Which of the following undergoes nucleophilic substitution by $S_N1$ mechanism?

An alkyl halide $C_3H_7Cl$,on reaction with a reagent $X$ gave the major product $Y$ $(C_4H_7N)$. $Y$ on hydrolysis released a gas,which turns red litmus to blue. What are $X$ and $Y$?

Given below are two statements :
Statement $I$ : $S_N2$ reactions are 'stereospecific',indicating that they result in the formation of only one stereo-isomer as the product.
Statement $II$ : $S_N1$ reactions generally result in the formation of product as racemic mixtures.
In the light of the above statements,choose the correct answer from the options given below :

Explain elimination reactions of alkyl halides.
or
Explain dehydrohalogenation ($\beta$-elimination) of alkyl halides.

Arrange the following alkyl halides in the increasing order of their reactivity towards nucleophilic substitution reaction:
$(I)$ $CH_3CH_2CH(Cl)CH_3$
$(II)$ $CH_2=CHCH(Br)CH_3$
$(III)$ $CH_2=CHCH(Cl)CH_3$
$(IV)$ $CH_3CH=C(Br)CH_3$

The compound which forms acetaldehyde when heated with dilute $NaOH$ is