Question

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

Answer 1

If you have such a carbon skeleton, then the formula is `H-C-=C-CH_3`, propyne, `C_3H_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_3H_8`; that is the number of hydrogen atoms is equal to `2n+2`, when `n` is the number of carbon atoms. Propane is of course a saturated hydrocarbon. Each 2 hydrogens LESS than `2n+2` represents 1 degree of unsaturation, which represents A DOUBLE BOND OR A RING.

For propyne, the saturated carbon chain would `C_3H_8`, but here the triple bond counts for `2""^@` of unsaturation, and hence the formula is `C_3H_4`. Both propene and cylcopropane (1 olefinic bond OR 1 ring, `1""^@` of unsaturation) have the formula `C_3H_6`.