Explain the following observation `:`
`a.` When pent `-1` en `-4-` yneistrested with `HBr` in molar proportion, the addition takes place on the double bond and not on the triple bond, thereby, yielding the porduct `,`

`b.` The `(C=-C)` bond length in propene is shorter than expected.
`c.` Whey benzyl carbocation is more stable than ethyl carbocation ?
`d.` Why does but `-2-` ene exhibit cis `-` trans isomerism but but `-2-` yne does not ?
`e.` `CI_(2)C=C C I_(2)` does not add `CI_(2)` under usual conditions but adds in the presence of `AICI_(3)`.

Explanation

`a.` Electrophilic addition `(EA)` such as `HBr` on `(C=C)` is more reactive than on `(C-=C)(` except hydrogenation `),EA` of `HBr` on `(C=C)` leads to the formation of `2^(@)C^(o+)` while that on `(C-=C)` leads to the formation of vinylic `C^(o+)(CH_(2)=CH^(o+))` as an intermediate. Vinyl `C^(o+)` being most unstable, whereas `2^(@)C^(o+)` being relative more stable, `EA` takes place on `(C=C)` rather than on `(C=C)`.
`EA` on `(C=C):`

`b.` Because of hyperconjugation, the `(C_(3)-C_(2))`
`c.` Benzyl carbocation is stabilised by resonance `(` five resonance contributing structures `)` and, thus, has lower energy and is more stable.

In comparison, `C_(2)H_(5)^(o+)` shows four hyperconjugative structures `(` more the resonance contributing structures, more is the stability, moreover, the resonance effect is stronger than the hperconjugation effect `).` Hence, benzyl conbocation is more stable than ethyl carbocation.

`d.`
`cis `or trans `-` But `-2-`ene has two dissimilar groups attached to an unsaturated `C` atom and shows geometrical isomers, whereas but `-2-`yne`(Me- -= Me)` is `sp-` hybridised and is linear, and so it dows not show geometrical isomers.
`e.` Due to the electron `-` withdrawing group `(E.W.G.)` of four `CI` atoms, the `bar(e)` density or nucleophilicity of `pi bar(e)` of the alkene decreases, sso `EA` reaction does not take place `(` even does not show the test for unsaturatio, `i.e.,` Beyer's test and decolouration of `Br_(2)` solution `)`.
`AICI_(3)` reacts with `Cl_(2)` and polarises `Cl_(2)` to `Cl^(o+)` or increases the electrophilic character of `Cl^(o+)` that can react efficiently.