IUPAC Nomenclature — Basic Rules for Hydrocarbons

The IUPAC (International Union of Pure and Applied Chemistry) system gives every organic compound exactly one systematic name, built up in a fixed sequence of steps.

Step 1 -- choose the parent chain. Identify the longest continuous chain of carbon atoms in the molecule; this becomes the 'parent chain' and is named with a word stem that states the number of carbon atoms it contains -- $\text{meth}$- (1), $\text{eth}$- (2), $\text{prop}$- (3), $\text{but}$- (4), $\text{pent}$- (5), $\text{hex}$- (6), $\text{hept}$- (7), $\text{oct}$- (8), $\text{non}$- (9), $\text{dec}$- (10) -- followed by a suffix that states how saturated the chain is: $-\text{ane}$ for a chain with only single bonds, $-\text{ene}$ for one carbon-carbon double bond, and $-\text{yne}$ for one carbon-carbon triple bond.

Step 2 -- number the chain. Number the carbon atoms of the parent chain from whichever end gives the lowest possible locants, applying this order of priority if there is a choice: first to the principal characteristic group (Section 11.4), then to any carbon-carbon multiple bond, and only then to substituent groups.

Step 3 -- name the substituents. Any carbon branch hanging off the parent chain is named as an alkyl substituent, formed by dropping the $-\text{ane}$ ending of the corresponding alkane and adding $-\text{yl}$: $\text{methyl}$ ($-\text{CH}_3$), $\text{ethyl}$ ($-\text{C}_2\text{H}_5$), $\text{propyl}$, and so on. Each substituent is written as a prefix to the parent name, preceded by the locant number of the carbon it is attached to.

Step 4 -- combine and order the name. When two or more different substituents are present, they are cited in alphabetical order of their names in the final written name -- never in order of their locant numbers. For instance, in 4-ethyl-2-methylhexane, 'ethyl' (attached at C-4) is written before 'methyl' (attached at C-2) purely because 'e' comes before 'm' alphabetically; the numbering itself is separately chosen (Step 2) to keep the overall set of locants $\{2,4\}$ as low as possible, which is lower than the alternative numbering that would give $\{3,5\}$ from the other end of the chain. …