$\mathrm{R}-\mathrm{OH}+{\mathrm{SOCl}}_{\mathrm{2}}\stackrel{\mathrm{Pyridine}}{\to }\mathrm{R}-\mathrm{Cl} + \mathrm{HCl} + {\mathrm{SO}}_{\mathrm{2}}$

Pyridine in the above reaction _____.

$\mathrm{A}$$\underset{413\mathrm{K}}{\overset{{\mathrm{H}}_{\mathrm{2}}/\mathrm{Raney} \mathrm{Ni}}{\to }} \mathrm{B} \underset{\mathrm{Room} \mathrm{temperature}}{\overset{\mathrm{HCl} + \mathrm{Anhydrous} {\mathrm{ZnCl}}_{\mathrm{2}}}{\to }}$ 

$\mathrm{A}$ in the above reaction is _____.

Four organic compounds have functional groups as shown below:

$\mathrm{A}: –\underset{\left|}{\overset{\right|}{\mathrm{C}}}-\mathrm{OH}, \mathrm{B}: -{\mathrm{CH}}_{\mathrm{2}}\mathrm{OH}, \mathrm{C}: \underset{\left|}{\overset{\right|}{\mathrm{C}}}\mathrm{HOH}, \mathrm{D}:{\mathrm{C}}_{\mathrm{6}}{\mathrm{H}}_{\mathrm{5}}-\mathrm{OH}$

The purple colouration of neutral ${\mathrm{FeCl}}_3$ will be given by ______.

Arrange the following compounds in order of decreasing acidity: