Assertion (A): SN1 reactions are generally carried out in polar protic solvents such as water, alcohol or acetic acid.
Reason (R): In the slow step of an SN1 reaction, the energy needed to break the carbon–halogen bond is obtained through solvation of the halide ion with the proton of the protic solvent.
In the light of the above statements, choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Both (A) and (R) are true and (R) is the correct explanation of (A).
An SN1 reaction takes place in two steps. In step I, the slow step, the polarised C–X bond breaks to give a carbocation and a halide ion. In step II the nucleophile attacks the carbocation.
\( \displaystyle \ce{(CH3)3C-Br <=> (CH3)3C+ + Br-} \qquad \text{(step I, slow)} \)
Assertion (A): SN1 reactions are generally carried out in polar protic solvents such as water, alcohol or acetic acid. — True. A protic solvent has a hydrogen attached to an electronegative atom (O–H), as in water, alcohols and acetic acid.
Reason (R): In the slow step of an SN1 reaction, the energy needed to break the carbon–halogen bond is obtained through solvation of the halide ion with the proton of the protic solvent. — True. As the bond breaks, the halide ion is surrounded and held by the solvent; this solvation releases the energy that pays for the bond cleavage.
Why R explains A?
Assertion (A) is about the choice of solvent; Reason (R) is about where the energy for the slow step comes from.
The slow ionisation step can take place only if the halide ion formed is solvated by the protons of a protic solvent, so R gives the cause of A.
NCERT (2026-27 edition):
6.7.1 Reactions of Haloalkanes – “(b) Substitution nucleophilic unimolecular (SN1) SN1 reactions are generally carried out in polar protic solvents (like water, alcohol, acetic acid, etc.).”
“Step I is the slowest and reversible. It involves the C–Br bond breaking for which the energy is obtained through solvation of halide ion with the proton of protic solvent.”