# What is the hydrocarbon formula for C-=C-C?

##### 1 Answer
Jan 1, 2016

If you have such a carbon skeleton, then the formula is $H - C \equiv C - C {H}_{3}$, propyne, ${C}_{3} {H}_{4}$.

#### Explanation:

I have written here before on the idea of degrees of unsaturation, and I am going to do it again. A carbon chain of 3 carbons that are fully saturated has a formula of ${C}_{3} {H}_{8}$; that is the number of hydrogen atoms is equal to $2 n + 2$, when $n$ is the number of carbon atoms. Propane is of course a saturated hydrocarbon. Each 2 hydrogens LESS than $2 n + 2$ represents 1 degree of unsaturation, which represents A DOUBLE BOND OR A RING.

For propyne, the saturated carbon chain would ${C}_{3} {H}_{8}$, but here the triple bond counts for 2""^@ of unsaturation, and hence the formula is ${C}_{3} {H}_{4}$. Both propene and cylcopropane (1 olefinic bond OR 1 ring, 1""^@ of unsaturation) have the formula ${C}_{3} {H}_{6}$.