Question

Explain :
a. Vinyl chloride is unreactive in nucliphillic `4-` subsitution reactions.
b. Neopenthyl bromide undergoes nucliphillic substituion reactions very slowly.
c. `3-` Bromocyclohexane is more reactive than `4-` bromovyclohexane in hydrolysis with aqueous `NaOH`.
d. `ter-` Butyl chloride reacts with aqueous sodium hydroxide by `SN^(1)` mechanishm while `n-` butuyl chloride reacts by `SN^(2)` medchanism.

Answer 1

a. Vinylic and aryl halides are unreactive towards `SN` reactions due to the resonance effects. Resonanace gives rise to partial double bond character to the `(C-X)` bond, making stonger and, therefore more diffcult to cleave than the `(c-X)` bond in `RX (R_(sp^(3)) - X)` bond. It also reduces te polarity of the `(c_X)` bond and, therefore, the heterolytic cleavage of `(C-X)` bond is difficult.

Neopentyl bromide gies `(1^(@) C^(o+))` which can rearrange to `3^(@) C^(o+)` by `1.2`, methyl shift, but is sterically hindered and so `SN` reaction takes place slowly.
C.
(i) gives allylic carbocation, while (ii) gives `2^(@)C^(o+)` and allylic `C^(o+)` is more stable than `2^(@) C^(o+)`. So (i) more reactive than (ii) in hydrolysis with aq. `NAOH`.

ii. `CH_(3) CH_(2) CH_(2) CH_(2) Cl rarr CH_(3) underset((1^(@) C^(o+)))(CH_(2) CH_(2)) overset(o+)(CH_(2))`
Reactivity of `SN^(1)` reaction is `3^(@)C^(o+) gt 2^(@) C^(o+) gt 1^(@) C^(o+)` and reactivity of `SN^(2)` reaction is `1^(@)C^(o+) gt 2^(@)C^(o+) gt 3^(@)C^(o+)` . So (i) reacts with aq. `NaOH` by `SN^(1)` mechanism because it gives `3^(@) C^(o+)`, while (ii) undergoes by `SN^(2)` reaction becuase it gives `1^(@)C^(o+)`, ions.