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Chemistry
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Carboxylic Acids
Session - 2
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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
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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.
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Common Names
CH3CH2CHC
Cl
OH
O
-chlorobutyric acid
CH3CH2CH2CHCH2COOH
Ph
-phenylcaproic acid
=>
Many aliphatic acids have historical names.
Positions of substituents on the chain are labeled with Greek letters.
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IUPAC Names
CH3CH2CHC
Cl
OH
O
2-chlorobutanoic acid
PhC
HC
H
COOH
trans-3-phenyl-2-propenoic acid (cinnamic acid)
• Remove -e from alkane (or alkene) name, add -oic acid.
• The carbon of the carboxyl group is #1.
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Naming Cyclic Acids
COOH
CH(CH3)2
2-isopropylcyclopentanecarboxylic acid
COOH
OH
o-hydroxybenzoic acid(salicylic acid)
Cycloalkanes bonded to -COOH are named as cycloalkanecarboxylic acids.
Aromatic acids are named as benzoic acids.
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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.
HOOCCH2CHCH2CH2COOH
Br
3-bromohexanedioic acid
-bromoadipic acid
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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.
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Boiling Points
Acetic acid, b.p. 118C
Higher boiling points than similar alcohols, due to dimer formation.
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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): 72C
Oleic acid (one cis double bond): 16C
Linoleic acid (two cis double bonds): -5C
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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.
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Acidity
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Resonance Stabilization
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Substituent Effects on Acidity
COOH
OCH3
COOH COOH
NO2
COOH
NO2
COOH
NO2
p-methoxy benzoic acid m-nitro p-nitro o-nitro
pKa = 4.46 pKa = 4.19 pKa = 3.47 pKa = 3.41 pKa = 2.16
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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
Preparation Reactions
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Carboxylic Acids via Oxidation
From Primary AlcoholsFrom AldehydesFrom Substituted Aromatics
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Oxidative Cleavage Reactions R CH CH R' RCOOH R'COOH+
KMnO4
²
R C C R' RCOOH R'COOH+
KMnO4²
ozonolysisor
R CH CH R' RCOOH R'COOH+
KMnO4
²
R C C R' RCOOH R'COOH+
KMnO4²
ozonolysisor
Alkene Cleavage
Hot Potassium Permanganate
Alkyne Cleavage
Hot Potassium Permanganate
Ozonolysis
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Grignard Synthesis
Grignard reagent + CO2 yields a carboxylate salt. R
Mg
X
C
O
O
+
R C
O
OMgX H+
H2OR C
O
OH
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R C
O
G
where G = X, OR, NH2 , NHR, NR2 , & O C
O
R- - - - - -
H+
H2OR C
O
OH +HG
R C
O
G H2OR C
O
O-OH-+G-
RC NH2O
R C
O
NH2 R C
O
OH R C
O
O-( )H+ OH-or
Hydrolysis of acid derivatives
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Hydrolysis of Nitriles
BrNaCN
CNH
+
H2O
COOH
Basic or acidic hydrolysis of a nitrile produces a carboxylic acid.
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C
H
OH
CH3 RX2
OH-/H 2ORCOO- +HCX3
X2
OH-/H 2ORCOO- +HCX3CH3 C
O
R
Haloform Reaction
Cleavage of methyl carbinols
Cleavage of methyl carbonyls
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H C
O
C
O
H HIO4HCOOH2 +HIO3
HIO4C
O
C
O
HR RCOOH+HCOOH +HIO3
HIO4C
O
C
O
R R' RCOOH+R'COOH+HIO3
Periodic Acid Cleavage of Vicinal Dials/Diketones
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Reduction of Carboxylic Acids RCOOH
LiAlH4ether
H+
H2ORCH2OH
RCOOHB2H6
diglymeRCH2OH
(selective reduction ofcarboxyl group in preference to carbonylof aldehydes/ketones)
RCOOH
LiAlH4ether
H+
H2ORCH2OH
RCOOHB2H6
diglymeRCH2OH
(selective reduction ofcarboxyl group in preference to carbonylof aldehydes/ketones)
Lithium Aluminum Hydride reduction
Diborane reduction
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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.
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Fischer Esterification
COOH
+ CH3CH2OHH
+ COCH2CH3
O
+ HOH
Acid + alcohol yields ester + water.
Acid catalyzed for weak nucleophile.
All steps are reversible.
Reaction reaches equilibrium.
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Conversion to Acid Chlorides
C
O
OHC
O
C
O
Cl Cl+
C
O
Cl+ + +HCl CO CO2
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.
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Conversion to Amides
C
O
OH +C
O
O-
+NH3CH3
+
CH3NH2heat
C
O
NHCH3
H2O
Amine (base) removes a proton from the carboxylic acid to form a salt.
Heating the salt above 100C drives off steam and forms the amide.
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Reduction to 1 Alcohols
Use strong reducing agent, LiAlH4.
Borane, BH3 in THF, reduces carboxylic acid to alcohol, but does not reduce ketone.
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Decarboxylation of RCOOH C
COOH
COOH
² CCOOH
H+ CO2
CCOOH
C
O
² CH
C
O
+ CO2
Thermolysis of beta-diacids
Thermolysis of beta-keto acids
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Substitution in the hydrocarbon part
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.
Hell-Volhard-Zelinsky reaction
2 4 2X / P , H O2RCH COOH RCH(X)COOH
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Ring substitution in aromatic acids
—COOH group is deactivating and meta directing.Aromatic carboxylic acids do not undergo Friedel-Crafts reaction.
C O O H
H N O (conc.)
H S O (conc .)2 4
3
C O O H
N O 2
C O O H
B r /FeB r3
C O O H
2
B r
Nitration
Bromination
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Some commercially important carboxylic acids
Methanoic acid; (HCOOH)
N aO H + C O473 K, 10 atm
H C O O N aH +
H C O O H
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.
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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 CH3COOHCH3OH + 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.
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Thank you