SN1 mechanism: two steps via a carbocationSN1 क्रियाविधि: कार्बोधनायन से होकर दो पदSN1 mechanism: two steps via a carbocation

Class 12 Chemistry · Chapter 6 · interactive diagram

Parts and what they mean

• tert-Butyl bromide (3° halide) (तृतीयक-ब्यूटिल ब्रोमाइड (3° हैलाइड)) — NCERT's SN1 example; carried out in polar protic solvents (water, alcohol, acetic acid).
• C–Br bond pair moves to Br (C–Br बंध युग्म Br पर जाता है) — Heterolytic cleavage: C becomes positive, Br becomes Br⁻.
• tert-Butyl carbocation (CH₃)₃C⁺ (तृतीयक-ब्यूटिल कार्बोधनायन (CH₃)₃C⁺) — sp², planar; stabilised by three electron-releasing CH₃ groups — 3° carbocations are the most stable, so 3° halides react fastest by SN1.
• Carbocation in step II (पद II में कार्बोधनायन) — Flat, so the nucleophile can attack from either face.
• Bromide ion (ब्रोमाइड आयन) — Solvated (stabilised) by the protic solvent.
• Hydroxide ion (nucleophile) (हाइड्रॉक्साइड आयन (नाभिकरागी)) — Its concentration does not appear in the rate law — it acts only in the fast step.
• O lone pair → C⁺ (O का एकाकी युग्म → C⁺) — Forms the new C–O bond.
• tert-Butyl alcohol (CH₃)₃COH (तृतीयक-ब्यूटिल ऐल्कोहॉल (CH₃)₃COH) — 2-Methylpropan-2-ol.
• Rate law: first order (वेग नियम: प्रथम कोटि) — Depends only on [alkyl halide] — unimolecular rate-determining step.
• Energy profile of SN1 (SN1 का ऊर्जा आरेख) — Two humps (two transition states) with a valley between them for the carbocation intermediate. (Explanatory sketch, not to scale.)
• Transition state of step I (पद I की संक्रमण अवस्था) — Highest point → step I is the slowest (rate-determining).
• Carbocation intermediate (कार्बोधनायन मध्यवर्ती) — A real (short-lived) species in a valley — unlike a transition state.
• Transition state of step II (पद II की संक्रमण अवस्था) — Small hump: step II is fast.
• Activation energy of step I (पद I की सक्रियण ऊर्जा) — Larger than that of step II.
• Axes (अक्ष) — Energy vs progress of reaction.

Steps

1. Step I (slow, reversible): the polarised C–Br bond breaks heterolytically — the bond pair goes to Br. A planar tert-butyl carbocation and Br⁻ form. The energy comes from solvation of Br⁻ by the protic solvent.
2. Step II (fast): OH⁻ uses a lone pair to attack the carbocation, giving tert-butyl alcohol. Only step I decides the rate, so rate = k[(CH₃)₃CBr].

Original diagram drawn by Simple Learning Hub, based on the NCERT textbook. Report a mistake