SN1 - Effect of the Substrate

The rate determining step (RDS) in S_N1 mechanism is dissociation of leaving group to form carbocation.  Thus, 3^o halides and tosylates react fast by this mechanism since they form relatively stable 3^o carbocation intermediates.  Imagine what would happen if methyl bromide (CH₃Br) were to react by this mechanism.  Do you think the cation formed would be very stable?

• 3^o > 2^o > 1^o > methyl
• Nucleophile not important (it aint in RDS)
• Rate=k[R-LG]
• no stereochemistry
• Favors polar solvents

I. Stability of the Carbocation Intermediate

The primary factor influencing the rate of an SN1 reaction is the stability of the carbocation intermediate. More stable carbocations form more readily, accelerating the reaction.

The general order of carbocation stability is:

• 3° > 2° > 1° > methyl

This trend is explained by hyperconjugation (the delocalization of electrons from adjacent C–H or C–C bonds) and inductive effects (electron-donating effects from nearby alkyl groups that stabilize the positive charge).

II. Resonance Stabilization

Some carbocations are stabilized not just by hyperconjugation, but also by resonance. Allylic and benzylic carbocations can delocalize their positive charge over multiple atoms through resonance structures.

This resonance stabilization greatly enhances their reactivity in SN1 pathways, often making them more reactive than even tertiary carbocations.

III. Carbocation Rearrangements

During the formation of carbocations, rearrangements may occur if a more stable carbocation can be formed. These rearrangements typically involve:

• Hydride shifts: migration of a hydrogen atom with its bonding electrons (a hydride) to the carbocation center.
• Alkyl shifts: migration of a methyl or larger alkyl group with its bonding electrons.

Such rearrangements affect product distribution and must be considered when predicting the outcome of SN1 reactions.