Stearic Acid (C₁₈H₃₆O₂) — Properties, Uses & Safety

Overview

Stearic acid (C18H36O2), systematically named octadecanoic acid, is a saturated fatty acid with a molar mass of 284.48 g/mol. It appears as a white, waxy solid at room temperature with a melting point of approximately 69 °C. The molecule consists of an 18-carbon straight chain with no double bonds, terminated by a carboxylic acid functional group (COOH). This long hydrocarbon chain makes stearic acid largely nonpolar and hydrophobic, while the carboxylic acid head is polar and hydrophilic — this amphiphilic nature is the key to its role in soap-making. Stearic acid is one of the most common saturated fatty acids found in nature, abundant in animal fats (beef tallow, lard) and some vegetable fats (cocoa butter, shea butter). When stearic acid reacts with a strong base like sodium hydroxide in a process called saponification (C17H35COOH + NaOH → C17H35COONa + H2O), it produces sodium stearate — one of the main components of bar soap. This reaction is a classic example of base-catalyzed ester hydrolysis and is one of the oldest known chemical processes. Beyond soap, stearic acid is used in candle-making (it hardens wax and raises the melting point), cosmetics, rubber manufacturing, and as a lubricant and release agent in pharmaceutical tablet production. Stearic acid has no significant GHS hazard classifications, making it one of the safest fatty acids to handle. For chemistry students, stearic acid is an excellent teaching molecule for carboxylic acid chemistry, fatty acid nomenclature, saturated versus unsaturated hydrocarbon chains, saponification reactions, micelle formation, and the concept of amphiphilic molecules. It connects organic chemistry to everyday products like soap and candles, making abstract concepts tangible and relatable.

IUPAC Name

octadecanoic acid

CAS Number

57-11-4

PubChem CID

5281

Molecular Formula

C₁₈H₃₆O₂

Molar Mass

284.477 g/mol

State at STP

solid

Appearance

White waxy solid