What is the decreasing order of reactivity of $HX$ in the reaction $ROH + HX \to RX + H_2O$?

Write the structures of different isomeric alkyl groups corresponding to the molecular formula $C_{5}H_{11}$. Write the $IUPAC$ names of the alcohols obtained by the attachment of $-OH$ groups at different carbons of the chain.

The reaction shown in the image proceeds by the $.......$ mechanism.
$CH_3CH_2CH(OH)CH_3 + SOCl_2 \rightarrow CH_3CH_2CH(Cl)CH_3 + SO_2 + HCl$

Consider the following alcohols:
$(I)$ $C_6H_5CH_2OH$
$(II)$ $O_2N-C_6H_4-CH_2OH$ (para-nitro)
$(III)$ $CH_3O-C_6H_4-CH_2OH$ (para-methoxy)
$(IV)$ $Br-C_6H_4-CH_2OH$ (para-bromo)
The order of decreasing reactivities of these alcohols towards nucleophilic substitution with $HBr$ is:

An alcohol $X$ $(C_4H_{10}O)$ does not give turbidity with conc. $HCl$ and $ZnCl_2$ at room temperature. $X$ on reaction with reagent $Y$ gives $Z$. What are $X, Y$ and $Z$ respectively?

$CH_3 CH_2 OH$ $\xrightarrow{Cl_2} CH_3 CHO$ $\xrightarrow{3 Cl_2} Cl_3 CCHO$. In the above reactions,the role of $Cl_2$ in step-$1$ and step-$2$ respectively is: