Why is that that a third order alkyl halide reacts faster in SN1 mechanism? Why is it the opposite for SN2? (is it just steric hindrance? )
http://www.cliffsnotes.com/WileyCDA/CliffsReviewTopic/SN1-versus-SN2-Reactions.topicArticleId-23297,articleId-23256.html
The reactivity of alkyl halides in SN1 (substitution nucleophilic unimolecular) and SN2 (substitution nucleophilic bimolecular) reactions is influenced by several factors, including steric hindrance.
In SN1 reactions, the rate-determining step involves the formation of a carbocation intermediate. The presence of more alkyl groups (tertiary alkyl halide) increases the stability of the resulting carbocation due to electron donation from adjacent alkyl groups through inductive effects. This stability reduces the energy barrier for carbocation formation, making the reaction faster. Additionally, the increased steric hindrance caused by the neighboring alkyl groups also helps to stabilize the positively charged carbocation.
On the other hand, in SN2 reactions, the rate-determining step involves a concerted one-step reaction where the nucleophile directly attacks the alkyl halide while the leaving group departs. Steric hindrance plays a crucial role in SN2 reactions because it affects the accessibility of the alkyl halide substrate and the nucleophile to one another. As steric hindrance increases with more alkyl groups attached to the carbon atom bearing the leaving group, it becomes challenging for the nucleophile to approach and attack the substrate. The bulky alkyl groups hinder the nucleophile's access and make the SN2 reaction slower. Hence, primary alkyl halides (with less steric hindrance) are more reactive in SN2 reactions compared to tertiary alkyl halides (with higher steric hindrance).
Therefore, it is primarily steric hindrance that governs the reactivity of alkyl halides in SN1 and SN2 mechanisms. In SN1 reactions, tertiary alkyl halides react faster due to increased stability of the carbocation intermediate, while in SN2 reactions, primary alkyl halides are more reactive due to decreased steric hindrance.