Chemistry

Carboxylic Acids

Session - 2

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

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.

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.

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.

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

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

Acetic acid, b.p. 118C

Higher boiling points than similar alcohols, due to dimer formation.

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

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

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

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

Carboxylic Acids via Oxidation

From Primary AlcoholsFrom AldehydesFrom Substituted Aromatics

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

Grignard Synthesis

Grignard reagent + CO2 yields a carboxylate salt. R

Mg

X

C

O

O

+

R C

O

OMgX H+

H2OR C

O

OH

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

Hydrolysis of Nitriles

BrNaCN

CNH

+

H2O

COOH

Basic or acidic hydrolysis of a nitrile produces a carboxylic acid.

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

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

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

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

COOH

+ CH3CH2OHH

+ COCH2CH3

O

+ HOH

Acid + alcohol yields ester + water.

Acid catalyzed for weak nucleophile.

All steps are reversible.

Reaction reaches equilibrium.

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.

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.

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 C

COOH

COOH

² CCOOH

H+ CO2

CCOOH

C

O

² CH

C

O

+ CO2

Thermolysis of beta-diacids

Thermolysis of beta-keto acids

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

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

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.

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.

Thank you