Consider the following compounds: (I) 2-Bromo-2-methylbutane (II) 1-Bromopentane (III) 2-Bromopentane (IV) Bromobenzene The correct decreasing order of their reactivity towards S _ N 1 reaction is:

Step-by-step solution

The rate of S _ N 1 reaction depends on the stability of the intermediate carbocation formed. The carbocations formed are: (I) 2-Bromo-2-methylbutane forms a 3^ carbocation. (II) 1-Bromopentane forms a 1^ carbocation. (III) 2-Bromopentane forms a 2^ carbocation. (IV) Bromobenzene forms a phenyl cation, which is highly unstable due to the partial double bond character of the C-Br bond and the instability of the positive charge on an sp^2 hybridized carbon. The order of stability of carbocations is 3^ > 2^ > 1^ > phe