Question

Which of the following is not a fatty acid [CPMT $1978$]
A.Stearic acid
B.Palmitic acid
C.Oleic acid
D.Phenyl acetic acid

Answer 1

Hint: Fatty acids are very essential to human body systems as they are building blocks of the fat in our body. These compounds consist of carboxylic acids with long aliphatic chains with saturated or unsaturated bonds. The number of carbon atoms in most fatty acids may vary from $2$to $30$.

Complete answer: In lipids, fatty acids are an important component found in animals, plants, and microorganisms. Fatty acids consist of a straight chain of an even number of carbon atoms or a straight aliphatic chain and a carboxyl group at one end of the compound. Few fatty acids have branched chains and ring structures.

If in a fatty acid all the carbon-carbon bonds are single then it is called saturated fatty acid such as stearic acid, palmitic acid, lauric acid, etc. When any of the bonds is double or triple then it is called unsaturated fatty acid. For example oleic acid, linolenic acid, arachidonic acid, etc.

Therefore from the options, we can see that stearic acid, palmitic acid, and oleic acid are examples of fatty acids. Stearic acid and palmitic acid are saturated fatty acids with $16$and 18 carbon atoms while oleic acid is an unsaturated fatty acid with $18$carbon atoms. Their structures are shown below:



Phenylacetic acid is an organic compound that has a phenyl group and a carboxylic acid group. But it is not a fatty acid.



Thus, option (D) is correct.

Note: Generally fatty acids are not found in a free state in nature, They always exist in a combination of alcohol called glycerol in the form of triglyceride. When glucose is not available in our body for energy, the fatty acids are used to provide fuel to our cell organisms.