Explain:
a. `alpha`-Halocarbonyl compounds, even the `3^(@)` types like , do not undergo `SN^(1)` reaction.
b. Which is more reactive: (i) PrCI or (ii) `MeCOCH_(2)CI` with NaI/acetone ?
c. Acetals and ketals are regenerated back to carbonyl. compounds with `H_(3)O^(o+)` but not with `overset(Θ)(OH)`.
d. Cyclohexane is more reactive than cyclopenatone with HCN.

Explanation

a. In the first step of `SN^(1)` reaction, a carbocation is formed. The carbocation formed is unstable is due to the presence of positive charges on adjacent C atoms.

b. Both are `1^(@)` and undergo `SN^(2)` reaction.
Reactivity of (ii) gtgt (i). The high reactivity of (ii) in `SN^(2)` reaction is due to the stabilising effect produced in the transition state of ketone by the overlap of adjacent `pi`-bond with the p-orbital of the `sp^(2)`-hybridised C atom. This type of overlap is also responsible for the high reactivity of allyl halides in `SN^(2)` reaction.
i.
ii.

c. The formation of hemiacetal or half hydrate takes place in acid only. Its not possible in base because a weaker base `(overset(Θ)(OH))` cannot displace stronger base `(RO^(Θ))`. But the formation of acetal form hemiacetal can take place by both acid and base. So reverse process of hemiacetal to `(C=O)` compound can occur only with acid but not with base.
d.
i. In cyclopentanone, the hybridisation of C atom of `(C=O)` group change form `sp^(2)rarr sp^(3)`, which increases the strain due to eclipsing interaction.
ii. In cyclohexane, the hybridisation of C atom of `(C=O)` group changes form `sp^(2)rarr sp^(3)`, but destabilisation effect is lost as there is no eclipsing interaction.
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