Give simple chemical test to distinguish between pentan- $2$-one and pentan- $3$-one pair of compounds. 

How will you prepare the following compound from benzene? You may use any inorganic reagent and any organic reagent having not more than one carbon atom: p-NitrobenzaIdehyde.

How will you bring about the following conversion in not more than two steps? 

Benzoic acid to m-Nitrobenzyl alcohol

Write the structure of the following compound.

$4$-Oxopentanal

Give plausible explanation for the following: 

During the preparation of esters from a carboxylic acid and an alcohol in the presence of an acid catalyst, the water or the ester should be removed as soon as it is formed.