Unsaturated hydrocarbons with triple bonds are called alkynes.
In systematic nomenclature, C2H2
would be ethyne, although a more common name for it is acetylene.The
simplest alkenes and alkynes are shown in the margin, on the previous
page, with the geometry that results from their carbon-carbon bonds.
A new kind of isomerism appears with butene, C4H8.
Butene has three different structural isomers, depending on whether
the four carbons are in a straight or branched chain, and where
the double bond is located.
These structural isomers are 1-butene, 2-butene (with the number
describing the position of the double bond), and isobutylene. (The
systematic name for isobutylene is 1-methylpropene.) 1-Butene, 2-butene,
and isobutylene are genuine structural isomers because their atoms
are connected to one another in different ways.
2-Butene has one methyl group on each double-bonded carbon, but
these methyl groups can be placed in two different ways relative
to the double bond: on the same side of the molecule, or diagonally
across the bond. These two possibilities result in two geometrical
isomers, cis2-butene and trans-2-butene, which are shown on the
next page ( page 11 ). These are geometrical rather than structural
isomers because they have the same connections between atoms.