2.02: Hydrocarbons
- Page ID
- 4227
Hydrocarbons are organic molecules that consist exclusively, or primarily, of carbon and hydrogen atoms. They come in two flavors: (1) aliphatic hydrocarbons that consist of linear chains of carbon atoms and (2) aromatic hydrocarbons that which consist of closed rings of carbon atoms.
Aliphatic Hydrocarbons
Methane has one carbon bonded to four hydrogens (CH4). Ethane has two carbons joined by a single bond, with three hydrogens attached to each carbon (C2H6). The two molecules are labeled beneath their structures. 
Three long chains extend horizontally from a three-carbon vertical backbone on the right. Each chain connects through a C(=O)???O ester linkage. Carbon and hydrogen atoms are explicitly labeled, and no carbon???carbon double bonds are drawn in the chains. The label beneath the structure reads Tristearin.
The simplest is methane, CH4. Next is ethane, C2H6.
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.
Example : Ethylene H2C=CH2
The fatty acids in trilinolein and linolenic acid are examples of unsaturated fatty acids
Aromatic Hydrocarbons
The building block of aromatic hydrocarbons is the benzene ring. 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 left representation labels the six ring carbons and their hydrogens. The middle orange hexagon omits atom labels and shows three alternating double bonds. The right orange hexagon contains a circle, representing the delocalized bonding in the ring. These are three representations of the same benzene molecule.
Some examples of biological molecules that incorporate the benzene ring:
- the amino acids tyrosine and phenylalanine
- cholesterol and its various derivatives, such as the sex hormones: estrogens and testosterone
- the herbicide, 2,4-D
The molecule is drawn horizontally with six-membered carbon rings at both ends. Alternating single and double bonds run along the central chain. Carbon and hydrogen atoms are labeled; methyl groups (CH3) branch from the chain and end rings. No nitrogen or oxygen atoms are shown.
The carotenoid, beta-carotene, is a hydrocarbon that has both aliphatic and aromatic portions.