An alkyl bromide $(A)$ forms a Grignard reagent which on treatment with water yields $n$-hexane. When $(A)$ is treated with sodium in dry ether,$4,5$-diethyloctane is formed. The structure of $(A)$ is:

The order of reactivities of methyl halides in the formation of Grignard reagent is

An 'Assertion' $(A)$ and a 'Reason' $(R)$ are given below. Choose the correct answer from the following options.
Assertion $(A)$: Vinyl halides do not undergo nucleophilic substitution easily.
Reason $(R)$: Even though the intermediate carbocation is stabilized by loosely held $p-$ electrons,the cleavage is difficult because of strong bonding.

Classify the following reactions into one of the reaction types studied in this unit:
$(a)$ $CH_3CH_2Br + HS^{-} \rightarrow CH_3CH_2SH + Br^{-}$
$(b)$ $(CH_3)_2C=CH_2 + HCl \rightarrow (CH_3)_2ClC-CH_3$
$(c)$ $CH_3CH_2Br + HO^{-} \rightarrow CH_2=CH_2 + H_2O + Br^{-}$
$(d)$ $(CH_3)_3C-CH_2OH + HBr \rightarrow (CH_3)_2CBr-CH_2CH_3 + H_2O$

Identify the chiral molecule from the following:

Choose the correct decreasing order of reactivity of alkyl halides towards $S_{N}1$ reaction.