Question

The following methylation is carried out in various solvents such as benzene, tetrahydrofuran (THF), dimethoxyethane (DME), dimethyl sulfoxide (DMSO) and N,N-dimethylformamide (DMF). Which one of the following is TRUE for the effect of solvent on the reaction rate? 

• A. DMSO>DMF>DME>THF>Benzene
• B. Benzene>THF>DME>DMF>DMSO
• C. DME>DMSO>DMF>THF>Benzene
• D. THF>Benzene>DME>DMSO>DMF

Hint:

In $S_N2$ reactions, use {polar aprotic solvents} (DMSO, DMF, acetone). They enhance nucleophilicity and speed up bimolecular substitution.

Answer 1

The Correct Option is A

Step 1: Identify the reaction type.
The given reaction is an $S_N2$ nucleophilic substitution, where the nucleophile (alkoxide ion) attacks the electrophilic carbon of CH$_3$I.
Step 2: Role of solvent polarity.
$S_N2$ reactions proceed faster in polar aprotic solvents, because such solvents stabilize the cation (Na$^+$) but not the nucleophile. This leaves the nucleophile more reactive.
Step 3: Rank solvents by polarity.
Polarity (and dielectric constant) roughly follows: DMSO>DMF>DME>THF>Benzene. Thus, the rate of $S_N2$ reaction increases in this order.
Step 4: Conclusion.
The correct trend for reaction rate is: \[ \boxed{\text{DMSO>DMF>DME>THF>Benzene}} \]