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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 acidsconsists of a C=O with -OH bonded to thesame carbon.
Carboxyl group is usually written -COOH.
Aliphatic acids have an alkyl groupbonded to -COOH.
Aromatic acids have an aryl group.
Fatty acids are long-chain aliphatic acids.
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Common Names
CH 3CH 2CHCCl
OHO
-chlorobutyric acid
CH 3CH 2CH 2CHCH 2COOH
Ph
-phenylcaproic acid
=>
Many aliphatic acids have historicalnames.
Positions of substituents on the chainare labeled with Greek letters.
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Naming Cyclic Acids
COOH
CH(CH 3)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 theend closest to a substituent.
Two carboxyl groups on a benzene ringindicate a phthalic acid.
HOOCCH 2CHCH 2CH 2COOH
Br
3-bromohexanedioic acid-bromoadipic acid
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Structure of Carboxyl
Carbon is sp 2 hybridized.
Bond angles are close to 120 .
O-H eclipsed with C=O, to get overlap of orbitalwith orbital of lone pair on oxygen.
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Boiling Points
Acetic acid, b.p. 118 C
Higher boiling points than similar alcohols,due to dimer formation.
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Melting Points
Aliphatic acids with more than 8 carbons are solidsat room temperature.
Double bonds (especially cis) lower the meltingpoint. Note these 18-C acids:
Stearic acid (saturated): 72 C
Oleic acid (one cis double bond): 16 CLinoleic acid (two cis double bonds): -5 C
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Solubility
Water solubility decreases with the length ofthe 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
OCH 3
COOH COOH
NO 2
COOH
NO 2
COOH
NO 2
p -methoxy benzoic acid m -nitro p -nitro o -nitro
p K a = 4.46 p K a = 4.19 p K a = 3.47 p K a = 3.41 p K a = 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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Oxidative Cleavage Reactions
R CH CH R' RCOOH R'COOH+KMnO 4
R C C R' RCOOH R'COOH+
KMnO 4
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 CO
OMgX H+
H 2OR C
OOH
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R CO
G
where G = X, OR, NH 2 , NHR, NR 2 , & O C
O
R - - - - - -
H+
H 2OR C
O
OH + HG
R C
O
G H 2OR C
O
O -OH-
+ G -
RC N H 2O R CO
NH 2 R CO
OHR C
O
O -( )H + OH -or
Hydrolysis of acid derivatives
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Hydrolysis of Nitriles
BrNaCN
CNH
+
H 2 O
COOH
Basic or acidic hydrolysis of a nitrile produces a carboxylic acid.
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C
H
OH
CH 3 R X
2OH -/H 2O
RCOO - + HCX 3
X2
OH -/H 2ORCOO - + HCX 3CH 3 C
O
R
Haloform Reaction
Cleavage of methyl carbinols
Cleavage of methyl carbonyls
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H C
O
C
O
HHIO 4
HCOOH2 + HIO 3
HIO 4C
O
C
O
HR RCOOH + HCOOH + HIO 3
HIO 4C
O
C
O
R R' RCOOH + R'COOH + HIO 3
Periodic Acid Cleavage ofVicinal Dials/Diketones
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Reduction of Carboxylic Acids
RCOOHLiAlH
4ether H +
H 2O RCH2OH
RCOOHB 2H 6
diglymeRCH 2OH
(selective reduction of carboxyl 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 theclass of compound:
-OH, carboxylic acid
-Cl, acid chloride
- OR, ester
-NH 2 , amide
These interconvert via nucleophilic acyl substitution.
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Fischer Esterification
COOH
+ CH 3 CH 2 OHH
+ COCH 2 CH 3
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 acylsubstitution easily.
To synthesize acid chlorides use thionyl chloride or oxalylchloride with the acid.
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Conversion to Amides
C
O
OH +C
O
O-
+NH 3 CH 3
+
CH 3 NH 2heat
C
O
NHCH 3
H 2 O
Amine (base) removes a proton from thecarboxylic acid to form a salt.Heating the salt above 100 C drives offsteam and forms the amide.
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Reduction to 1 Alcohols
Use strong reducing agent, LiAlH 4 .
Borane, BH 3 in THF, reduces carboxylic acid toalcohol, but does not reduce ketone.
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Decarboxylation of RCOOH
CCOOH
COOH
C
COOH
H+ CO 2
C
COOH
C
O
C
H
C
O
+ CO 2
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 ontreatment with chlorine or bromine in the presence of smallamount of red phosphorus to give -halocarboxylic acids.
Hell-Volhard-Zelinsky reaction
2 4 2X / P , H O
2RCH 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.
COOH
HNO (conc.)
H SO (conc.)2 4
3
COOH
NO 2
COOH
Br /FeBr 3
COOH
2
Br
Nitration
Bromination
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Some commercially importantcarboxylic acids
Methanoic acid; (HCOOH)
NaOH + CO473 K, 10 atm
HCOONaH+
HCOOH
It is colourles, pungent smelling liquid.It is powerful reducing agent. It reduces Tollens reagentand Fehling solution.Used in rubber, textile, dyeing, leather and electroplating
industries.
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Ethanoic acid(acitic acid, CH 3 COOH)
Main constituent of vinegar and is obtained by fermentationof molasses in presence of air.
Industrially, it is obtained in pure form by oxidation of ethanalwith air in the presence of cobalt acetate catalyst or bycarbonylation of methanol in the presence of rhodium catalyst.
CH 3 CHO + CO CH 3COOHCH 3 OH + CO CH 3COOH
Colourless liquid with pungent odour.Freezes at 289 K forming ice like crystal.Water free acetic acid, obtained by melting of the crystals is calledglacial acetic acid .
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