides and alkalis)
(c) esterification of:
(i) carboxylic acids with alcohols in the presence
of an acid catalyst (e.g. concentrated H2SO4)
(ii) acid anhydrides with alcohols
(d) hydrolysis of esters:
(i) in hot aqueous acid to form carboxylic acids
and alcohols
(ii) in hot aqueous alkali to form carboxylate
salts and alcohols
(e) the formation of acyl chlorides from carboxylic
acids using SOCl2
(f) use of acyl chlorides in synthesis in formation of
esters, carboxylic acids and primary and secondary
amides.
1
Carboxylic acids
Carboxylic acids are organic compounds which contain both a carbonyl (C=O)
and a hydroxy group (-OH) attached to the same carbon, this is called the
carboxyl group.
RCOOH
The skeletal, displayed and structural formulas are shown above. R represents the
rest of the chain, which could be an alkyl group including mixture of branches
and/or other functional groups.
Naming carboxylic acids
To name a carboxylic acid, start with the alkane, remove the ‘e’ and replacing this
with’ –oic acid’. When numbering, the carbon atom which is part of the
carboxylic acid functional group is always carbon 1.
E.g.
O
C2H5
C
H
OH
Cl
H
C
C
C
H
H
H
O
C
OH
3-chlorobutanoic acid
Propanoic acid
CH3
H
O
CH3CH2CH2CH2CH2CH2CH2COOH
OH
3-methylhexanoic acid
Octanoic acid
2
Student activity 1
a) Name each of the following
O
C4H9
C
pentanoic acid
………………………………………………………………………………
OH
CH3
H3C
C
O
C
2-methyl propanoic acid
………………………………………………………………………………
OH
H
O
C2H5
C
propanoic acid
………………………………………………………………………………
OH
O
OH
butanoic acid
………………………………………………………………………………
O
OH
2,3-dimethyl octanoic acid
………………………………………………………………………………
b) Draw each of the following, using full skeletal formula
2-methylnonanoic acid
methanoic acid
Butan-1,4-dioic acid
3-chloro-3-methylpentaoic acid
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Physical properties of carboxylic acids
Carboxylic acids can form hydrogen bonds with other molecules due to the
hydroxy group. This means the small carboxylic acids are able to dissolve in water.
Draw a diagram to show the interaction between ethanoic acid and water
Reactions of the carboxylic acids.
Carboxylic acids, as the name suggests, are acids. You know from Year 1 that
they are weak acids because they only partially dissociate in water. The react in
the same way as other acids.
E.g. ethanoic acid (CH3COOH)
Reaction with NaOH(aq)
CH3COOH(aq) + NaOH(aq)  CH3COONa(aq) + H2O(l)
Explain why CH3COONa is a salt.
Because the hydrogen of the acid has been replaced by a
sodium ion.
……………………………………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………………………………
Reaction with Na2CO3(aq)
2CH3……………………………………………………………………………………………………………………………………………
COOH (aq) + Na2CO3 (aq) → 2CH3COONa (aq) + CO2 (g) + H2O (l)
What would you observe during this reaction?
……………………………………………………………………………………………………………………………………………
Fizzing / bubbling / effervescence
……………………………………………………………………………………………………………………………………………
Reaction with Na(s)
CH3COOH(aq) + Na(s) → CH3COONa(aq) + ½H2(g)
……………………………………………………………………………………………………………………………………………
Write an ionic equation for the reaction with Na
……………………………………………………………………………………………………………………………………………
+
H (aq)
+ Na(s) →
+
Na
(aq)
+
½H2(g)
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Reaction with Na2O(s)
……………………………………………………………………………………………………………………………………………
2CH
COOH(aq) + Na2O(s) → 2CH3COONa(aq) + H2O(l)
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The carboxylate ion
H
H
H
O
C
C
+
OH-
H
OH
H
O
C
+
C
H2O
-
O
H
ethanoate
Ethanoic acid
The ion formed when carboxylic acids react with in alkali conditions is called the
carboxylate ion. It is named after the carboxylic acid that forms it, replacing the ‘–
oic acid’ with ‘–oate’.
Reaction with alcohol (esterification)
When an alcohol and carboxylic acid are warmed in the presence of
concentrated sulphuric acid (H2SO4) an ester is formed. This reaction is both slow
and reversible.
E.g. ethanoic acid with methanol.
H
H
C
H
O
+
C
H
H
O
H
O
H
ethanoic acid
C
CH3
H
methanol
O
C
+
O
H2O
CH3
methylethanoate
water
Write an equation for butanoic acid reacting with ethanol
……………………………………………………………………………………………………………………………………………
CH3……………………………………………………………………………………………………………………………………………
CH2CH2COOH + CH3CH2OH → CH3CH2CH2COOCH2CH3 + H2O
……………………………………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………………………………
Esters
Esters contain the carbonyl (C=O) and alkyl (O-R) group.
They can be formed from the reaction between a carboxylic
acid and an alcohol (see above) or an acid anhydride and
an alcohol (see below) or an acyl chloride and an alcohol.
5
Naming esters
The name of the ester is derived from the carboxylic acid and alcohol that would
form it. They are named ‘back to front’ i.e. start with the alkyl part and end with
the acid.
H
H3C
O
C
From carboxylic acid……………………………oate
C
O
H
C2H5
From alcohol…………………………………………yl
The name for this ester is …………………………………………………………………………
Student activity 2
Name these esters
H
H
O
C
C
O
H
H3C
H
H
C
C
H
H
C
H
propyl ethanoate
O
C
O
H
H
C3H7
C3H7
propyl butanoate
O
C
O
C2H5
ethyl ethanoate
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Making esters from acid anhydrides
We have already seen how esters can be made from carboxylic acids. Another
way to make esters is by reacting them with acid anhydrides. Acid anhydrides are
derivatives of carboxylic acids, where the H of the hydroxy
group has been replaced with a carbonyl group. When an
acid anhydride is reacted with an alcohol in the presence of
concentrated sulphuric acid an ester and a carboxylic acid
form.
E.g. ethanoic anhydride and methanol.
ethanoic anhydride
methanol
methyl ethanoate
ethanoic acid
Student activity 3
1. Draw the structure of these acid anhydrides.
a) Butanoic anhydride
b) Methylpropanoic anhydride
2. Name these acid anhydrides
a)
Propanoic anhydride
b)
2-chloropropanoic anhydride
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Write fully balanced equation using the type of formula stated and name the
products in the following reactions.
1. Ethanoic anhydride and propan-1-ol (Displayed)
→
+
+
propylethanoate
ethanoic acid
2. Propanoic anhydride and methanol (Skeletal)
+
→
methylpropanoate
+
propanoic acid
3. butanoic anhydride and methanol (Structural)
(CH3CH2CH2CO)2O + CH3OH → CH3CH2CH2COOCH3 + CH3CH2CH2COOH
methylbutanoate
butanoic acid
Sometimes the acid anhydride used is not symmetrical. E.g. ethanoic propanoic
anhydride. In this case a mixture of two esters and two carboxylic acids is formed.
Draw the two carboxylic acids and two esters that
could form from the reaction of ethanoic propanoic
anhydride with butan-1-ol.
Ester 1
Carboxylic acid 1
Cont. overleaf
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Ester 2
Carboxylic acid 2
Stretch & Challenge extension: Which acid anhydride and alcohol combination
would make these esters and carboxylic acids.
2-methylpropanoic acid
Propyl methylpropanoate
Ethanoic acid
Propyl ethanoate
Making esters from phenols.
Phenols are a bit like alcohols, but due to the benzene ring, react slightly
differently. We shall look at phenol, the simplest of the phenols.
Phenols do not readily react with carboxylic acids to form esters, but acid
anhydrides and acyl chlorides do react with phenols. (We will look at acyl
chlorides later.).
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This is due to the comparative non-availability of the lone pair on the O atom of
the OH group. You will also come across phenols in 6.1.1 Aromatic compounds.
With acid anhydride
Complete this equation:
Reactions of esters
Esters can be hydrolysed (broken down using water) into their carboxylic acid and
alcohol. There are two types of hydrolysis depending on the conditions used.
Acid hydrolysis
The ester is reacted with dilute hot aqueous acid. The acid is acting as a catalyst.
The reaction is reversible.(Note concentrated acid to make an ester, dilute to
hydrolyse one.). Write an equation to show how propylethanoate will be
hydrolysed in acidic conditions.
CH3COOCH2CH2CH3 + H2O → CH3COOH + CH3CH2CH2OH
………………………………………………………………………………………………………
Base hydrolysis
Base hydrolysis requires hot aqueous alkali. This reaction is not reversible. Because
it is done in alkali conditions, instead of producing the carboxylic acid the
carboxylate salt is made. This process is called saponification because if you
hydrolysed a large ester such as those found in animal fats or vegetable oils, the
sodium salt produced would be the same as those found in soap.
http://estream.loreto.ac.uk/view.aspx?id=4755~4v~6cmREL7c
10
Write an equation to show how methylpropanoate will be hydrolysed in alkali
conditions.
-
CH3CH2COOCH3 + OH → CH3CH2COO
-
+ CH3OH
………………………………………………………………………………………………………
Esters have many uses including as solvents and as adhesives. Nail varnish remover
and board pens contain ethylethanoate as a solvent. Different esters have
different smells and are used to flavour foods and perfumes.
Demo
Your teacher is going make some esters. Record in the table the alcohol and
carboxylic acid used. Deduce the ester that has been made and record its smell.
Alcohol
Carboxylic acid
Ester
Smell
Acyl chlorides
Acyl chlorides are derivatives of carboxylic acids where the hydroxy group has
been replaced by a chlorine atom. Acyl chlorides have very polar carbonyl
bonds and therefore react readily with other molecules. They form permanent
dipole interactions with other molecules.
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Making acyl chlorides
Acyl chlorides can be prepared from carboxylic acids using SOCl2 (sulfur dichloride
oxide or thionyl chloride or sulfurous dichloride)
e.g. reaction with ethanoic acid
CH3COOH(l) + SOCl2(l)  CH3COCl(l) + SO2(g) + HCl(g)
Student Activity 4
Write full balanced equations to show how the following would react with SOCl 2
a) propanoic acid
CH CH COOH(l) + SOCl (l)  CH CH COCl(l) + SO (g) + HCl(g)
3
2
2
3
2
2
…………………………………………………………………………………………………………
b) 2,2-dimethylpropanoic acid
…………………………………………………………………………………………………………
c)
…………………………………………………………………………………………………………
Reactions of acyl chlorides
The chlorine bonded to the carbon of the carbonyl group (C=O) is extremely
electronegative. This makes the bond very polar, and the chlorine (as a chloride
ion) an excellent leaving group, and therefore when acyl chlorides react the Cl is
usually substituted for something else.
Reaction with water to form carboxylic acids.
Acyl chlorides react readily with water to produce carboxylic acids. A product of
the reaction is HCl(g) which can be seen as white fumes.
e.g. ethanoyl chloride
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CH3COCl + H2O  CH3COOH + HCl
Write an equation to show how (CH3)2CHCOCl would react with water. Name the
organic product.
(CH ) CHCOCl
+ HO
→ (CH ) CHCOOH + HCl
3 2
2
3 2
…………………………………………………………………………………………………………
Reaction with alcohols to form esters.
When acyl chlorides react with alcohols an ester is formed, again HCl is a product.
e.g. ethanoyl chloride with methanol
CH3COCl + CH3OH  CH3COOCH3 + HCl
Write an equation to show how (CH3)2CHCOCl would react with propan-2-ol.
(CH3)2CHCOCl + CH3CH(OH)CH3 → (CH3)2CHCOOCH2(CH3)2 + HCl
…………………………………………………………………………………………………………
Reaction with phenols to form esters.
13
Reaction with ammonia to form amides.
Ammonia reacts with acyl chlorides in a similar way. However ammonia is basic so
will react with the HCl formed to make a salt. We use two ammonia molecules in the
equation – the first NH3 reacts with the acyl chloride to produce the amide and HCl. The
second NH3 mops up the HCl produced forming the salt NH4Cl.
Then:
NH3 +
HCl
→
NH4Cl
Overall : RCOCl + 2NH3  RCONH2 + NH4Cl
e.g. ethanoyl chloride with ammonia
CH3COCl + 2NH3  CH3CONH2 + NH4Cl
Write an equation to show how CH3CH2CH2COCl would react with excess
ammonia.
CH CH CH COCl + 2NH  CH CH CH CONH + NH Cl
3
2
2
3
3
2
2
2
4
…………………………………………………………………………………………………………
Reaction with amines to form amides.
Amines, which you met in Year 1, are formed when haloalkanes are reacted with
ammonia.
They react with acyl chlorides in a similar way to ammonia, again producing a salt
from the reaction of the amine starting material with HCl produced in the reaction.
R‘
R‘
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Then:
R’NH2 + HCl  R’NH3Cl
Overall: RCOCl + 2R’NH2  RCONHR’ + R’NH3Cl
e.g. propanoyl chloride with methylamine
CH3CH2COCl + 2CH3NH2  CH3CH2CONHCH3 + CH3NH3Cl
Write an equation to show how (CH3)2CHCOCl would react with an excess of
propylamine
(CH ) CHCOCl
+ 2CH NH  (CH ) CHCONHCH + CH NH Cl
3 2
3
2
3 2
3
3
3
…………………………………………………………………………………………………………
Student Activity 5
1. For each reaction, write a full balanced equation, clearly showing the structure
of the product formed when
is reacted with:
a. Excess ammonia
…………………………………………………………………………………………………………
b. SOCl2
…………………………………………………………………………………………………………
c. 2-methylpropan-2-ol (with an acid catalyst)
…………………………………………………………………………………………………………
d. Na2CO3
…………………………………………………………………………………………………………
e. Propyl-1-amine
15
…………………………………………………………………………………………………………
2. Draw the products formed when the following is heated with aqueous HCl.
Carboxylate salts
The carboxylate ion has been formed in several of these reactions. It is a
derivative of a carboxylic acid. The salt of this ion is formed when a metal ion is
present. The carboxylate salt is an ionic compound and therefore we can predict
its properties.
Remember, this will form whenever a carboxylic acid is in alkali conditions.

Solubility in water:……………………………………………………..
O

Electrical conductivity:…………………………………………...
C4H9
C
-
+
O Na

Melting point:………………………………………………………..
16
Student Activity 6
Complete the flowchart below. Note that phenols are also weak acids and will
react with strong bases.
Warm with
aqueous NaOH
Warm with
aqueous HCl
Warm with propanoyl anhydride and
concentrated H2SO4
Warm with propanoic acid
and concentrated H2SO4
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Independent Learning –
Task
Completed?
If you need more support…
Read the text book pages 463-471 and complete the summary questions on page
471. Check your answers on pages 586.
If you want more practice…
Answer the practice questions on page 472-473. Answers on the Weebly.
If you want to challenge yourself…
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