Q3(c). Simmons–Smith; SOCl₂ on crotyl alcohol; Beckmann rearrangement

(i) Simmons–Smith cyclopropanation

Scheme: The zinc carbenoid delivers CH₂ to both carbons at once.

1. The zinc–copper couple inserts into a C–I bond: CH₂I₂ + Zn → ICH₂ZnI, a carbenoid (it behaves like singlet CH₂ but is not a carbene itself).
2. The carbenoid transfers CH₂ to the alkene in a concerted, three-centred (“butterfly”) transition state; ZnI₂ is released. Both new C–C bonds form on the same face: stereospecific syn addition.
3. The electron-rich tetrasubstituted alkene reacts well; product: 1,1,2,2-tetramethylcyclopropane (C₇H₁₄). No C–H insertion products form, unlike with uncomplexed CH₂.

(ii) Thionyl chloride and crotyl alcohol: SNi'

Scheme: Cl is delivered intramolecularly to C-3 while the double bond shifts to C1=C2 (allylic rearrangement).

1. The OH attacks S of SOCl₂; loss of HCl gives the alkyl chlorosulfite ROS(O)Cl.
2. In the absence of a base the chlorosulfite reacts intramolecularly: the Cl on sulfur is delivered to the far end of the allyl unit (C-3) through a six-membered cyclic transition state, while the C1–O bond breaks and SO₂ leaves (SNi').
3. Major product: 3-chlorobut-1-ene, CH₃CHClCH=CH₂, formed with allylic rearrangement. This SNi' course has been shown for both but-2-en-1-ol and but-3-en-2-ol, which each give the rearranged chloride.
4. If pyridine is added, uncombined Cl⁻ attacks C-1 of the chlorosulfite (SN2) and the unrearranged 1-chlorobut-2-ene is the main product.

(iii) Oxime formation and Beckmann rearrangement

Scheme: The group anti to the leaving oxygen migrates from C to N.

1. Hydroxylamine adds to C=O and water is lost: acetophenone oxime. The (E)-oxime, with OH anti to the bulky phenyl, is the main isomer.
2. PCl₅ converts the oxime OH into a good leaving group (-OPCl₄).
3. As the leaving group departs, the group anti-periplanar to it, the phenyl, migrates from C to N in one concerted step (Beckmann rearrangement), giving a nitrilium ion CH₃C#N+Ph. The migrating group keeps its configuration.
4. Water adds to the nitrilium carbon and the imidic acid tautomerises: product acetanilide, C₆H₅NHCOCH₃ (N-phenylacetamide).

Conclusion. (i) 1,1,2,2-tetramethylcyclopropane (Simmons–Smith, syn, concerted). (ii) 3-chlorobut-1-ene (SNi' with allylic shift; 1-chlorobut-2-ene with pyridine). (iii) acetanilide (Beckmann: phenyl anti to OH migrates).

Application. Simmons–Smith reactions make cyclopropane rings of drugs and pyrethroids; the Beckmann rearrangement of cyclohexanone oxime is the industrial route to caprolactam for nylon-6; acetaminophen (paracetamol) has been made by a Beckmann route from 4-hydroxyacetophenone oxime.