Are all alkenes and alkynes unsaturated hydrocarbons?

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 #2n+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.

#DoU = (2C+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)