Which of the following alkyl halides is hydrolyzed most rapidly by the $S_N2$ mechanism?

The following reaction,$(CH_3)_3CBr + H_2O \rightarrow (CH_3)_3COH + HBr$,is an example of:

Consider the following reaction:
$CH_3-CH_2-CH(Br)-CH_3 \xrightarrow{alc. KOH, \Delta} CH_3-CH_2-CH=CH_2 (I) + CH_3-CH=CH-CH_3 (II)$
Which of the following statements are correct?
$a.$ $I$ is the major product of the reaction
$b.$ $II$ is the major product of the reaction
$c.$ Formation of $I$ is in accordance with Saytzeff's rule
$d.$ $II$ is more stable because it is more substituted

Enlist the main points of difference between $S_{N}1$ and $S_{N}2$ reactions.

Write a note on nucleophilic substitution reactions.

Which of the following is likely to undergo racemization during alkaline hydrolysis by $SN^1$ mechanism?