Astraight-chain hydrocarbon that has only one double bond in each molecule has the general formula
(1) cnh2n-6 (3) cnh2n
(2) cnh2n-2 (4) cnh2n+2

- 3)

Explanations:- is not a valid formula for an alkane, alkene or alkyne.

is the general formula for an alkyne that has a triple bond between carbon atoms. The example for an alkyne is acetylene and it's formula is .

In these general formulas, n is an integer number and for alkene and alkyne the minimum value of n is 2.

Acetylene has two carbons so the value of n for it is 2. If we plug in the value of n in the formula then:

=

In acetylene, there is a triple bond between the carbon atoms and each hydrogen is bonded to each carbon.

If we plug in n = 2 in the formula, then:

It could be written as:

There is a has a double bond between the carbon atoms and two H are bonded to each carbon atom so, is the right formula of an alkene.

If we plug in n = 2 in the fourth formula then:

=

It could be written as , it has a single bond between carbons and so it is an alkane.