The reactivity of the compounds (i) MeBr (ii) PhCH₂Br (iii) MeCl (iv) p - MeOC₆H₄Br decreases as

ii > i > iii > iv

The reactivity of an alkyl halide depends on the stability of the carbocation that is formed when the halide is cleaved. The more stable the carbocation, the more reactive the alkyl halide.

The stability of a carbocation can be affected by the following factors:

• The electronegativity of the atom bonded to the carbon atom. The more electronegative the atom, the more it will withdraw electrons from the carbon atom, making the carbocation less stable.
• The presence of electron-withdrawing groups. Electron-withdrawing groups withdraw electrons from the carbon atom, making the carbocation less stable.
• The presence of electron-donating groups. Electron-donating groups donate electrons to the carbon atom, making the carbocation more stable.

In the case of the compounds listed, the stability of the carbocation decreases in the order:

PhCH₂⁺ > Me⁺ > p-MeOC₆H₄⁺ > MeCl⁺

This is because the phenyl group in PhCH₂Br is an electron-withdrawing group, which destabilizes the carbocation. The methyl group in MeBr is an electron-donating group, which stabilizes the carbocation. The methoxy group in p-MeOC₆H₄Br is an electron-donating group, but it is less electron-donating than the methyl group. The chloride ion in MeCl is an electron-withdrawing group, but it is less electron-withdrawing than the phenyl group.

Therefore, the reactivity of the compounds decreases in the order:

(4) ii > i > iii > iv

Here is a table that summarizes the stability of the carbocations and the reactivity of the compounds: