Yes, ALKYL is a Scrabble word!

Letter values in Scrabble US: A 1, L 1, K 5, Y 4, L 1.

Is it a valid word?

ALKYL is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

any of a series of univalent groups of the general formula CnH2n+1 derived from aliphatic hydrocarbons

Also: alkyl group, alkyl radical.