Which of the following halogen derivatives forms poisonous phosgene gas when it comes in contact with air and light?

If all the nucleophilic substitution reactions at saturated carbon atoms in the above sequence of reactions follow $S_N2$ mechanism, then $\underline{E}$ and $\underline{E}$ will be respectively,

$3$-Phenylpropene on reaction with $HBr$ gives (as a major product)

Which of the following chiral compounds undergoes nucleophilic substitution by $OH^-$ ions to form a racemic mixture?
$(a)$ $CH_3-CH(Br)-C_2H_5$
$(b)$ $CH_3-C(Br)(CH_3)-C_2H_5$
$(c)$ $CH_3-CH(C_2H_5)-CH_2Br$

In the $S_{N}2$ substitution reaction $R-Br + Cl^- \xrightarrow{DMF} R-Cl + Br^-$,which of the following will have the highest rate?

$A$ solution of $(+)-1$-chloro-$1$-phenylethane in toluene racemizes slowly in the presence of a small amount of $SbCl_5$ due to the formation of which of the following?