CHAPTER

6 Carborylic Acid
1 .IntrodUction

ch
compounds which contain a functional group COOH. are calle carbenylie eids Among he
which
nic amples arefomic acd COOH. which occurs ants, and acetic acid H,
in
p l e s t a a r its sour taste. >ome nore examples include a benzoic acid or a cyclopropane c a r t o n y i
COOH.

givesvinegarits
s h o w n b e l o w

OH OH
Benzoic acid Cyclopropane carboxylic acid
acids a r e
Bronsted-Lowry
Bronsted acids because they are proton (H") donors. They are the most common
Carboxytic
f organic acid. The wordcarboxyl is a contraction of the words carbonyi(C=O) and hydroxyi (O
e in the carboxylic group, both the (C=0) and (OH) group are combined as shown below
becau

-OH OH
Carbonyl Hydroxyl Carboxyl
The may contain one or
compound more carboxyl (-COOH) groups. Compounds
with two or more
cartboxyl groups are called dicarboxylic, tricarboxylic acid etc. The simplest dicarboxylic acid is oxaiic acid
coOH, which contains just two connected carboxyl groups together. Mellitic acid is an exampie of a
hexacarboxylic acid. Other important natural examples are citric acid (in lemons) and tartaric acid (in
tamarinds). These are shown below:

COOH
COOH O OH
HOOC OH O OH
HO
COOH o OH OH
HOOC OH
COOH
OH
Mellitic acid Citric acid Tartaric acid
Slts and esters of carboxylic acids are called carboxylates. When a carboxyl group is deprotonated, its
onugate base forms a carboxylate anion. Carboxylate anions are resonance stabilized and these
aroylate anions are more stable than the corresponding carboxylic acid. This increased stability makes
Grlic acids highly willing to lose proton and makes them more acidic than alcohols. Under certain
stances these carboxylic acids can be decarboxylated to yield carbon dixide.
 /0-272 Organic Chemistry-l|
2 .Nomenclature
it is important to know that for carboxylic acids, the
common names are arely used
IUPAC-recommer
acids are
nended names follow a pattern. The word root is followed by oic acid suffix. Broad The
known as alkanoic acids. For
example, butyric acid (C3H,CO,H) is termed as butanoic thes
according to lUPAC guidelines. The carboxylate anion R C00 is usually named with the
suffix-at
ethanoic
of the
acid, becomes ethanoate ion. In the
following table, both the common and lOPAC names of soso
carboxylic acids are listed. some
Table 1: IUPAC and common names of straight-chained, saturated carboxylic acids
Carbon Common name IUPAC name Chemical formula Common location or
atoms
use
1 Formic acid Methanoic acid HCOOH Insect stings
Acetic acid Ethanoic acid CH3COOH Vinegar
3 Propionic acid Propanoic acid Preservative for stored
CHCHCOOH
grains
Butyric acid Butanoic acid CHCH2hCOOH Rancid butter
Valeric acid Pentanoic acid CH(CH}3COOH Valerian
6 Caproic acid Hexanoic acid I1,(1,),00OH Goat fat

1 Enanthic acid Heptanoic acid CHy(CE2)5C0OH

Caprylic acid Octanoic acid CH,Cl,COOH Coconuts and breast milk

Pelargonic acid Nonanoic acid CH(CHrCOOH Pelargonium

10 Capric acid Decanoic acid CH3(CH2 COOH
11 Undecylic acid Undecanoic acid CH(CH2yC0OH
12 Lauric acid Dodecanoic acid CH CH2 oCOOH Coconut oil and hand
wash soaps

13 Tridecylic acid Tridecanoic acid CH(CH2COOH

14 Myristic acid Tetradecanoic acid Nutmeg
CH(CH2)C0OH
15 Pentadecyclic acid Pentadecanoic acid CH,CH23COOH
16 Palmitic acid Hexadecanoic acid Palm oil
CH CH2)4C9OH
Margaric a id Heptadecanoic acid CHCH h5COOH
Stearic acid Octadecanoic acid CH3CH 6COOH Chocolate, waxes, soaps
and oils
19 Arachidic acid lcosanoic acid
CH(CH2)COOH Peanut oil
 Carboxylic Acid o275

3 . Structure

fraction studies of carboxylic acids have shown tiat in carboxylic group carbon and oxygen atom
Electron o

one plane li. e. planar). This shows that carboxylic carbon and both the oxygen atorn are sp hybridised
orbitals of carboxylic carbon two overlap with sp hybridised orbital of each
of the three sp hybridised
out o

ugen and
third overlap with sp" hybrid orbital of alkyl group making a (sigrna bonds) An sp hybridised
oxyge

Each of
cbital of one of the two oxygen
orbi
overlaps with s orbital of a hydrogen atorm to form a a bond.
atoms

atoms and the carboxylic carbon have one porbital lying prependicular to
the a bonding
the two oxygen
delocalized between
these three unused orbitals overlap so that the resulting n bond is partly
keleton. All
on one side and the carbon and the second oxygen on the
other side as shown in fig
carbon and one oxygen A) than the
explains why : (1) the C - O single bond of carboxylic group is shorter (1.36
This delocalization carboxylic
bond (1.43 A) in alcohols and ethers ; and (2) the C = O double bord in

normal CO single and aldehydes

qroup is slightly longer
(1.23 A) than the normal C=O double bond (1.20 A) in ketones

Unused
P-orbitals
Sp2

R sp R
sp OD or

- H

sp
(a)

or R-
R-C

H
-H

b)
electron cloud
Fig. 1. (a)G skeleton of the carboxylic acid molecule (bi Delocallised n
 0274

H I 43A 123A
Organic Chemitr
OH R 0
12A
R
1.36A
C Lo
O-H
Alcohol
Carboxylic acid
Ketone
4. Physical Properties of Carboxylic Acids
Odour: Carboxylic acids often have strong odour,
especially the volatile
are acetic acid and butanoic acid
(rancid butter). Acetic acid is the chief derivati,
butanoic acid is the main component of rancid componers
ent
tend to have
butter. On the other
hand, esters oh
pleasant odors and many are used in perfumes.
Boiling point: Carboxylic acid have quite high boiling points due to
the presence of
intermolecular hydrogen bonding which results in the R- H-
formation of dimeric structures.
Moreover, O-H bond in
0-H-
carboxylic acids is more polar than O-- H bond in alcoholk
electron withdrawing effect of carbonyl
group on O-H Henc, H-bonds in This s
relatively stronger than those in alcohols. carbore
3 Melting points: In first ten memb?rs
of the homologous series, the alternation effect is ches
alternation effect implies that the melting point of an acid with even number of
carbon atr
than the acid with odd number of carbon atoms above and it.
be low However, no such s'shi
observed in homologous with more than ten carbons. The alterna ion effect can be ee
explained
basis of the fact that in the carboxylic acids with even number of carbon atoms, the
temina
group and carboxylic group are on the oppos.te side. of zig-zag carbon chain. Hence. they
the crystal lattice and it results in stronger intermolecular forces. On the other hand.
acids
number of C atoms have carboxyl and terminal methyl groups on the same side
ofig2s
chain. Therefore, such molecules being relatively unsymmetrical, fit poorly atica
in the crystal
causes weaker intermolecular forces and accounts for the relative y lower melting points

CH2 COOH CH2 CH

CH, CH2 CH CH COOH
Even number of C-atoms, fit Odd number of C-atoms, fit poorly.
better in crystal lattice, have in crystal lattice have lower m.
higher m. pts. (Terminal groups Dts. (Terminal groups are on
are on opposite side) same side)

The melting and boiling points of aromatic acids are usually higher than those of aliphati: aci
comparable molecular masses. This is presumably due to the fact that planar benzene ring in these 2d

can pack closely in the crystal lattice than zig-zag aliphatic acids.
 Carboxylic Acid o275

5 .R e a c t i

Reaction
Due to Acidic Hydrogen Atom
5. Acids
o f Carboxylic
Acidity

cids are typically weak acids, meaning that they only partially dissociate into H cations and
carboxvlh
ons in aqueous solution. For example, at room temperature, in a 1 M (molar) sohution of acetic

a4% ofthe acid molecules dissociated. Strength of acid its acidity is proportional
rd,only0 , 4 g
are an or to the

owith which it loses a proton leaving behind the anion. Acid strength chiefly determined is by the

tve estability of the

ference in stabilit
and its anion. The
acid carboxylic acid and the carboxylate ion are each

two resonance structures

and are therefore, resonance hybrids.
nted by
presen

C
R-C R-C H +|R-C, R-C
OH OH|

(1) (I1) (III)

Carboxylic acid Carbo ylate acid

resonance, stablisation is far greater for

the anion
and carboxylate anion are stabilised by
ah bothacid to two equivalent structures contributing
to the carboxylate anion. So. as the
This is due
Jr the acid. acid
resonance stablisation of the anion, carboxylic
is more stable due to the powerful
haate anion
arboxy1
acid.
like a n
ehaves

6 .Acid Strength

of Substitutents on Acidity
Effect more than it stablizes
the acid.
it is clear that any factor that stablizes the anion
above discussion, the
o m the
should decrease the acidity of
rom and factor that makes the anion less stable
increase the acidity
any
hnuld the negative charge and
substituent stablizes the anion by dispering
acid. An electron withdrawing
arboxylic substituents intensify the negative
the acidity. On the other hand, electron-releasing
herefore, increases decrease the
anion and therefore
in decrease of stability of the carboxylate
harge on the anion resulting
acid.
cidity of the
R--C
W--C
substituent
substituent An electron releasing
An electron withdrawing R release electrons
and
electrons and
stabilizes
W withdraws the arnion
the anion to
is definitely important
along with their pKa values. It
the following table,
carboxylic acids are given
some
value.
n m o r e is the Ka value and smaller is the pKa
Tow that stronger the acid,
 0276
Organie Che
Table 2: Diferent earbonylie acids and their aceid strengths
id stren
hemita
Carboxylle acid pK
Fomic acik (HCO0 375
Acetic acid (CHC0O) 4 76

Chloroacetic acid (CH,CICO,1D 2 86

Dichlornacetic acid (CHCICO,1) 129

Trichloroacetic acid (CCI,C0,1) 065
Trifluoroacetic acid (CF^CO,H) 0.5

Oxalic acid (HO,CCO,H 127

Benzoic acid (C%H5cO21) 42

The effect of various substituents on the strength of acids has been furtt er ilustrated with the
the help of
examples.
The effect of number of the substituents is illustrated by the chloro sutistituted acetic acids
1. The acd stne
increases from chloroacetic acid to trichloroacetic acid.
CICHCOOH<ClCHCOOH< Cl3CCOOH
be visuilised as,
The impact of Cl-atoms as an e withdrawing group can

CI

< Cl -C-«-C
Cl-CH2 < Cl--CH-«-C
O o
CI CI

As observed, increase in the number of chloro substituents on a-carbon atom of acetic acid ma
electron withdrawing effect more pronounced and increases the stability of correspondina co
makes th
base - the carboxylate ion. Similarly, the impact of e releasing roups can be visualised as,
onjugam

R -CH2 -Co0> R->-CH-»-cod> k »-Co0

As observed, more is the number of e releasing group, more is the de-stabilisation of anion. This reuah
into reluctance to lose proton by the conjugate acid, so acid strer gth decreases.
 Acid

0277
o f the
s
sbstituent is ilkustrated by the
ure

various halo
alo scetic
scetic acids
aids Their
Ther streng
ICH,COOH
<BrCH;COOH CICHcoOH FCH,COOH
ne
electro ne gative, its stabilising the anion
gative
most ability is the most, hercemost
most acidic
te substituent
t tne
osition of
position is
ilustrated by the
- c h i o r o
and p-chilo o propanoic acids CHCHOOH CHEHCOOH
CHCHCOOH CHCHA >

a a
uent de creases a its distance
from the -coOH
nB-chlo
efect in
groupincreases.
ine
8-chloro ropanoic acd is less pronounced because-Clgroup
Thus. electron
is relativey aua
Thus. 0-chloropropanoic acid stronger acid than
Thus. 0-chlo is
OOH
group.

Substitutec Benzoic Acids

3-chioropropanoc
Srengtho f
Su
ithdrawing ups like -CL- N0 increase the acid strength whereas electron

s
such as NH etc..
decreases the acid strength.
COOH
COO

w W
Wis elec tron withdrawing group stabilised anion

CCOH COO

R is electron releasing group destabilised anion

ample ptoluic acid is veaker acid than benzoic acid while pnitrobenzoic acid is stronger acid
an benzoic acid.

COOH CCOH COOH

CH3 NO2
Acid strength INCREASES

he efect is more pronounce d at para position as compared to that at meta position. For example,
 Organic Che

The e
withcrawig «
song acd

NO
A stmth INUREASES
6.3 Ortho Eftect
The ortho
substituted
electron uithdrawing)isomer of every sub
ubsntutad benaoi acid (uhether the sushtaent s
Ortho Effect.
is
relativel stronger than the aNresnniing na ani meta

nene is pushod out o nlane

Itis believed to be due
to the joint application of inductive and sten tactoS
the The
carbonulate forves the catnylate to bendsteN
substituent and the hinderane bete
neighbounng
plane of the aromatie ring. This out a
confommational shift inhibits the deloralisaton f the charepushed ot
nng. making it easier for the negative change to be delacaliaedi into the bene
Non-pardcipation of benzene into conjugation with C- knoan thraughout the sistem ot cabanydae an
is as sterc inhibton to
resonane SR
7. Methods of
Preparation of Carboxylic Acid
There are a number of methods of preparation of carboxylic acids are availabk Some of the
promie
methods are discussed here
7.1 By Oxidation of
Primary Alcohols
Carboxylic acids can be easily prepared by oxidation of primary alcohols with acxdic
KCrO AMn), or a

MnO H
RCHOH (O) RCHO.H0

RCOOH
For example, ethanol is undergoing the above sequenoe of reactons yielding ethanoic acid eentual
 boyieA
Acid
cid

0279

CH-CH20H + [0] KMnO4/H*

KCrO7/H
KCrO7/H* CHg-CHO CIl-C0OH
alcoholis
zyl
onverted into
benzoic acid.
Irohol
KMnO4/H
0/H
CHs CHOH+ [O] Cr CgHs-CHO CH5-COO
K2Co07/H
d a t i o no fAldehydes

Oridotio
Oxdation ive carbo:ylic acids with same number of carbon atomss as
g i u

a in the aldehyde. The

o n
Tollen's ragent or a
KMnO4 in acidic medium.
des

Tollen's Reagent
R-CHO+ [0].
R-COOH
a of aa ben
of aldehyde into benzoic acid
conversion or ethanol into ethanoic acid.
e i s
t h

hee

Tollen's Rezgent
CHCHO [0)
Tollen's Reagent
CgH5-CoOH
CH,CHO10) » CH-C0OH

J t h eH
drolysis
y d r
of Followi sg Compounds
the ertain
a0ounds like e lkyi cyarnides, esters, acid amides, acid chlorides and acid anhydrides.
compound

acids give carboxylic acids

dilute.

h
rolysis with
o nh y a

R-CN R-COOH NH
R-CONH 1 0 R-COOH NH
R-COC R-COOH HCI
(R-COO 1 0 2R-COOH
R-COOR 10 R-COOHRcOH

thanenitrie is unders Oing acid hydrolysis below yielding ethanoic acid

ircuarple

HO
CH,CN -
CH,COOH +
NH
H
is producing benzcic acid
Nonitrile
,

CN COOH

H
HO Ni4

Both the groups are

e olowng example. there
are two hydrolysable groups-CN and CONH2.
osed to carboxyic acids
 0-280
HO
Organic Chemitn
CH2-CH-CH,CONH2 H CH2-CH2-CH2COOH
CN COOH
In these
hydrolysis reactions, the rate of reaction is governed by the
substituted. Moreover; the conjugate bases of strong acid are good leaving group ability otc the
leaving groups. So, goup
acid chloride> order
anhydride > ester > amide.

Solved Examples

Example 1: The corect order of ease of of acid

(a) ester > amide > acid hydrolysis derivatives is:
chloride (b) amide > ester >
(c) amide> acid >
acid
chloride ester (d) acid chloride > ester >chlorida
Solution: During hydrolysis
leaving groups are (CI, alcohol and NH3 among which amide
hence it will be a
good leaving group. C is aueal
veakest bas
Ans:(d).
7.4 By Trihalo Alkanes
Trihalo alkanes on reaction with
aqueous KOH give salts of carboxylic acid which
carboxylic acid. on
acidification c
(i) KOH (aq)
R CX3 R-COOH
(ii) HT (Here X is ahalogen)
Reaction proceeds as

OH
R-CX3+ 3KOH (aq)
R-C-OH+3KX
oH
Unstable
 O280
CH CH
Org
CH COOH
anie
H CH, CONH2
COOH
N
reaction is
he leaving
govermed by the
n these hudrolysis reactions the rate of actd are good leaving
substit conkgate
bases of strong
Moreover the
cid chloride anhydride ester a

Solved Examples

of acid derivative.
of hydrolysis es is:
ENomple The c o r e c t order of e a s e
1: (b) amide >
(a) esteramide > acid chloride > ester
(c) acid chloride ester >
d)acid chloride > acidid chi
among which*sto,
amide
Cl, alcohol and NH, among.
Soution: During hydrolysis leavinggroups are

hence it will be a good leaving group

Ans. (d).
7.4 By Trihalo Alkanes
KOH give salts ot carboxylic acid
ihalo alkanes on reaction with aqueous nich on ac
carboxylic acid. (i) KOH (aq)
R - COOH
R CX3 (ii) HT Here is
Reaction proceeds as
OH

R-C-OH3KX
R-CX3+3KOH (aq)
Unstable

H
R-COOH
+KOHR-COOK
R-COOH

Some of the applications of the above sequence

of reaction is shown below. In the first examnia

derivative.
is obtained by the hydrolysis of the trichloro

COOH
CCl3
(i) KOH(aq) + KCI + H20
(ii) H
 o28

the similar heatment as shown in ths example The reac oms 1

H-COOH KU H

y Cnboxyvlotion Orignard Reagent

latien of dioxke a
carteon
wn beoblaned hy cartoxulatton of CGrignard reagents h this red
itionai
one
hsequent ackd huvinodsis gives rise to a carbewylie ackd uhich contaims
cheme
of the rnction is shown beow
he

tsD 0>0=(-0+MgK OH

MgOH
carboxic act
to snthesize
in which adove scheme is applied
ane shown
yehonë eNAMp

ygMgC)

COOR
MgCl

(i) H0

i)CO -c0OH

MgC
sxtium
and p r e s u r e ,
C a r b o n m o n o x i d e

and temperature

with CO at high
Alkoxide

acid.
Sodium alkoxide is treated carbonylie
16
From sodium rise to the
alcohol or
aciditication gives
salt of
sodium obtained.
The salt upon
Uhen
acid is RCOOH+Na
carboxylic 140°C

sat of CO
RCOONa
RCOONa
sxtium
nmethonide
is

RONa
reaction,
- 7 am
below. In this
shown
reaction is
of
sequence
above CHCOOH+ Nac?
of the HC
acid.
An example ethanoic
acetic or 140CC H C O O N a
onverted
into

C H ON a +CO
6-7a
atm same sheme
the
fomic
acid by
into
hydroxide
of
sodium HCOOH+ Nac1
c o n v e r s i o n

the
1 4 0 C

CO
iniguing is
HCOONa reaction,

More
But in this
nature.
H O ' N a +CO in
6-/ atn neutral

m o n o x i d e - C
is
O

produce
salt.
carbon to
(NaOH)
observe
that alkali
to with
t is too i nteresting

cause
it
c o m b i n e s

exhibits acidic nature

 o282
Organie Chem
7.7 From Alkene minta
arboxylik acid of hhgher molar masses can be obtained by heating an alkene with Co
A t 350°T in presence ofphosphoric acid. Phosphoric acid acts as catalyst CO and s
HgPO
R-CH CH
R-CHCH CO+H,0 350C
COOH
HPO4
CHCH=CH +OO H,0 - 350°C CH-CH CH
COOH
2 Methyl propanoic acid
7.8 From
Alky! Benzene
Alkyl groups are usually fairly resistant to oxidation. However, when they are at
attached
are
easily oxidised by an alkaline solution of
OXIdation of alkyl benzenes
potassium permanganate. So, benzoic acid car
with alkaline potassium permanganate tollowed by acid be
aberzeneprepared
ring, the
sulphuric acid. Only. primary and secondary alkyl groups directly attaeched to a benzene rina diication with
KMnO, to a-CO,H group. Tertiary alkyl group attached directly to benzenes ring does dilute
response to thbiys
oxidized
oxidised process. ring not

-CH-CH3
CH3
KMnO4/KOH
Heat
O-co O -co,H
isopropyl benzene

KMnO4/KOH
-CH,CH,CH,CH3 H0co -COOK
n-butyl benzene

HSO
COOK

CH3 COOH

KMnO4/KOH

Toluene
CH3 COOH

Ethyl benzene
AMnO4/OH
O)
 arboxylic Acid
o283

rom these examp amples. allylated benzene is converted to

benzoic acid respective of the k
ime
s k d echain

KMnO4 OH
OH

3 alkyl attached directly to benzene ring is not

H o w e
oxidisable.
KMnO4
No reaction

So. one
can conclude that presence of benzylic hydrogen is essential for the
ca

place.
to take
reaction
ortant to remember that these are flow schemes -notfullequations. And it is
t o h difficult whether one could write an accurate single equation for anything
r e complicated than a methyl group attached to the ring. In other cases apart
methyl group, one will certainly get some carbon dioxide produced at the benzylic Hydrogen
rbon atoms present in alkyl group. The purple colour of the
of other carb.
ense
potassium permang nganate(VII)
is replaced by a dark brown precipitat of manganese(TV) oxide
eventually
is the inorganic by product
which
Example 2: A hydrocarbon (A) of the formula CsHo. on oz2onolysis gives compound (B).

CHO2 only. The compound (B) can also be obtained from the alkyl bromide (C), C3H_Br
upon treatment
with magnesium in dry ether followed by carbondioxide and acidification.
ldentify (A). (B), (C) and also give equations for the reaction.
the products are carboxylic
Solution: Since, there is no loss of carbon content on ozonolysis and the fact that
reaction is,
acid, the compound (A) may be represented as C3H5C= CC3H5. The ozonolysis
O3
CgHsC=CCgH5 2CHCOOH
(A) (B)
The compound (B) may be obtained as follows
Mg CO2
C3HBr CgH5MgBr CzH,COOH
ether H
The species C3H5 must be a saturated alkyl group. The only possibility is the cycloproyl group. Hence.

A -c=C-A AA (A) (B)

COOH
(C)
Br
 Ovganie Chemi
8.Reactions of Carboxylit Aclds
hamitry
A mumber
of ail These van he
chemical eaf av eshibited by vatonylie ecategoviaed
Rea tiann tue to avde hyubugen ahm
ea on inobng repMavemnent o Otpu
eatione mvohng OMop
ewtons invohig alts of varbxylie achls
atho ioh ing alkyl gpn of eatonylie acht
hew caepvies of reacthons ave diseusseod below, one y t
8.1 Reactions
Due to Aidie Nydrogen Atem
Neactton with active metals Cabonylie aido react with active metals Na K, Zn ete
h e n as This behaviour ia shown by phewl and aliphalic alcohols as well and ihe
RCOOn Na RCOONa
Asa matter of fact, carboxylic acids are slronger ackd than phenol which in turn is stronge
algphatik aleohol than
Reaction with alkalies: Due to their strong acidie nature,carboxylic aclkd can react
with
aryta Ba(O1); or caustic soda Naclete yiekding comesponding salts

RCOONaO RCOO Na' +lLO

2R°OOU Ca) (RCOO),Ca )

HCOOl+ Ca) (CHOO)a +L)

CHCOOH+ Ba(OH), (CHOO),Ba + H,O
On a
comparative scale, phenol like carboxylic acids reacts with alkali but aliphatic alcohol
are
reluctant to react with alkalies.
3 Reaction with bicarbonates and carbonates: Carboxylic acids react with
bicarbonates and
carbonates and proxduce brisk effervesence dlue to liberation CO,. Both aliphatic alcohol
of and phenol d
not react with bicarbonates and
carbonates
HCO
RCOOH RCOO t CO + 1,0
Examples showing this behaviour are,
CHCOOH+ NatCOD CHCOONa +CO +HO
2CHCOOH+ CaCO1 (CHCOO),Ca +CO, +H,O
The reaction sequence is
R COOH+ NalHCOD »
R COO Na +HCO3

COIHO+ CO,
 /O 286 Organie Chei
d)Mg/ether
CU C'11
(i) CO, (Y)

> C1 CLC1 C=11,

(Allyl carboration)

O/1,0/7n
Br-CH,CHO+ CH,=0 CH=C1,
Br

8.2 Reactions Involving Replacement o f - 0H Group

Esterification: Carboxylic acids react with alcohols in the presence of conc. H,5o,

R- -OH + H-OR' R-COR'+Ho

>d into
An example ofthis sequence is shown in which ethanoic acid is converted into an
an ester
ester when
with ethanol. reacor
O O
H
CH-COH+H-OCH; CH-C-OC15 + 1,0

cture.
The relative reactivity of alcohols and acids is markedly dependent on their structure. The
The qrea greateris the
bulk of the substituents around the-OH group in alcohol or-COOH group in acid, the sloweru
be the reaction rate. So, for alcohols the ease of esterification decreases as,
CH,OH>CH3CHOH> (CH3)%CHOH > (CH3),COH

Similarly, for carboxylic acid the ease of esterification order is,

HCOOH>CH,COOH>(CH,>CHCOOH>(CH;)CCOOH
Carboxylic acids whose molecules have a hydroxyl group at y or ð carbon undergo an intramolecular
esterification to give cyclic esters known as y or ö lactones. The reaction as usual is acid catalyzed
O
HO7HO
R
O
H/H2O
OH
hydroxyacid anioon T-lactone

OH O
HO/H2O
R H/HO R
y-hydroxvacid anion
6-lactone
boyleAcid
Ester (A)+
o H + C1,OH Water (B)
(B)
are
n o n n da ,
a n d

(b)
(A)
e
(d) None
otharecorrect lacesOH
rep lace, group in ester formation
group

ethoxy
ta
As .forldes: Carboxylic
cid chlori
acld
can be converted into acid halides with the help
Some o
off
these reagents are shown below
p m a t i o no f S o m e

reagents.
+PCly
amen
of RCOCI + HCI+POC
P y

+PCly
RCOCI + HPO,

R - C O O H

+SOCl2
RCOCI+ SO2 + HC
CsHsN

Carboxyiic acids on treatment with any dehydrating agent POs

as

acid anhydrides:
of acid a n h y d r i d e s

Formation of water molecule from two molecules of the carboxylic acid.

by elimination
dride by
elin
a n h y d r i d e

id

R-C
2R-C-OH
P2Os +H20
A
R-C

of the above scheme are shown below.

Came of the examples
O

P205 CH-C
2CH-C-OH
A
O+H,0
CH-C

CHCOOH P0s CHCO

A
+H20
CH2CO
CH,COOH
Succinic acid Succinic anhydride
 0-288 Organic Chemintri
8.3 Reaction InvolvingCOON Group
Schmidt reaction: It is one of the important reactions shown by carboxylic acids. As.
of conc. H,SO, at 90°C to form nri a
acids react with hydrazoic acid in the presence
primary amine Th
reaction has an additional advantage that it is used tor stepping down because one raua

the acid is lost as carbon dioxide.

Conc. Hs4RNH2 +N2+C0
R COOH+NgH 90°C
Hvdrozoic acid

Mechanism:
OH
OH
R-C-OH RC-OH H-N-NEN R-CLOH
OH
HN-NSN

R-C-OH/ -H20

R-N
0H

H
R-NHLc=O> RNH,+CO
OH2

For example, acetic acid combines with hydrazoic acid to yield methanamine.

Conc. HSO4
CH3 COOH+ NgH- CH3 NH2 + Na + CO2
90°C

Similarly, benzoic acid can be transformed into aniline via Schmidt reaction,
COOH NH2

+NgH +N +C0,
2 Sodalime decarboxylation: Carboxylic acids on heating with sodalime give alkane with one carbo
less than the parent acids. Soda lime is a mixture of NaOH and CaO.
Sodalime
RCOOH R-H+ CO2
Over all reaction proceeds as:
CaO
RCOOH+ NaOH RCOONa+ H,O
 b o n y l i cA c Acid
Organie Chem
emistryy-IN decarboxylation
RCOONa +NaOt
acids.
As
such,amine
om primary R- OH
se one carbo
carbon atom
d2 +CO2 from 1 R-
les of the reaction are,
e x a m

the

HOOC COOH
COOH
HOOC
COOH uaieee
CoOH
Mellitic acid
or

Benzenehexacarboxylic acid
i gtNeSame
same
scheme
woaction, lactic acid
of rea

can be converted
=N into
ethanoic acid
CH3-CH- OH Sodalime
COOH CHCHO
Lactic acid
+CO2
Amdt-Eist nthesis Named after the
hosis is
German chemists Fritz
Eistert
a
popular method of
producing
Arndt and Bema Eistert
A m d t - 4

erally, the synthesis. osis allows the formation of Bamino acids from a-amino acids
a reaction is considered as a higher carboxylic acid homologue from
acid.
That is why the
r
homologation Tne cabosjic
process.
a

process alongwith reagents

is s h o w n
below,
required
2
R
SOC%R CHN
ether
R
A
OH dioxane OH
hiomologated carboxyli acids other derivatives can also be synthesized by reactiono the
Apartfrom of the
activate carboxylic acids with diazomethane. It is followed with Wolff-rearrangement
alcohols, or amines to give
the presence of nucleophiles such as water,
diazoketones in
intermediate
desired product.
with onecarbon rise to the
Arndt-Eistert
Synthesis acylated by an acid
carbon is

Wechanism of the homologation, the

diazomethane
by
one-carbon diazomethane can
be destoyed
of this
the first step The excess can be
stable and
kep 1: In
o-diazoketone.
are
an
to give stiring. Most
a-diazcketones

mixed anhydride, vigorous

idoride or acid or
acetic
amounts of
dflon of small

odted and purified by

column
chromatography.

4
 -y Organic Chem
290

1
R- :(1-N=N:- Ro

R R
N=N EN
Step 2: The key step of the Arndt-Eistert homologation is the Wolff-rearrangement of the diazoke

ketenes, which can be accomplished thermally over the range between r)om temperature and
ketones te
photochemically or by silver() catalysis. It is followed by treatment with nucleophiles such as water 150PC
carboxylic acids), ohols (to give esters) or amines (to give amides). These r ucleophiles capture tha yield
the ketene
intermediate and inhibit the competing formation of diketenes.

Na Ag (cat.) OH
nucleophile
Wolff
rearrangement
ROH
- OH

tautomerism

R
0-R N-R OH
H

The method is widely used now a days for the synthesis of B -amino acids. Peptides that contain -amino
acids feature a lower rate of metabolic degradation and are therefore of interest for pharmaceutcal
applications.
8.4 Reaction Involving Salts of Carboxylic Acids
1 Kolbe's electrolytic decarboxylation: The aqueous solution of sodium or potassium salt of

2RCOONa + HO
Electrolysis
- R-R+ 2CO% + 2 NaOH + H2

at anode at calthode

ca:boxylic acid on electrolysis gives alkane at anode. This reaction is called Kolbe's electrolysis
The sequence of the reactions at the two electrodes are shown here,

At anode

R 0
lons of electron is taking place in the first step. Subsequently R bond undergoes homolyti
cleavage.
 C r b o x y l i cA
Acci
id

Organie P
s o d i h u m
but
reac+

with Pz It CHC
with
id n acid
dification a

o292 does
3-methylbut-2-enoic
not
r e a c t
acid

C H O

(A) ?
compound
What
is (A)
2-methylpentane. nd thus (A)
(A) may be carb
ma

andthus
7:A gives
n e
and
PCIs
reection with

gorodine Hunsdi
butreacts
d i e c k s

haloform
genation (A)
gives
Na
not
reactwsth CHH-CO

unit give
a

i)A)
does shoukd have CCl4
it
thus.
of
S e b t i o m :

a c dn o a k o h o l )

ard

h a i o f o m
r e a c t i o n

methylpentane)
prsend R
u n d e r g e s

CH, CH-(2-
Al ab
the
CH.CH, ot

CH example
bromomethane,
A
CH formed
A n

is cid salt)
acid salt 2- enoic :
reaction.
methylbut-
haioform

DuningCH-C=CHCOONa (3- has a

reaction
the
Hu
The
in

CH (A) is
radical is pr
chanism:

Keeping
CH3C=C
in v i e w
H.CO
of a b o v e
facts
. C H , (4-methylbut-3-en-2-one)
carbanya
v d
through

radical
a l k y l
ib r o
Reactions ui a n

Na
N o reaction uce

C H 3 2 C = C H . C O C H 3

(A)
CH3hC=CH.COCH PC5 CHs)2C=CH.CCh CH
) (CH3)2C=HCOONa+
CH3hC= CHx
CHs2C=CH.COCH3
+X2 + NaOH -

(CH3 2 C=CH.COOH
S e p2 (Initiatic

acids: When ammonium salt of

salts of carboxylic
a

of am1nonium further heating viels

Dry distillation The amide on

acid is subjected to heat, decomposition

yields an amide.

R-COONH4 RCONH2 + H20

alone Acid am.de
Step3+4
P205.a
-2H20 P20O5 -H20
R-CN

Some of the examples are st.own here. In the first example ammonium acetate is convene
ethanenitrile. Similarly. in the second example, ammonium si ccinate yields succino nitrle
POs
CHCOONH4 CHCONH2- CHCN
A
 Or
scation
NYN,

n has a ertaih meh mism. R s disussed thekw

n te n t w e r wNtn, the siher
sall ofa
varbonyth nnt n heateed
witth bwowewave n e t

l
a l is }xt Na o sep NAess Subeuenit the vartoxl raleal n eowestot we

h hdeatlakein The resuiting alklradkal aburatha brwnne atm eanmROBe

a bromie
k a d e e ate a c a t l adival

Al

ypuobramite

reaction)
(initiation
salt of a Ser 2
camon
ing yields nimie R-
R-C

(Chain propagation)
e 34
RCO
R-C

s convere > R-Br +R-

nitrik
R
O-294

Decarth arboxylation of carboxylle acids: The reaction in which a cartoxyle

Organie Chemi
a decarboxylation reaction
i s
O
decarboxylation
RC-OH RH+(0,
Although the unusual stability of carbon dioxide means that decarbonylatiea
exothermic and hence, highly spontaneous. In practice, the reaction is not er
tion
emy to ca
the reacti is very slow. Special groups have to be attached in the molecule f
be rapid enough to be synthetically useful One such compound attached with.
etor derntn jat
B-keto acid. When the carboxylate ion decarboxylates, it forms a rexonance-stabil
as shown.

- CO2
R-C-CH, HH >R-C-CH
R-C-H2
Acyl acelate ion
B-keto acid

R-C=CH2
Resonance-stabilized anion

heated to 100-150°C. Some B-et.

-keto acids, decarboxylate readily when they
are
arid
decarboxylate slowly at room temperature.

O 100 - 150 °C
R-C-CH, +CO2
R-CCH2-C -OH
A B-keto acid
on heating is shown below. It does so thrnroug
Mechanism : The mechanism of decarboxylation
state:
six-membered cyclic transition
H

O
Keto-eno
-CO2
C=0
CHCH2
tautomerism

R
CH
R CH2 Enol
Ketone
B-Keto acid
anionic
intermediate. The enol
and avoids an
an enol directhy heati
This reaction produces readily upon
derivative decarboxylates
acid and its
ketone. Malonic
tautomerizes to a methyl

as shown here,