SN2 mechanism: backside attack and inversionSN2 क्रियाविधि: पृष्ठ आक्रमण और प्रतिलोमनSN2 mechanism: backside attack and inversion
NCERT Fig 6.2 · Class 12 Chemistry · Chapter 6 · interactive diagram
Parts and what they mean
- Hydroxide ion HO⁻ — the nucleophile (हाइड्रॉक्साइड आयन HO⁻ — नाभिकरागी) — Electron-rich species; uses a lone pair on O to form the new C–O bond.
- Chloromethane CH₃Cl — the substrate (क्लोरोमेथेन CH₃Cl — क्रियाधार) — The δ+ carbon is attacked; Cl is the leaving group. Methyl halides react fastest by SN2 (only three small H atoms around C).
- Curved arrows = movement of electron pairs (वक्र तीर = इलेक्ट्रॉन युग्मों की गति) — Each arrow starts at an electron pair (lone pair or bond) and ends at the atom that receives it.
- Transition state [HO···CH₃···Cl]⁻ (संक्रमण अवस्था [HO···CH₃···Cl]⁻) — Partial bonds (dashed) to both O and Cl; the three C–H bonds lie in one plane (trigonal bipyramidal arrangement).
- Methanol CH₃OH (product) (मेथेनॉल CH₃OH (उत्पाद)) — OH is on the side opposite to where Cl was.
- Chloride ion Cl⁻ (leaving group) (क्लोराइड आयन Cl⁻ (अवशिष्ट समूह)) — Leaves with the bond pair of the old C–Cl bond.
- Inversion of configuration (विन्यास का प्रतिलोमन) — Like an umbrella turned inside out in a strong wind. For a chiral halide, SN2 always gives the inverted product.
- Rate law: second order (वेग नियम: द्वितीय कोटि) — Rate depends on both [CH₃Cl] and [OH⁻] — both are in the single (slow) step: bimolecular.
Steps
- HO⁻ (nucleophile) attacks the δ+ carbon of CH₃Cl from the side exactly opposite to Cl. Red curved arrows: the O lone pair goes to C; the C–Cl bond pair goes to Cl.
- Transition state: C–O bond partly formed, C–Cl bond partly broken, all three C–H bonds in one plane. Carbon is bonded to five atoms at once — unstable, cannot be isolated.
- CH₃OH and Cl⁻ form in the same single step (no intermediate). The three H atoms have flipped to the other side: inversion of configuration.
Original diagram drawn by Simple Learning Hub, based on NCERT Fig 6.2. Report a mistake