Naming Alkanes, Alkenes and Alkynes

0:00How do chemists name a hydrocarbon, a compound of only carbon and hydrogen?

0:06Each group here is one carbon atom with its hydrogens.

0:10A name joins a prefix, for the carbons in the longest chain, and an ending, for the bonds.

0:17Prefixes count carbons: one is methane, two ethane, three propane, four butane, five pentane, six hexane, seven heptane, eight octane, nine nonane, ten decane.

0:37These are alkanes: only single bonds between carbons, so the names end in -ane.

0:44Alkenes have a carbon-carbon double bond, and their names end in -ene.

0:49Alkynes have a carbon-carbon triple bond, and their names end in -yne.

0:55Our molecule has only single bonds, so its name ends in -ane.

1:00Now find the longest chain, the most carbons joined in one line. Straight across there are only three.

1:08But a chain can bend on the page. Follow it down: one, two, three, four carbons.

1:17Four carbons and only single bonds make butane.

1:21The carbon left over is a branch, called methyl, from meth for one.

1:27Number from the end that gives the branch the lowest number: two from the top, not three from the bottom.

1:35So the name is 2-methylbutane, and its formula is C5H12.

1:42Now picture no branch, and a double bond between the first two carbons: a butene.

1:48Number from the end nearer the double bond: one, not three. So it is but-1-ene, not but-3-ene.

1:57Find the longest chain, give the lowest numbers, then join the prefix and the ending.