Download Document

Chemistry
Session - 2
Carboxylic Acids
Session Objectives
1.
Introduction to carboxylic acids
2.
Physical properties and structure
3.
General method of preparation
By oxidation
Carbonation of grignard reagent
Hydrolysis of acid derivatives
From nitriles
4.
Reactions of the carboxylic acids
Introduction
The functional group of carboxylic acids
consists of a C=O with -OH bonded to the
same carbon.
Carboxyl group is usually written -COOH.
Aliphatic acids have an alkyl group
bonded to -COOH.
Aromatic acids have an aryl group.
Fatty acids are long-chain aliphatic acids.
Common Names
Many aliphatic acids have historical
names.
Positions of substituents on the chain
are labeled with Greek letters.
Ph
Cl O
CH3CH2CHC OH
CH3CH2CH2CHCH2COOH
-phenylcaproic acid
-chlorobutyric acid
=>
IUPAC Names
•
Remove -e from alkane (or alkene) name,
add -oic acid.
•
The carbon of the carboxyl group is #1.
Ph
Cl O
H
H
C C
COOH
CH3CH2CHC OH
2-chlorobutanoic acid
trans-3-phenyl-2-propenoic acid
(cinnamic acid)
Naming Cyclic Acids
Cycloalkanes bonded to -COOH are named as
cycloalkanecarboxylic acids.
Aromatic acids are named as benzoic acids.
COOH
CH(CH3)2
2-isopropylcyclopentanecarboxylic acid
COOH
OH
o-hydroxybenzoic acid
(salicylic acid)
Dicarboxylic Acids
Aliphatic diacids are usually called by their
common names (to be memorized).
For IUPAC name, number the chain from the
end closest to a substituent.
Two carboxyl groups on a benzene ring
indicate a phthalic acid.
Br
HOOCCH2CHCH2CH2COOH
3-bromohexanedioic acid
-bromoadipic acid
Structure of Carboxyl
Carbon is sp2 hybridized.
Bond angles are close to 120.
O-H eclipsed with C=O, to get overlap of  orbital
with orbital of lone pair on oxygen.
Boiling Points
Higher boiling points than similar alcohols,
due to dimer formation.
Acetic acid, b.p. 118C
Melting Points
Aliphatic acids with more than 8 carbons are solids
at room temperature.
Double bonds (especially cis) lower the melting
point. Note these 18-C acids:
Stearic acid (saturated): 72C
Oleic acid (one cis double bond): 16C
Linoleic acid (two cis double bonds): -5C
Solubility

Water solubility decreases with the length of
the carbon chain.

Up to 4 carbons, acid is miscible in water.

More soluble in alcohol.

Also soluble in relatively nonpolar solvents like
chloroform because it dissolves as a dimer.
Acidity
Resonance Stabilization
Substituent Effects on Acidity
COOH
COOH
COOH
COOH
COOH
NO2
NO2
OCH3
p-methoxy benzoic acid
pKa = 4.46
m-nitro
pKa = 4.19 pKa = 3.47
NO2
p-nitro
pKa = 3.41
o-nitro
pKa = 2.16
Preparation Reactions

Oxidation of Primary Alcohols

Oxidation of Aldehydes

Oxidation of Substituted Aromatics

Carbonation of Grignard reagents

Hydrolysis of Acid derivatives and Nitriles

Haloform reaction

Periodic acid Cleavage of Vicinal Dials/Diketones

Oxidative Cleavage of Alkenes/Alkynes
Carboxylic Acids via Oxidation
From Primary Alcohols
From Aldehydes
From Substituted Aromatics
Oxidative Cleavage Reactions
Alkene Cleavage
Hot Potassium Permanganate
Alkyne Cleavage
Hot Potassium Permanganate
Ozonolysis
R
R
CH
C
CH
C R'
KMnO
R'
²
4
ozonolysis
or
KMnO 4
²
RCOOH +R'COOH
RCOOH +R'COOH
Grignard Synthesis
Grignard reagent + CO2 yields a carboxylate salt.

O
R
C +
Mg
O
X
O
+
O
H
R C OMgX
R C OH
H 2O
Hydrolysis of acid derivatives
H+
R C G
H 2O
O
C OH + HG
R
O
OH R
R C G
H 2O
O
where G = -X,- OR, -NH
2
C O - + GO
, - NHR, NR
2
, &- O C
R
O
H + or OH RC N
R
H 2O
C NH
O
2
R
C OH ( R
C O- )
O
O
Hydrolysis of Nitriles
Basic or acidic hydrolysis of a nitrile produces a carboxylic acid.
Br
NaCN
CN
+
H
H2O
COOH
Haloform Reaction
Cleavage of methyl carbinols
Cleavage of methyl carbonyls
H
CH 3
C R
OH
CH 3 C R
O
X2
-
OH /H 2O
X2
-
OH /H 2O
-
RCOO + HCX 3
RCOO - + HCX 3
Periodic Acid Cleavage of
Vicinal Dials/Diketones
H C C H
O O
R C C H
O O
R C C R'
O O
HIO 4
HIO 4
HIO 4
2 HCOOH + HIO 3
RCOOH + HCOOH + HIO 3
RCOOH + R'COOH + HIO 3
Reduction of Carboxylic Acids

Lithium Aluminum Hydride reduction

Diborane reduction
RCOOH
LiAlH 4
ether
RCOOH
B 2H 6
diglyme
H+
H 2O
RCH 2OH
RCH 2OH
(selective reduction of
carboxyl group in
preference to carbonyl
of aldehydes/ketones)
Conversion to acid derivatives
The group bonded to the acyl carbon determines the
class of compound:
-OH, carboxylic acid
-Cl, acid chloride
-OR’, ester
-NH2, amide
These interconvert via nucleophilic acyl substitution.
Fischer Esterification
Acid + alcohol yields ester + water.
Acid catalyzed for weak nucleophile.
All steps are reversible.
Reaction reaches equilibrium.
O
COOH
+ CH3CH2OH
+
H
COCH2CH3
+ HOH
Conversion to Acid Chlorides
An activated form of the carboxylic acid.
Chloride is a good leaving group, so undergoes acyl
substitution easily.
To synthesize acid chlorides use thionyl chloride or oxalyl
chloride with the acid.
O
O
C OH
+
O O
C Cl
C C
Cl
Cl
+ HCl + CO + CO2
Conversion to Amides
Amine (base) removes a proton from the
carboxylic acid to form a salt.
Heating the salt above 100C drives off
steam and forms the amide.
O
O
O
C OH CH NH
+
3
2
C O- +NH CH
3
3
C NHCH
3
heat
+ H2O
Reduction to 1 Alcohols
Use strong reducing agent, LiAlH4.
Borane, BH3 in THF, reduces carboxylic acid to
alcohol, but does not reduce ketone.
Decarboxylation of RCOOH

Thermolysis of beta-diacids

Thermolysis of beta-keto acids
COOH
C
COOH
H
²
COOH
COOH
C
C
O
+ CO 2
C
H
²
+ CO 2
C
C
O
Substitution in the hydrocarbon part
Hell-Volhard-Zelinsky reaction
Acids having an -hydrogen are halogenated at the -position on
treatment with chlorine or bromine in the presence of small
amount of red phosphorus to give -halocarboxylic acids.
X2 / P4 , H2O
RCH2COOH 
 RCH(X)COOH
Ring substitution in aromatic acids
—COOH group is deactivating and meta directing.
Aromatic carboxylic acids do not undergo Friedel-Crafts reaction.
Nitration
COOH
COOH
HNO3(conc.)
H2SO4(conc.)
NO2
Bromination
COOH
COOH
Br2 /FeBr3
Br
Some commercially important
carboxylic acids
Methanoic acid; (HCOOH)
473 K, 10 atm
NaOH + CO
+
HCOONa
H
HCOOH
It is colourles, pungent smelling liquid.
It is powerful reducing agent. It reduces Tollen’s reagent
and Fehling solution.
Used in rubber, textile, dyeing, leather and electroplating
industries.
Ethanoic acid(acitic acid, CH3COOH)
Main constituent of vinegar and is obtained by fermentation
of molasses in presence of air.
Industrially, it is obtained in pure form by oxidation of ethanal
with air in the presence of cobalt acetate catalyst or by
carbonylation of methanol in the presence of rhodium catalyst.
CH3CHO + CO  CH3COOH
CH3OH + CO  CH3COOH
Colourless liquid with pungent odour.
Freezes at 289 K forming ice like crystal.
Water free acetic acid, obtained by melting of the crystals is called
glacial acetic acid.
Thank you