Explain why neither of the following compounds undergoes Diels `-` Alder reaction with dienophile `(e.g.` maleic anhydride`).`
`a.` `HC-=C-C-=CH`
`b.`
`c.`

Explanation

Necessary conditions for Diels`-` Alder reaction are `S-` cis conformation and non`-` linearity of dienes.
`a.` `1,3-` Butadiyne is linear and cannot assume `S-cis` conformation
`b.` is held in an `S-` trans conformation and cannot assume `S-cis` conformation by the requirement of the ring.
`c.` The diene portion of the molecule is blocked into an `S-trans `conformation and cannot assume `S-cis` conformation by the presence of the ring.