Unimolecular nucleophilic substitution (SN1) is generally carried out in
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → polar protic solvents.
An SN1 reaction takes place in two steps:
The C–X bond breaks slowly, giving a carbocation and a halide ion.
The nucleophile quickly attacks the carbocation.
The first step needs energy to separate the charges. Polar protic solvents, such as water, alcohols and acetic acid, surround and stabilise both the carbocation and the halide ion through ion–dipole attraction and hydrogen bonding. The energy released in this solvation helps the C–X bond to break. A non-polar solvent cannot stabilise ions, so the carbocation hardly forms.
So SN1 reactions are carried out in polar protic solvents.
Why the other options are incorrect
Option (2) polar aprotic solvents. — Incorrect. Polar aprotic solvents, such as acetone, DMSO and DMF, do not hydrogen bond strongly with anions. The nucleophile stays free and reactive, so these solvents favour SN2 reactions, not SN1.
Option (3) non-polar protic solvents. — Incorrect. Protic solvents such as water and alcohols are polar; a non-polar solvent cannot stabilise the carbocation.
Option (4) non-polar aprotic solvents. — Incorrect. A non-polar solvent cannot stabilise the ions, so the carbocation hardly forms.
NCERT (2026-27 edition):
Substitution Nucleophilic Unimolecular (SN1)
