Fatty Acids Unbranched carboxylic acids with 12-20 carbons. Most contain an even number of carbons because they are built from acetic acid units. Melting points increase with increasing molecular weights. Unsaturation greatly lowers the melting point. Chapter 25 5Chapter 25 5 Fatty Acids ▪ Unbranched carboxylic acids with 12–20 carbons. ▪ Most contain an even number of carbons because they are built from acetic acid units. ▪ Melting points increase with increasing molecular weights. ▪ Unsaturation greatly lowers the melting point