Question

Which of the following is Crotonic acid?
A. ${\text{C}}{{\text{H}}_{\text{2}}} = {\text{CH}} - {\text{COOH}}$
B. ${{\text{C}}_6}{{\text{H}}_5} - {\text{CH}} = {\text{CH}} - {\text{COOH}}$
C. ${\text{C}}{{\text{H}}_3} - {\text{CH}} = {\text{CH}} - {\text{COOH}}$
D. ${{\text{C}}_6}{{\text{H}}_5} - {\text{COOH}}$

Answer 1

Hint: Crotonic acid is the trivial name of the acid derivative. The acid derivative has one carboxylic group and one double bond. We should know the IUPAC name of crotonic acid to determine its structure. We will write the name of each given compound to determine the structure of crotonic acid.

Complete step-by-step answer:
but$ - 2 - $enoic acid is known as crotonic acid. We will write the IUPAC name of all given compounds as follows:
-We will select the longest carbon chain.
-Then we will determine the substituents. We will arrange the substituents in alphabetical order. After that we will specify the position of unsaturation.

The first given compound is as follows:
${\text{C}}{{\text{H}}_{\text{2}}} = {\text{CH}} - {\text{COOH}}$
The above compound has three carbon in the longest chain. One double bond at carbon-I so, the name of the compound is propionic acid. So, it is not crotonic acid.

The second given compound is as follows:
${{\text{C}}_6}{{\text{H}}_5} - {\text{CH}} = {\text{CH}} - {\text{COOH}}$
The above compound has three carbon in the longest chain. It has a double bond and phenyl group at carbon-I so, the name of the compound is $1 - $ phenyl propionic acid. So, it is not crotonic acid.

The third given compound is as follows:
${\text{C}}{{\text{H}}_3} - {\text{CH}} = {\text{CH}} - {\text{COOH}}$
The above compound has four carbon in the longest chain. It has a double bond and methyl group at carbon-II, so the name of the compound is but $ - 2 - $enoic acid. So, it is the crotonic acid.

The fourth given compound is as follows:
${{\text{C}}_6}{{\text{H}}_5} - {\text{COOH}}$
The above compound benzene ring and carboxylic group so, the name of the compound is benzoic acid. So, it is not crotonic acid.
So, ${\text{C}}{{\text{H}}_3} - {\text{CH}} = {\text{CH}} - {\text{COOH}}$ is the Crotonic acid.

Therefore, option (C) ${\text{C}}{{\text{H}}_3} - {\text{CH}} = {\text{CH}} - {\text{COOH}}$ is correct.

Note: Crotonic acid is a trivial name but$ - 2 - $enoic acid is an IUPAC name. There is no rule for the trivial naming. We have to remember the trivial name of the compound. Generally trivial names are related to the source of that compound. The trivial name of the compound ${{\text{C}}_6}{{\text{H}}_5} - {\text{CH}} = {\text{CH}} - {\text{COOH}}$, whose IUPAC name is $1 - $ phenyl propionic acid, is cinnamic acid.