Show examples and structures the order of reactivity in SN2?
The correct answer and explanation is :
Order of Reactivity in SN2 Reactions
General Order of Reactivity:
The reactivity of alkyl halides in SN2 reactions follows this trend:
Methyl > Primary > Secondary > Tertiary (almost unreactive)
Relative Reactivity of Alkyl Halides:
[
CH_3X > RCH_2X > R_2CHX > R_3CX
]
Explanation (300 words):
SN2 (Substitution Nucleophilic Bimolecular) is a one-step mechanism where the nucleophile attacks the electrophilic carbon, leading to the simultaneous departure of the leaving group. This occurs via a backside attack, resulting in an inversion of configuration (stereochemical inversion, also called Walden inversion).
The order of reactivity in SN2 is determined primarily by steric hindrance around the electrophilic carbon.
- Methyl Halides (CH₃X) – Most Reactive:
- No steric hindrance around the carbon.
- Nucleophile can attack freely, leading to a fast reaction.
- Primary Alkyl Halides (RCH₂X) – Highly Reactive:
- Minimal steric hindrance from only one alkyl group.
- Nucleophile can still easily approach and attack.
- Secondary Alkyl Halides (R₂CHX) – Moderately Reactive:
- Increased steric hindrance from two alkyl groups.
- Reaction slows down compared to methyl and primary halides.
- Tertiary Alkyl Halides (R₃CX) – Least Reactive (Almost Unreactive):
- Severe steric hindrance from three alkyl groups.
- Nucleophile cannot effectively attack the carbon.
- SN2 does not occur efficiently, and SN1 is preferred.
Conclusion:
The order of reactivity in SN2 reactions is dictated by steric hindrance. Methyl halides react the fastest, followed by primary, then secondary alkyl halides, while tertiary alkyl halides are nearly unreactive due to excessive crowding around the reaction center.