The correct answer is option 1. Primary halide.
The reactivity of alkyl halides in \(S_N1\) (substitution nucleophilic unimolecular) reactions is largely influenced by the stability of the carbocation intermediate that is formed during the reaction. The \(S_N1\) reaction involves a two-step mechanism:
Formation of Carbocation: The leaving group (halide ion) departs, resulting in the formation of a carbocation intermediate. The stability of the carbocation determines the overall reactivity of the reaction.
Nucleophilic Attack: A nucleophile attacks the carbocation, completing the substitution.
Now, let's examine the reactivity in \(S_N1\) reactions for different types of alkyl halides:
1. Tertiary Halide:
Tertiary halides form the most stable carbocations because the positive charge is distributed over three alkyl groups.
They exhibit high reactivity in \(S_N1\) reactions.
2. Secondary Halide:
Secondary halides form moderately stable carbocations.
They show reactivity in \(S_N1\) reactions, but generally, they are less reactive than tertiary halides.
3. Benzylic Halide:
Benzylic halides have the benzene ring adjacent to the halide, providing resonance stabilization to the carbocation.
They show good reactivity in \(S_N1\) reactions.
4. Allylic Halide:
Allylic halides have a double bond adjacent to the halide, allowing resonance stabilization of the carbocation.
They exhibit reactivity in \(S_N1\) reactions.
5. Primary Halide:
Primary halides form the least stable carbocations.
They show lower reactivity in \(S_N1\) reactions compared to secondary, benzylic, allylic, and tertiary halides.
In summary, the order of reactivity in \(S_N1\) reactions is generally as follows:
\[ \text{Tertiary > Secondary > Benzylic > Allylic > Primary} \]
Therefore, the species that do not show high reactivity towards \(S_N1\) reactions is a Primary Halide. | 
