Final answer:
Isopropyl benzene reacts quicker with bromine than ethyl benzene because isopropyl has a secondary carbon which forms a more stable carbocation intermediate through hyperconjugation and inductive effects. A secondary carbocation being more stable results in lower activation energy for the reaction, leading to a faster reaction rate.
Step-by-step explanation:
The reason isopropyl benzene reacts quicker than ethyl benzene with bromine has to do with the degree of alkyl substitution on the benzene ring. Isopropyl benzene has a secondary carbon attached to the ring, while ethyl benzene has a primary carbon. Secondary carbocations, which can form during the reaction with bromine, are more stable than primary carbocations due to better hyperconjugation and inductive effects. These effects result in a lower activation energy for the formation of the carbocation intermediate, making the reaction with isopropyl benzene faster.
Hyperconjugation is the delocalization of electrons from a σ-bond (usually C-H or C-C) to an adjacent empty or partially filled p-orbital or a pi-bond, which helps to stabilize the carbocation intermediate. A secondary carbocation can be hyperconjugated with more neighboring C-H bonds compared to a primary carbocation. Inductive effects also stabilize the carbocation through the donation of electron density along the sigma bonds from the alkyl group to the positively charged carbon. The greater the alkyl substitution, the more pronounced the inductive effect, making secondary carbocations more stable than primary ones.