(a) How many signals would you expect in the ¹H NMR spectrum of above compounds ? Mark these protons. 10 marks (b) Compare the…

The spectroscopic and photochemical behaviour of the given structures follows from symmetry, ring strain, radical stability and cage effects.

(a) ¹H NMR signals. For 4-allyloxyacetophenone (A), the para-disubstituted ring has a mirror plane, so H-2/H-6 and H-3/H-5 are each equivalent; these give two aromatic signals, usually an AA′BB′ pair of doublets from ortho coupling. The acetyl methyl is one singlet. In the allyloxy chain, the OCH₂ protons are enantiotopic and equivalent in an achiral solvent, giving one doublet coupled to the vinyl CH; the internal vinyl CH is one multiplet, often a ddq; and the terminal =CH₂ protons are nonequivalent (cis and trans to the CH), giving two dd signals. Thus A shows seven signals. Mark them as Hₐ = Ar-H ortho to COCH₃, H_b = Ar-H ortho to O-allyl, H_c = COCH₃, H_d = OCH₂, H_e = CH=, H_f/H_g = =CH₂. For B, 1,2-dimethylcyclopropane, the count depends on the stereochemistry actually drawn. If the common cis isomer is shown, the two methyl groups are equivalent (6H, doublet), the two substituted ring CH protons are equivalent (2H), and the C-3 CH₂ protons are diastereotopic, one cis and one trans to the methyls, so B gives four signals. If the drawing is trans, the C-3 CH₂ protons are related by a C₂ axis and are equivalent, so B gives three signals; the methyl and ring-CH assignments remain the same. The usual cis drawing therefore gives four signals.

(b) C=C stretching. Methylenecyclopropane shows a higher C=C stretching frequency (about 1650 cm⁻¹) than methylenecyclobutane (about 1620 cm⁻¹). Both have an exocyclic terminal C=C and similar reduced mass, so the difference is mainly force constant; since ν ∝ √(k/μ), a higher wavenumber means a larger force constant. In methylenecyclopropane the three-membered ring is highly strained; the bent C–C bonds force the ring carbon to use orbitals with greater s-character for the exocyclic double bond, strengthening and shortening the C=C and raising the force constant. In methylenecyclobutane the ring strain is smaller, the exocyclic C=C is less strengthened, and the stretching frequency is lower. Conjugation is absent in both, so ring strain/rehybridization controls the order.

(c)(i) A and B. 2 CH≡CH dimerizes in the presence of Cu₂Cl₂/NH₄Cl to A, vinylacetylene (but-1-en-3-yne), CH₂=CH–C≡CH. The Cu₂Cl₂/NH₄Cl system promotes coupling of acetylene molecules. Addition of HCl to the terminal alkyne gives a resonance-stabilized allenyl cation; chloride capture at the terminal carbon gives B, 4-chloro-1,2-butadiene, CH₂=C=CH–CH₂Cl. B is not the final rubber monomer; it rearranges by an allylic/allenic shift to the more stable conjugated diene 2-chloro-1,3-butadiene, chloroprene, CH₂=CCl–CH=CH₂. The rearranged product polymerizes without a catalyst by 1,4-addition to give neoprene, poly(chloroprene), –[CH₂–CCl=CH–CH₂]–ₙ.

(c)(ii) Rubber embrittlement. Rubber objects contain unsaturated C=C bonds and allylic C–H bonds. Exposure to air, light and heat initiates radical autoxidation: an allylic H is abstracted, O₂ adds to the allylic radical to give peroxyl and hydroperoxide species, and these decompose to carbonyls, chain ends and crosslinks. Chain scission reduces molecular weight and elasticity, while crosslinking makes the material stiff; hence rubber balls crack. Polyethylene has a saturated –CH₂–CH₂– backbone, no C=C and no weak allylic C–H bonds. Oxidation requires stronger C–H cleavage and does not generate the same conjugated peroxide chain reactions, so it remains comparatively flexible.

(d) Camphor reduction. Camphor is a rigid bicyclic ketone. The C-7 gem-dimethyl bridge and the C-4 methyl group shield the face of the C-2 carbonyl that is syn to the bridge. LiAlH₄ therefore delivers hydride preferentially from the less hindered exo face, opposite the bridge. The new C–H bond is formed on that exo face, while the alkoxide/oxygen remains on the bridge side; after work-up the OH is cis to the bridge, giving isoborneol as the major (about 90%) product. The minor product, borneol, arises from attack on the more hindered face. This steric approach control is consistent with the Cieplak model: the nucleophile approaches anti to the larger σ* orbital of the adjacent C–C bond, and in camphor that preferred trajectory coincides with the less hindered exo approach. The rigid framework prevents conformational averaging, so facial selectivity is high.

(e) Photochemical products. (i) In the vapour phase, hν excites acetone to an n,π* state. The vapour-phase reaction follows the Norrish Type II cleavage pattern (excited-state C–C cleavage with H-transfer): α-cleavage gives CH₃• and CH₃CO•, and subsequent H-transfer gives methane. Because the vapour is dilute, there is no solvent cage and the radicals escape. Acetyl radicals recombine to biacetyl: 2 CH₃CO• → CH₃COCOCH₃. Methyl radicals abstract H from another acetone molecule: CH₃• + CH₃COCH₃ → CH₄ + CH₃COCH₂•, so methane is formed. Thus the characteristic vapour-phase products are biacetyl and methane. (ii) At room temperature acetone is liquid, and the same excited-state chemistry is trapped in a solvent cage. Intermolecular H-transfer, for example (CH₃)₂C=O* + (CH₃)₂C=O → (CH₃)₂C•–OH + CH₃COCH₂•, gives ketyl radicals, (CH₃)₂C•–OH. The cage effect suppresses radical escape and favours coupling of two ketyl radicals at carbon: 2 (CH₃)₂C•–OH → (CH₃)₂C(OH)–C(OH)(CH₃), pinacol. Hence liquid-phase/room-temperature photolysis gives pinacol as the major product, whereas vapour-phase photolysis gives cage-escape products such as biacetyl and methane.