In carboxylic acids, The carbon of the -COOH group is Sp2 hybridised and hence the carbon along with two oxygen atoms lie in one plane and are separated by about 120°. The orbital structure of -COOH is shown below:
group and QH group. The 1t pair of electrons of the C group are in conjugation with the lone pair of electrons on the -OH group. As a result of this, the -COOH group can be represented by different electronic structures, called canonical forms or mesomers.
This phenomenon in which a compound/group can be written in two or more than two structures is known as resonance or mesomerism.
Due to resonance, the electron density around oxygen of O- H group decreases due to displacement of electrons towards –C- group. The displacement of electrons is due to resonance is known as mesomeric effect or resonance effect. The phenomenon of resonance effect or mesomeric effects in carboxylic acid group helps to explain the acidic character of these compounds (see acidic nature).
The common or trivial names are derived from the source of individual acids. Formic acid, for example, was so named because it was first obtained by the distillation of ants (Latin fomica meaning ·ants). Similarly, acetic acid has derived its name from vinegar (Latin : acetum meaning vinegar) Butyric acid is present in butter fat (Latin : butyrum meaning butter). The positions of substituents present is specifted by Greek letters a, β, y, etc. The carbon atom next to the carboxyl carbon is considered as a-carbon.
1 In IUPAC system, the monocarboxylic acids are named as alkimoic acids. The name of the acid is derived by replacing the terminal ‘e-’ of the corresponding alkane with ‘-oic acid’.
1 Carboxyl carbon is always given number one in chain of carbon atoms.
The trivial and IUPAC names of some common alkanoic acids are given in Table 46.2 .
Table 46.2. The Trivial and IUPAC Names of Some Acids
NOMENCLATURE Of POLYCARBOXYLIC ACIDS
In order to write the name IUPAC names of polycarboxylic acids i.e;, when two or more carboxylic acids groups are present the following rules are followed:
(a) When Two COOH Groups an! Present. In such co1npound< the Carbon atom.’ of both the carboxylic acid group are to be counted in the chain. For example.
(b) When Three or More COOB Groups are Present. According to 1993 recommendations when three or more COOH groups are present group like ram rule is followed According to this rule the carbon at of the carboxylic group are not counted in the chain. Some examples are discussed as follow:
ISOMERISM OF ALKANOIC ACIDS
(i) Chain (nuclear isomerism):
(ii) Functional isomerism. CnH2NO2 represents saturated monocarboxylic acids. Este hydroxy carbonyl compounds and hydroxy oxiranes For example.
(iii) Optical isomerism. Carboxylic acids with one chiral atom can show optical isomerism.