# What are the general formulae for alkanes, and for alkenes?

Feb 8, 2017

$\text{Alkanes.........} {C}_{n} {H}_{2 n + 2}$

#### Explanation:

$\text{Olefins.........} {C}_{n} {H}_{2 n}$

Alkanes are saturated molecules. Their formulae correspond to ${C}_{n} {H}_{2 n + 2}$. Try it out with methane, ethane,.....hexane. Olefins have ONE so-called $\text{degrees of unsaturation}$ with respect to equivalent alkanes. For instance, compare the formula of ethylene and propylene, ${H}_{2} C = C {H}_{2} ,$ and ${H}_{3} C - C H = C {H}_{2}$, with that of their saturated precursors, ${H}_{3} C - C {H}_{3}$, and ${H}_{3} C - C {H}_{2} C {H}_{3}$. What are the ${C}_{n} {H}_{m}$ formula? How many degrees of unsaturation does acetylene have?

A $\text{degree of unsaturation}$ corresponds to the presence of an olefinic bond, $H C = C H$, OR a ring junction (the ring junction reduces the overall hydrogen count by 2). Halogens count for 1 hydrogen, oxygens we ignore (though a carbonyl bond could be the source of unsaturation), and if there is nitrogen present we subtract $N H$ from the formula before we assess saturation.

For more details, see here. For a practical example of how we can use this idea to interrogate a formula see here.