Hydrocarbons

• aliphatic hydrocarbons, which consist of linear chains of carbon atoms;
• aromatic hydrocarbons, which consist of closed rings of carbon atoms.

Aliphatic Hydrocarbons

The fatty acids in fats are aliphatic hydrocarbons. If a chain holds all the hydrogen atoms it can, the molecule is said to be saturated. The fatty acids in tristearin are all saturated.

If two adjacent carbon atoms each lose a hydrogen atom, a double bond forms between them. Such a molecule is said to be unsaturated.

Ethylene is an example.

H₂C=CH₂

The fatty acids in trilinolein and linolenic acid are examples of unsaturated fatty acids.

Aromatic Hydrocarbons

The arrangement of atoms is shown on the left. The version in the center is often used to simplify diagrams of molecular structures.

The three double bonds are not restricted to the positions shown but are free to pass around the ring. This is sometimes indicated by drawing the benzene ring as it is on the far right.

• the amino acids tyrosine and phenylalanine
• cholesterol and its various derivatives, such as the sex hormones
  • estrogens
  • testosterone
• the herbicide, 2,4-D

The carotenoid, beta-carotene, is a hydrocarbon that has both aliphatic and aromatic portions.