A carboxylic acid is called a fatty acid when it has a long hydrocarbon chain attached to the carboxyl group (-COOH).

where R is a long hydrocarbon chain, usually containing 4 to 36 carbon atoms (most common are 12–22 carbons).

Examples

Compound	Formula	Fatty Acid?	Reason
Formic acid	HCOOH	❌ No	No hydrocarbon chain
Acetic acid	CH₃COOH	❌ No	Only 2 carbons
Propionic acid	CH₃CH₂COOH	❌ No	Too short
Butyric acid	CH₃(CH₂)₂COOH	✔️ Yes (short-chain fatty acid)	4 carbons
Palmitic acid	C₁₅H₃₁COOH	✔️ Yes	16 carbons
Stearic acid	C₁₇H₃₅COOH	✔️ Yes	18 carbons
Oleic acid	C₁₇H₃₃COOH	✔️ Yes	18 carbons, one double bond

Classification of fatty acids

• Short-chain: 4–6 carbons
• Medium-chain: 8–12 carbons
• Long-chain: 14–20 carbons
• Very long-chain: 22 or more carbons

Saturated vs Unsaturated

Saturated fatty acid (no C=C bond)

• Palmitic acid (C₁₆:0)
• Stearic acid (C₁₈:0)

Unsaturated fatty acid (one or more C=C bonds)

• Oleic acid (C₁₈:1)
• Linoleic acid (C₁₈:2)
• Linolenic acid (C₁₈:3)

NEET/JEE Definition

Fatty acids are aliphatic monocarboxylic acids containing a long hydrocarbon chain. They may be saturated or unsaturated and are the building blocks of fats and oils (triglycerides).

Important Note

There is no strict carbon-number cutoff in chemistry, but in biology and biochemistry, carboxylic acids with long hydrocarbon chains (typically C₄ and above, especially C₁₂–C₂₂) are referred to as fatty acids. Acetic acid and propionic acid are carboxylic acids, but they are generally not considered fatty acids in the context of fats and oils.