64. carboxylic acids

8/11/2019 64. Carboxylic Acids

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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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Thank you

64. carboxylic acids

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