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. 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