[ifrah]

where would we place 1,5-hexadiyne ?in alkynes or not

[sjb]

where would we place 1,5-hexadiyne ?in alkynes or not

Well, depends what you have on your list of things you can classify compounds under.

[ifrah]

since general formula for alkynes is not applicable on it i think its not possible to put it under alkynes

[Arctic-Nation]

I'm not really sure what you mean by 'general formula is not applicable'. 1,5-Hexadiyne contains two alkyne units, so unless you have the option of diyne compounds, you should classify it under alkynes.

[dunno260]

General formula for an alkyne is only for a molecule that contains one alkyne function group in the molecule and the rest of the molecule is a saturated hydrocarbon.  A molecular formula can only give you so much information and using a formula to classify a molecule without knowing its structure will lead you down dark and dangerous avenues. 

For say 1-octyne the molecular formula is C₈H₁₄.  But you can make a number of structures using that formula that don't have an alkyne, say 1,3-octadiene or cyclooctene or bicyclobutane (1-cyclobutylcyclobutane), or bicyclo[2.2.2]octane or a host of other structures.