1. In t-butyl chloride, there is niore steric hindrance and it involves formation of a stable tertiary carbocation. Therefore it reacts with KOH by SN1 mechanism rather than SN2 mechanism because SN1 mechanism is faourable in case of steric crowding and is directly proportional to partial positive charge on carbon atom. In n-butyl chloride, there is least steric hindrance and involves formation of less stable primary carbocation. Thus it takes place in one step and is favoured by SN2 mechanism.
2. Melting point of p – dichiorobenzene is higher than that of ortho and meta-dichiorobenzene. This is due to the fact that is has a symmetrical structure and therefore, its molecules can easily pack closely in the crystal lattice. p-dichlorobcnzene being more symmetrical fits closely in the crystal lattice and has stronger intermolecular attraction than o & m isomers. So p-isomer has high melting point than the corresponding o & m-isomers