Question

Why can we not form a `:CH_2` molecule?

Answer 1

Well methylene, `:CH_2` certainly can be formed.....

Explanation:

But it is not something that you could store in a bottle or cylinder. Olefins feature a `C=C` double bond by definition, and the simplest such species is `H_2C=CH_2`; this is simply not available for a one carbon chain. And the same applies to acetylene, another TWO CARBON chain, i.e. `HC-=CH`, which is supplied as a gas.

Are you happy with this?

Answer 2

Because there is no other carbon atom with which to bond!

Explanation:

The "ene" and "yne" suffixes denote olefins and acetylenes (or alkenes or alkynes). These compounds contain at least one carbon to carbon double bond, or at least one carbon to carbon triple bond, respectively.

But the prefix "meth" denotes 1 carbon (e.g. methanol, `CH_3-OH`, methanoic acid `H-COOH`) and as there is only 1 carbon, there is no other carbon atom with which to form a double or triple bond.

So you cannot form a methene nor a methyne.