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

(a) Number of ¹H NMR signals

Compound A, 4-allyloxyacetophenone, has seven signals. The para-disubstituted ring has a mirror plane, so the two protons ortho to COCH₃ (Hₐ) are equivalent and the two ortho to O-allyl (H_b) are equivalent, giving two aromatic doublets (AA′BB′). Then: COCH₃, a singlet (H_c); OCH₂, a doublet (H_d); internal =CH, a multiplet (H_e); and the two terminal =CH₂ protons, non-equivalent because one is cis and one trans to the CH (H_f and H_g).

Compound B, 1,2-dimethylcyclopropane: the drawing shows no stereochemistry, so both isomers are given. In the cis isomer the two CH₃ groups are equivalent (one signal, 6H) and the two ring CH are equivalent (one signal, 2H). The ring CH₂ protons are diastereotopic, one cis and one trans to the methyls, so there are four signals. In the trans isomer a C₂ axis makes the CH₂ protons equivalent, so there are three signals.

(b) C=C stretching in methylenecyclopropane and methylenecyclobutane

Methylenecyclopropane absorbs at a higher wavenumber (about 1780 cm⁻¹) than methylenecyclobutane (about 1680 cm⁻¹). Both are exocyclic, unconjugated C=C with a similar reduced mass, so the difference comes from the force constant, since ν ∝ √(k/μ).

The bonds inside the three-membered ring are bent and use orbitals of high p-character. The ring carbon therefore uses orbitals of higher s-character for the exocyclic C=C, which makes it shorter and stronger and raises the frequency. Cyclobutane has less strain, so this effect is much smaller and the frequency is close to that of a normal terminal alkene. The wavenumbers are typical literature values.

(c) Acetylene to synthetic rubber, and ageing of rubber

(i) Cu₂Cl₂/NH₄Cl dimerises two acetylene molecules to A, vinylacetylene (but-1-en-3-yne), CH₂=CH–C≡CH. HCl adds 1,4 across the ene-yne to give B, 4-chlorobuta-1,2-diene, CH₂=C=CH–CH₂Cl. This allene rearranges by an allylic shift to the conjugated diene chloroprene (2-chlorobuta-1,3-diene), CH₂=CCl–CH=CH₂. Chloroprene polymerises, without added catalyst, by 1,4-addition to neoprene, –[CH₂–CCl=CH–CH₂]ₙ–.

(ii) Rubber has C=C bonds and weak allylic C–H bonds in its chains. In air, light and heat, O₂ abstracts allylic hydrogen and sets off radical autoxidation, and ozone attacks the C=C directly. Hydroperoxides form and decompose, cutting chains and forming cross-links. The result is a hard, brittle surface that cracks. Polyethylene is a saturated –CH₂–CH₂– chain with no C=C and no allylic C–H, so these reactions are very slow and it stays flexible.

(d) Reduction of camphor with LiAlH₄

Camphor is a rigid bicyclic ketone, so hydride reaches the C=O from one of two faces only. The exo face is shielded by the syn methyl group on the C-7 bridge, while the endo face is relatively open. Hydride therefore attacks from the endo face (steric approach control). The oxygen is pushed to the exo side, cis to the bridge, giving isoborneol (exo-OH) as the major product, about 90%. The minor product, borneol (endo-OH), comes from exo attack. The rigid skeleton permits no conformational change, so the face selectivity is high.

(e) Photolysis of acetone

In both cases acetone is excited to its n,π* state and undergoes Norrish Type I α-cleavage to CH₃• and CH₃CO•. There is no γ-hydrogen, so Type II is not possible.

(i) Vapour phase: there is no solvent cage and the radicals escape. The acetyl radical loses CO, and methyl radicals couple: CH₃CO• → CH₃• + CO, then 2CH₃• → C₂H₆. The main products are carbon monoxide and ethane, with some methane.

(ii) Room temperature (liquid): the radicals stay in the solvent cage and decarbonylation is slow, so acetyl radicals survive and dimerise to biacetyl (2CH₃CO• → CH₃COCOCH₃), with ethane from methyl coupling. The excited ketone can also abstract H from another acetone to give (CH₃)₂C•OH, which couples to pinacol. The product mix in the liquid depends on conditions; the commonly quoted products are given here.