When benzyl chloride is treated with sodium iodide in acetone, it reacts much faster than 1-chlorobutane, even though both compounds are primary alkyl chlorides. Explain this rate difference.
The rate difference observed between the reactions of benzyl chloride and 1-chlorobutane with sodium iodide in acetone can be explained by the stability of the carbocation intermediate formed during the reaction.
In the case of benzyl chloride, when it reacts with sodium iodide in acetone, the chloride ion is substituted by the iodide ion, resulting in the formation of benzyl iodide. During this substitution reaction, a carbocation intermediate is formed. The benzyl carbocation is relatively stable due to resonance stabilization. The positive charge on the carbon atom can be delocalized onto the benzene ring through resonance, involving the lone pairs of electrons on the adjacent carbon and the pi electrons of the aromatic ring.
On the other hand, in the reaction of 1-chlorobutane with sodium iodide, the carbocation intermediate formed upon nucleophilic substitution is a primary carbocation. Primary carbocations lack the stability provided by resonance, as they do not have any adjacent atoms with lone pairs of electrons or pi-bonds that can help delocalize the positive charge.
The difference in stability between the carbocation intermediates in the two reactions dictates the rate of the substitution reaction. A more stable carbocation intermediate allows for a faster reaction rate. Therefore, the benzyl chloride reacts much faster than 1-chlorobutane in this specific reaction.