Are all alkenes and alkynes unsaturated hydrocarbons?

Nov 9, 2014

Yes, alkenes and alkynes are both classified as unsaturated hydrocarbons.

Saturation refers to the number of hydrogens attached to each carbon in a molecule. In general, for $n$ number of carbon atoms in a molecule, there can be a maximum of $2 n + 2$ hydrogen atoms. Take hexane, 1-hexene and 1-hexyne as examples. The hex- term means that the molecules have six carbon atoms and can therefore have a maximum of 14 hydrogen atoms. Looking at the structures, we see that only hexane has the full 14 hydrogens. 1-hexene is missing two hydrogens and 1-hexyne is missing four hydrogens. Therefore, both hexene and hexyne are unsaturated hydrocarbons; we say that 1-hexene has one degree of unsaturation and 1-hexyne has two degrees of unsaturation.

In general, the following equation can be used to determine *degrees of unsaturation * (DoU) for a given molecule. As a reference point, anything with more than zero degrees of unsaturation is technically unsaturated.

$D o U = \frac{2 C + 2 + N - X - H}{2}$

C - number of carbon atoms
N - number of nitrogen atoms
X - number of halide atoms
H - number of hydrogen atoms
(Oxygen and sulfur do not generally effect the DoU formula)