Question

In the reaction given below, what is the reactivity of different alcohols?
  \[R - OH + HX \to R - X + {H_2}O\]
A. ${1^ \circ } > {2^ \circ } > {3^ \circ }$
B. ${1^ \circ } < {2^ \circ } > {3^ \circ }$
C. ${3^ \circ } > {2^ \circ } > {1^ \circ }$
D. ${3^ \circ } > {1^ \circ } > {2^ \circ }$

Answer 1

Hint: The ability of an atom, molecule, or radical to engage in a chemical reaction with another atom, molecule, or substance is known as reactivity. Greater the stability of intermediate, faster is the reactivity. Therefore, for the given reaction we get, \[3^\circ > 2^\circ > 1^\circ \] this reactivity series.

Complete step-by-step answer:We get \[{H^ + }\] ions from \[H - X\] . Therefore, the loan pairs of oxygen from \[R - OH\] will attack them and we will get \[R - O{H_2}^ + \] . Further we get \[{R^ + }\] with the release of water. \[{R^ + }\]. R is actually an alkyl group. In the reaction, we are getting a carbo - cation. Now, the reactivity depends on this carbo - cation obtained. Greater the stability of carbo - cation, faster is the reaction. Its stability depends on the alpha hydrogen. Greater the number of alpha hydrogen, greater will be the stability of the carbo - cation. The number of carbon atoms directly attached with the carbo - cation tells us whether the carbo - cation is primary \[(1^\circ )\] , secondary \[(2^\circ )\] or tertiary \[(3^\circ )\] . Tertiary carbo - cations have greater numbers of alpha hydrogen than secondary and primary carbo - cations \[(1^\circ )\]. Similary, secondary carbo - cations \[(2^\circ )\] have more alpha hydrogen as compared to primary carbo - cations. Hence tertiary carbo - cations \[(3^\circ )\] are most stable and primary carbo - cations \[(1^\circ )\] are least stable.
Stability series becomes \[3^\circ > 2^\circ > 1^\circ \] .
We know that greater the stability, faster is the reactivity. Thus we get the reactivity series as \[3^\circ > 2^\circ > 1^\circ \] .

Option ‘C’ is correct

Note: The stability of the carbocation that is produced during the reaction via the \[{S_N}1\] mechanism will determine the reactivity of \[ROH\] in the given reaction which is a nucleophilic substitution reaction. Due to the + Inductive effect of the alkyl groups, tertiary carbocations are more stable than secondary carbocations while primary carbocations are the least stable. Alcohols will react in the sequence listed below
Primary alcohol < Secondary alcohol < Tertiary alcohol