Classification of Fatty Acids

A carboxylic acid with a long, unbranched, saturated or unsaturated aliphatic tail (or chain) is called a fatty acid. Natural fatty acids found abundantly carry carbon atoms in even number because they are biosynthesised using acetyl-CoA (a co-enzyme having a two carbon atom group).

Hydrolysis of the ester linkages in fat or biological oil (both are triglycerides) removes glycerol, thus yielding fatty acids.

Fatty acids are classified on the basis of the following two parameters:

On the Basis of Saturation: Fatty acids are categorised into:

Unsaturated Fatty Acids: These fatty acids are of similar form; but one or more alkenyl functional groups are present along the chain, with each alkene replacing a single-bonded “- $CH 2 -CH 2 -$ ” part of the chain with a double-bonded “- $CH=CH-$ ” part (i.e., a carbon double-bonded to another carbon). The two next carbon atoms bound to either side of the double bond in the chain can occur either in cis or trans configuration:

Cis: When adjacent hydrogen atoms lie on the same side of the double bond, a cis configuration is attained. The double bond rigidity freezes its conformation, and in case of cis isomer it bends the chain and restricts the conformational freedom of the fatty acid. More the number of double bonds in the chain in cis configuration, lesser is its flexibility.

Trans: When next two hydrogen atoms are bound to opposite sides of the double bond, a trans configuration is attained. As a result, they do not bend the chain much, and they are similar to straight saturated fatty acids in their shape.

Some common examples of unsaturated fatty acids are:

Common Name	Chemical Structure
Myristoleic acid	C $H 3$ (C $H 2$ ) $3$ CH=CH(C $H 2$ ) $7$ COOH
Palmitoleic acid	C $H 3$ (C $H 2$ ) $5$ CH=CH(C $H 2$ ) $7$ COOH
Sapienic acid	C $H 3$ (C $H 2$ ) $8$ CH=CH(C $H 2$ ) $4$ COOH
Oleic acid	C $H 3$ (C $H 2$ ) $7$ CH=CH(C $H 2$ ) $7$ COOH
Linoleic acid	C $H 3$ (C $H 2$ ) $4$ CH=CHC $H 2$ CH=CH(C $H 2$ ) $H 7$ COOH
α-Linolenic acid	C $H 3$ C $H 2$ CH=CHC $H 2$ CH=CHC $H 2$ CH=CH(C $H 2$ ) $7$ COOH
Arachidonic acid	C $H 3$ (C $H 2$ ) $4$ CH=CHC $H 2$ CH=CHC $H 2$ CH=CHC $H 2$ CH=CH(C $H 2$ ) $3$ COOH
Eicosapentaenoic acid	C $H 3$ C $H 2$ CH=CHC $H 2$ CH=CHC $H 2$ CH=CHC $H 2$ CH=CHC $H 2$ CH=CH(C $H 2$ ) $3$ COOH
Erucic acid	CH $3$ (CH $2$ ) $7$ CH=CH(C $H 2$ ) $11$ COOH

Saturated Fatty Acids: These fatty acids are long chain carboxylic acids having 12-24 carbon atoms carrying no double bonds. Thus, these fatty acids are saturated with hydrogen as the double bonds reduce the number of hydrogen atoms on each carbon.

Some common examples of saturated fatty acids are:

Lauric acid (12 C),
Myristic acid $(14C)$ ,
Palmitic acid (16 C).
Stearic acid ( $18C$ ), and
Arachidic acid (20 C).

On the Basis of Carbon Chain Length: Fatty acids are categorised into:

Short-Chain Fatty Acids (SCFA): These fatty acids have aliphatic tails carrying fewer than 6 carbons.
Medium-Chain Fatty Acids (MCFA): These fatty acids have aliphatic tails carrying 6-12 carbons, which can form medium-chain triglycerides.
Long-Chain Fatty Acids (LCFA): These fatty acids have aliphatic tails longer than 12 carbons.
Very Long-Chain Fatty Acids (VLCFA): These fatty acids have aliphatic tails longer than 22 carbons.

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