IIT-JEE 2010 - Career Point

KEY CONCEPT
Organic
Chemistry
Fundamentals
CARBOXYLIC ACID
Acidity of carboxylic acids.
Fatty acids are weak acids as compared to inorganic
acids. The acidic character of fatty acids decreases
with increase in molecular weight. Formic acid is the
strongest of all fatty acids.
The acidic character of carboxylic acids is due to
resonance in the acidic group which imparts electron
deficiency (positive charge) on the oxygen atom of
the hydroxyl group (structure II).
O
R C O H
I
Non-equivalent structures
O –
R C
+
O H
II
(Resonance less important)
O –
R C O + H +
The positive charge (electron deficiency) on oxygen
atom causes a displacement of electron pair of the
O—H bond towards the oxygen atom with the result
the hydrogen atom of the O—H group is eliminated
as proton and a carboxylate ion is formed.
Once the carboxylate ion is formed, it is stabilised by
means of resonance.
O
O –
R C
R C
O – O
Resonating forms of carboxylate ion (Equivalent structures)
(Resonance more important)
R
C
O
O
Resonance hybrid of carboxylate ion
Due to equivalent resonating structures, resonance in
carboxylate anion is more important than in the
parent carboxylic acid. Hence carboxylate anion is
more stabilised than the acid itself and hence the
equilibrium of the ionisation of carboxylic acids to
the right hand side.
RCOOH RCOO – + H +
The existence of resonance in carboxylate ion is
supported by bond lengths. For example, in formic
acid, there is one C=O double bond (1.23 Å) and one
C—O single bond (1.36Å), while in sodium formate
both of the carbon-oxygen bond lengths are identical
–
(1.27Å) which is nearly intermediate between C O
and C—O bond length values. This proves resonance
in carboxylate anion.
H
C
O
Formic acid
OH
H
C
O
O
–
Sodium formate
Na +
It is important to note that although carboxylic acids
and alcohols both contain –OH group, the latter are
not acidic in nature. It is due to the absence of
resonance (factor responsible for acidic character of
–COOH) in both the alcohols as well as in their
corresponding ions (alkoxide ions).
R—O—H R—O – + H +
Alcohol
Alkoxide ion
(No resonance)
(No resonance)
Relative acidic character of carboxylic acids with
common species not having —COOH group.
RCOOH > Ar—OH > HOH > ROH >
HC CH > NH 3 > RH
Effect of Substituents on acidity.
The carboxylic acids are acidic in nature because of
stabilisation (i.e., dispersal of negative charge) of
carboxylate ion. So any factor which can enhance the
dispersal of negative charge of the carboxylate ion
will increase the acidity of the carboxylic acid and
vice versa. Thus electron-withdrawing substitutents
(like halogens, —NO 2 , —C 6 H 5 , etc.) would disperse
the negative charge and hence stabilise the
carboxylate ion and thus increase acidity of the
parent acid. On the other hand, electron-releasing
substituents would increase the negative charge,
destabilise the carboxylate ion and thus decrease
acidity of the parent acid.
X
C
The substituent X withdraws electrons, disperses negative
charge, stabilises the ion and hence increases acidity
–
O
Y C
O
The substituent Y releases electrons, intensifies negative
charge, destabilises the ion and hence decreases acidity
O
O
–
XtraEdge for IIT-JEE 34 DECEMBER 2009