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Carboxylic Acids and Esters

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AN INTRODUCTION TO
CARBOXYLIC ACIDS
AND THEIR DERIVATIVES
KNOCKHARDY PUBLISHING
Carboxylic Acids
A carboxylic acid
 Contains a carboxyl group, which is a carbonyl group
(C=O) attached to a hydroxyl group (—OH).
 Has the carboxyl group on carbon 1.
carbonyl group
O

CH3 — C—OH hydroxyl group or CH3COOH
carboxyl group
2
Models of Carboxylic Acids
 The three-dimensional models show the geometry
of atoms in carboxylic acid molecules.
Copyright © 2007 by Pearson Education, Inc.
Publishing as Benjamin Cummings
3
IUPAC Names
The IUPAC names of carboxylic acids
 Replace the -e in the alkane name with -oic acid.
CH4
methane
HCOOH
CH3—CH3 ethane
methanoic acid
CH3—COOH ethanoic acid
 Number substituents from the carboxyl carbon 1.
CH3
O
|
║
CH3—CH—CH2—C—OH
4
3
2
1
3-methylbutanoic acid
4
Naturally occurring acids
Alpha hydroxy acids
(AHAs)
 Occur naturally in
fruit, milk, and
sugarcane.
 Are used in skin
care products.
5
Names and Sources of Some
Carboxylic Acids
TABLE 16.1
Copyright © 2007 by Pearson Education, Inc.
Publishing as Benjamin Cummings
6
Common Carboxylic Acids
Methanoic acid (formic acid)
O
║
H─C─OH
ethanoic acid (acetic acid)
O
║
CH3─C─OH
Copyright © 2007 by Pearson Education, Inc.
Publishing as Benjamin Cummings
7
Aromatic Carboxylic Acids
Benzoic acid
 Is the aromatic carboxylic acid.
 Locates substituents by assigning 1 to the carbon
attached to the carboxyl group.
 Has common names that assign prefixes ortho,
meta, and para for 2 substituents.
ortho
1, 2 location
meta
1, 3 location
para
1, 4 location
8
Aromatic Carboxylic Acids
O
C
OH
O
O
C OH
C OH
Cl
NH2
benzoic acid
3-chlorobenzoic acid
m-chlorobenzoic acid
4-aminobenzoic acid
p-aminobenzoic acid
9
Learning Check
Give the IUPAC and common names:
A. CH3—COOH
B.
C.
CH3
|
CH3—CH—COOH
COOH
Br
10
Learning Check
Give the IUPAC and common names for the following:
A. CH3─CH2─COOH
CH3
|
B. CH3─CH─CH2─COOH
C.
O
C
OH
Br
11
12
Polarity of Carboxylic Acids
Carboxylic acids
 Are strongly polar.
 Have two polar groups:
hydroxyl (−OH) and
carbonyl (C=O).
δO
║δ+ δ- δ+
CH3CO H
13
Boiling Points of Carboxylic Acids
 Higher than alcohols, ketones, and aldehydes of
similar mass.
 Are high because they form dimers in which
hydrogen bonds form between the polar groups
in the two carboxyl groups.
O
||
CH3—C
|
O—H
H—O
|
C—CH3
||
O
A dimer of acetic acid
14
Comparison of Boiling Points
Compound
O
║
CH3−CH2−C−H
Molar Mass
Boiling Point
58
49°C
CH3−CH2−CH2−OH
60
97°C
O
║
CH3−C−OH
60
118°C
15
PHYSICAL PROPERTIES
BOILING POINT
Increases as size increases - Stronger van der Waals forces
Carboxylic acids have high boiling points for their relative mass
The effect of hydrogen bonding on the boiling point of compounds of similar mass
Compound
ethanoic acid
propan-1-ol
propanal
butane
Formula
CH3COOH
C3H7OH
C2H5CHO
C4H10
Mr
60
60
58
58
b. pt. (°C)
118
97
49
- 0.5
Comments
h-bonding
h-bonding
dipole-dipole
basic V der W
PHYSICAL PROPERTIES
BOILING POINT
Increases as size increases - due to increased van der Waals forces
101°C
118°C
141°C
164°C
PHYSICAL PROPERTIES
BOILING POINT
Increases as size increases - due to increased van der Waals forces
101°C
118°C
141°C
164°C
Boiling point is higher for “straight” chain isomers.
164°C
154°C
Greater branching = lower inter-molecular forces = lower boiling point
PHYSICAL PROPERTIES
BOILING POINT
Increases as size increases - due to increased van der Waals forces
Carboxylic acids have high boiling points for their relative mass
• arises from inter-molecular hydrogen bonding due to polar O—H bonds
HYDROGEN
BONDING
AN EXTREME CASE... DIMERISATION
• extra inter-molecular attraction = more energy to separate molecules
Boiling Points
Higher boiling points than similar alcohols, due to dimer
formation.
Acetic acid, b.p. 118C
Chapter 20
=>
20
Solubility in Water
Carboxylic acids
 Form hydrogen
bonds with many
water molecules.
Water molecules
 With 1-4 carbon
atoms are very
soluble in water.
 carboxylic acids
are soluble in
organic solvents
21
PHYSICAL PROPERTIES
SOLUBILITY
• carboxylic acids are soluble in organic solvents
• they are also soluble in water due to hydrogen bonding
HYDROGEN
BONDING
• small ones dissolve readily in cold water
• as mass increases, the solubility decreases
•
Boiling Points and Solubility
TABLE 16.2
Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings
23
Preparation of Carboxylic Acids
 Carboxylic acids can be prepared by oxidizing
primary alcohols or aldehydes.
 The oxidation of ethanol produces ethanoic acid
(acetic acid).
OH
O
|
[O]
||
CH3—CH2
CH3—C—H
ethanol
(ethyl alcohol)
O
[O]
||
CH3—C—OH
ethanal
(acetaldehyde)
ethanoic acid
(acetic acid)
24
25
PREPARATION OF CARBOXYLIC ACIDS
Oxidation of aldehydes
RCHO
+
[O]
Hydrolysis of esters
RCOOR + H2O
Hydrolysis of acyl chlorides
RCOCl +
Hydrolysis of nitriles
RCN
Hydrolysis of amides
——>
RCOOH
RCOOH + ROH
H2 O
——>
RCOOH + HCl
2 H2O
——>
RCOOH + NH3
RCONH2 + H2O ——>
RCOOH + NH3
+
Learning Check
What alcohol could be used to prepare the following:
1. butanoic acid
2. propanoic acid
27
28
Acidity of Carboxylic Acids
Carboxylic acids
 Are weak acids.
 Ionize in water to produce carboxylate ions
and hydronium ions.
O
O
║
║
CH3−C−OH + H2O
CH3−C−O– + H3O+
Copyright © 2007 by Pearson Education, Inc.
Publishing as Benjamin Cummings
29
Acid Dissociation Constants
Carboxylic acids
 Have small Ka values.
 Exist mostly as molecules
and a few ions in aqueous
solutions.
TABLE 16.3
30
Substituent Effects on Acidity
pKa = 4.46
pKa = 4.19 pKa = 3.47
pKa = 3.41
Chapter 20
pKa = 2.16
=>
31
32
CHEMICAL PROPERTIES
ACIDITY
weak acids
RCOOH + H2O(l)
RCOO¯(aq)
+ H3O+(aq)
1. React with bases
RCOOH + NaOH(aq) ——> RCOO¯Na+(aq) + H2O(l)
2. React with carbonates and hydrogen carbonates
3. React with metals
RCO2H + Na  RCO2-Na+ + H2(g)
QUALITATIVE ANALYSIS
Carboxylic acids are strong enough acids to liberate CO2 from carbonates
and dissolve well in alkalis
Neutralization of Carboxylic Acids
Carboxylic acid salts
 Are a product of the neutralization of a carboxylic acid
with a strong base.
CH3—COOH + NaOH
CH3—COO– Na+ + H2O
acetic acid
sodium acetate
(carboxylic acid salt)
 Are used as preservatives and flavor enhancers.
Copyright © 2007 by Pearson Education, Inc.
Publishing as Benjamin Cummings
34
Learning Check
Write the equation for the reaction of propanoic acid with
A. water
B. KOH
35
CHLORINATION
Chlorination
involves replacing the OH with a Cl
Product
Acyl chloride
Reagent
PCl3 , PCl5 or SOCl2 (most popular reagent)
Conditions
dry conditions
Equation
CH3COOH
+
SOCl2
——> CH3COCl +
SO2 +
HCl
white
fumes
Esters
In an ester,
 The H in the carboxyl group is
replaced
with an alkyl group.
O

CH3 — C—O—CH3
ester group
Copyright © 2007 by Pearson Education, Inc.
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37
Naming Esters
The name of an ester contains the names of
 The alkyl group from the alcohol.
 The carbon chain from the acid with –ate ending.
from alcohol
from acid
O
methyl

CH3— O—C —CH3
ethanoate (acetate)
IUPAC: methyl ethanoate
common: methyl acetate
38
Ester Products
Aspirin
 Is used to relieve pain and
reduce inflammation.
 Is an ester of salicylic acid
and acetic acid.
Oil of wintergreen
 Is used to soothe sore
muscles.
 Is an ester of salicylic acid
and methanol.
O
C OH
O
O C CH3
O
C O CH3
OH
39
Esters in Plants
Esters give
flowers and
fruits their
pleasant
fragrances
and flavors.
40
Learning Check
Give the IUPAC and common names of the following
compound, which is responsible for the flavor and odor
of pears.
O

CH3—C—O—CH2—CH2—CH3
41
Learning Check
Name the following esters:
O
║
A. CH3—CH2—CH2—C—O—CH3
B.
O

CH3—CH2 —C—O—CH2—CH3
42
Learning Check
Write the structure of the following esters:
A. Ethyl pentanoate
B. Propyl butyrate
43
Solution
A. Ethyl pentanoate
O
║
CH3—CH2—CH2—CH2—C—O—CH2—CH3
B. Propyl butyrate
O
║
CH3—CH2—CH2—C—O—CH2—CH2—CH3
44
STRUCTURAL ISOMERISM – FUNCTIONAL GROUP
Classification
Functional Group
Name
CARBOXYLIC ACID
ESTER
R-COOH
R-COOR
PROPANOIC ACID
METHYL ETHANOATE
Physical properties
O-H bond gives rise
to hydrogen bonding;
get higher boiling point
and solubility in water
No hydrogen bonding
insoluble in water
Chemical properties
acidic
reacts with alcohols
fairly unreactive
hydrolysed to acids
Esterification
Esterification is
 The reaction of a carboxylic acid and alcohol in the
presence of an acid catalyst to produce an ester.
O

H+
CH3—C—OH + H—O—CH2—CH3
O

CH3—C—O—CH2—CH3 + H2O
ethyl acetate (an ester)
46
Learning Check
Write the equation for the reaction of propanoic acid
and methanol in the presence of an acid catalyst.
47
PREPARATION OF ESTERS - 1
Reagent(s)
alcohol + carboxylic acid
Conditions
reflux with a strong acid catalyst (e.g. conc. H2SO4 )
Equation
Notes
e.g. CH3CH2OH(l) + CH3COOH(l)
ethanol
ethanoic acid
CH3COOC2H5(l) + H2O(l)
ethyl ethanoate
Conc. H2SO4 is a dehydrating agent - it removes water
causing the equilibrium to move to the right and thus
increases the yield of the ester
For more details see under ‘Reactions of carboxylic acids’
PREPARATION OF ESTERS - 2
Reagent(s)
alcohol + acyl chloride
Conditions
reflux under dry conditons
Equation
Notes
e.g.
CH3OH(l) + CH3COCl(l) ——> CH3COOCH3(l) + HCl(g)
methanol
ethanoyl
methyl
chloride
ethanoate
Acyl chlorides are very reactive but must be kept dry
as they react with water
PREPARATION OF ESTERS - 3
Reagent(s)
alcohol + acid anhydride
Conditions
reflux under dry conditons
Equation e.g. CH3OH(l) + (CH3CO)2O(l) ——> CH3COOCH3(l) +
CH3COOH(l)
methanol
ethanoic
methyl
anhydride
ethanoate
Notes
Acid anhydrides are not as reactive as
acyl chlorides so the the reaction is slower.
The reaction is safer - it is less exothermic.
Acid anhydrides are less toxic.
ethanoic
acid
HYDROLYSIS OF ESTERS
Hydrolysis is the opposite of esterification
ESTER + WATER
CARBOXYLIC ACID + ALCOHOL
HCOOH
METHANOIC
ACID
ETHYL METHANOATE
+
C2H5OH
ETHANOL
HYDROLYSIS OF ESTERS
Hydrolysis is the opposite of esterification
ESTER + WATER
CARBOXYLIC ACID + ALCOHOL
HCOOH
METHANOIC
ACID
ETHYL METHANOATE
METHYL ETHANOATE
+
C2H5OH
ETHANOL
HYDROLYSIS OF ESTERS
Hydrolysis is the opposite of esterification
ESTER + WATER
CARBOXYLIC ACID + ALCOHOL
HCOOH
+
METHANOIC
ACID
C2H5OH
ETHANOL
ETHYL METHANOATE
CH3COOH
ETHANOIC
ACID
METHYL ETHANOATE
+
CH3OH
METHANOL
HYDROLYSIS OF ESTERS
Hydrolysis is the opposite of esterification
ESTER + WATER
CARBOXYLIC ACID + ALCOHOL
The products of hydrolysis depend on the conditions used...
acidic
CH3COOCH3
+ H2 O
alkaline
CH3COOCH3 + NaOH
CH3COOH
+
CH3OH
——> CH3COO¯ Na+ + CH3OH
HYDROLYSIS OF ESTERS
Hydrolysis is the opposite of esterification
ESTER + WATER
CARBOXYLIC ACID + ALCOHOL
The products of hydrolysis depend on the conditions used...
acidic
CH3COOCH3
+ H2 O
alkaline
CH3COOCH3 + NaOH
CH3COOH
+
CH3OH
——> CH3COO¯ Na+ + CH3OH
If the hydrolysis takes place under alkaline conditions,
the organic product is a water soluble ionic salt
HYDROLYSIS OF ESTERS
Hydrolysis is the opposite of esterification
ESTER + WATER
CARBOXYLIC ACID + ALCOHOL
The products of hydrolysis depend on the conditions used...
acidic
CH3COOCH3
+ H2 O
alkaline
CH3COOCH3 + NaOH
CH3COOH
+
CH3OH
——> CH3COO¯ Na+ + CH3OH
If the hydrolysis takes place under alkaline conditions,
the organic product is a water soluble ionic salt
The carboxylic acid can be made by treating the salt with HCl
CH3COO¯ Na+ +
HCl
——>
CH3COOH
+
NaCl
USES OF ESTERS
Despite being fairly chemically unreactive, esters are useful as ...
• flavourings
apple
pear
banana
pineapple
rum
• solvents
nail varnish remover - ethyl ethanoate
• plasticisers
2-methylbutanoate
3-methylbutylethanoate
1-methylbutylethanoate
butylbutanoate
2-methylpropylpropanoate
58
Acid Hydrolysis of Esters
In acid hydrolysis
 An ester reacts with water to produce a carboxylic
acid and an alcohol.
 An acid catalyst is required.
O

H+
H—C—O—CH2—CH3 + H2O
O

H—C—OH
+ H—O—CH2—CH3
59
Base Hydrolysis (Saponification)
Base hydrolysis (also called saponification)
 Is the reaction of an ester with a strong base.
 Produces the salt of the carboxylic acid and an
alcohol.
O
||
CH3—C—O—CH2—CH3 + NaOH
O

CH3—C—O– Na+ + HO—CH2—CH3
salt of carboxylic acid
alcohol
60
NATURALLY OCCURING ESTERS - TRIGLYCERIDES
• triglycerides are the most common component of edible fats and oils
• they are esters of the alcohol
glycerol (propane-1,2,3-triol)
CH2OH
CHOH
CH2OH
Saponification
•
•
•
•
alkaline hydrolysis of triglycerol esters produces soaps
a simple soap is the salt of a fatty acid
as most oils contain a mixture of triglycerols, soaps are not pure
the quality of a soap depends on the oils from which it is made
“Soaps”
The base hydrolysis of long chain fatty acids
produces acid salts called “soaps”.
62
Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings
Cleaning Action of Soap
A soap
 Contains a nonpolar end
that dissolves in
nonpolar fats and oils,
and a polar end that
dissolves in water.
 Forms groups of soap
molecules called
micelles that dissolve in
water and are washed
away.
Copyright © 2007 by Pearson Education, Inc.
Publishing as Benjamin Cummings
63
Learning Check
Write the organic products when methyl acetate reacts
with
A. Water and an acid catalyst
B. KOH
64
Solution
Write the organic products when methyl acetate reacts with:
A. Water and an acid catalyst
O

CH3—C—OH + HO—CH3
B. KOH
O

CH3—C—O– K+
+ HO—CH3
65
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