Alkyl iodides on keeping become dark in color due to ...........

The rate of $\beta$-dehydrohalogenation in the following halides under treatment with a strong base follows the order:

The decreasing order of reactivity of the following compounds towards nucleophilic substitution $(S_{N}2)$ is :

What is the relative rate of $S_{N}1$ reaction for $(CH_{3})_{2}CHBr$?

Identify the correct statement for the below given transformation:
$CH_{3}CH_{2}CH_{2}CH(N^{+}(CH_{3})_{3})CH_{3}$ $\xrightarrow{C_{2}H_{5}ONa / C_{2}H_{5}OH} A (\text{major}) + B (\text{minor})$

Hydrolysis of an alkyl bromide $X$ $(C_5H_{11}Br)$ follows first-order kinetics. Reaction of $X$ with $Mg$ in dry ether followed by treatment with $D_2O$ gives $Y$. What is $Y$?