Classification of Hydrocarbons

A hydrocarbon is an organic compound whose molecule is built from carbon and hydrogen atoms

only -- no oxygen, nitrogen, halogen or any other element. Hydrocarbons form the backbone of

organic chemistry: every other class of organic compound is, structurally, a hydrocarbon with

one or more hydrogens replaced by a functional group. They are classified first by whether they

contain only single carbon-carbon bonds or also multiple bonds.

Saturated hydrocarbons (alkanes) contain only carbon-carbon single bonds; every carbon is

$sp^3$ hybridised and, apart from the ends of the chain, bonded to four other atoms (carbon or

hydrogen). Their general formula is $\text{C}_n\text{H}_{2n+2}$. Because every bond is a single

sigma bond, alkanes are relatively unreactive and undergo mainly substitution reactions.

Unsaturated hydrocarbons contain one or more carbon-carbon multiple bonds and are further

divided into two families. Alkenes contain at least one carbon-carbon double bond

($\text{C}{=}\text{C}$), general formula $\text{C}_n\text{H}_{2n}$ for one double bond;

alkynes contain at least one carbon-carbon triple bond ($\text{C}{\equiv}\text{C}$),

general formula $\text{C}_n\text{H}_{2n-2}$ for one triple bond. The extra pi bond(s) in both

families make them far more reactive than alkanes, and they typically react by addition rather

than substitution -- a reagent adds across the multiple bond instead of replacing a hydrogen.

Hydrocarbons are also classed by whether their carbon skeleton is a straight/branched

open chain (acyclic or aliphatic, covering the alkanes, alkenes and alkynes above) or a

closed ring. Within ring compounds, aromatic hydrocarbons are a chemically distinct group

built around the benzene ring: a planar, cyclic, fully conjugated system of alternating

carbon-carbon bonds whose pi electrons are delocalised all the way around the ring rather than

localised into fixed double bonds. This delocalisation makes the ring unusually stable, so

aromatic hydrocarbons undergo substitution reactions that preserve the ring's aromatic character,

in sharp contrast to the addition reactions of alkenes and alkynes.

This chapter takes up each of the four families in turn -- alkanes, alkenes, alkynes and aromatic

hydrocarbons -- covering how each is named, how its functional carbon(s) are hybridised and

shaped, how it is made, and its characteristic chemical reactions, closing with the health

hazards posed by benzene and the polycyclic aromatic hydrocarbons.