JEE MainChemistryHaloalkanes and Haloarenes
The reaction of (2R)-2-bromopentane with NaI is carried out to form a major product. Identify the stereochemistry of the major product formed. Furthermore, what will be the effect on the rate of this reaction if the solvent is changed from acetone to methanol?
Options
- AMajor product is (2S)-2-iodopentane; the reaction rate increases in methanol.
- BMajor product is (2R)-2-iodopentane; the reaction rate decreases in methanol.
- CMajor product is (2S)-2-iodopentane; the reaction rate decreases in methanol.
- DMajor product is a racemic mixture of 2-iodopentane; the reaction rate increases in methanol.
Correct answer
C. Major product is (2S)-2-iodopentane; the reaction rate decreases in methanol.
Step-by-step solution
The reaction of an alkyl halide with NaI is the Finkelstein reaction, which proceeds via an S _ N 2 mechanism. Since S _ N 2 reactions occur with complete inversion of configuration (Walden inversion), the nucleophile ( I ^- ) attacks from the side opposite to the leaving group ( Br ^- ). Because the relative priority of the groups around the chiral centre remains unchanged ( I > propyl > methyl > H ), the (2R)-2-bromopentane substrate is converted exclusively into (2S)-2-iodopentane. Acetone is a polar aprotic sol