Haloalkanes and Haloarenes Test

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Comprehension # 3
Nucleophilic aliphatic substitution reaction is mainly of two types : $$S_{N}1$$ and $$S_{N}2$$. The $$S_{N}1$$ mechanism is a two step process. Reaction velocity of $$S_{N}1$$ reaction depends only on the concentration of the substrate. Since product formation takes place by the formation of carbocation, optically active substrate gives $$(+)$$ and $$(-)$$ forms of the product. In most of the cases the product usually consists of $$5-20\%$$ inverted product and $$80-95\%$$ racemised species. The more stable the carbocation, the greater is the proportion of racemisation. In solvolysis reaction, the more nucleophilic the solvent, the greater is the proportion of inversion.

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Comprehension # 2
Nucleophilic substitution reactions generally expressed as
$$Nu^-+R\,-\,L\longrightarrow R-\,Nu+L^-$$
Where $$Nu^-\longrightarrow Nucleophile\:;\:R-L\longrightarrow substrate\:;\:L\longrightarrow leaving\:group$$
The best leaving groups are those that become the most stable ions after they depart. Since most leaving groups leave as a negative ion, the best leaving groups are those ions that stabilize a negative charge most effectively. A good leaving group should be
(a) electron-withdrawing to polarize the carbon
(b) stable once it has left (not a strong base)
(c) polarisable- to maitain partial bonding with the carbon in the transition state (both $$S_{N}1$$ and $$S_{N}2$$). This bonding helps to stabilise the transition state and reduces the activation energy.

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