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Joined 2026-09-21 Message
Incoming links: Degree of unsaturation
Past exam of the chemistry B course of the University of Cambridge 2021 Paper 1 B1 a i Solution 2026-10-20
The product is 1,4-dimethoxy-2,5-bis(2-methylbutan-2-yl)benzene. Each new substituent is , a tert-amyl group, rather than the unrearranged secondary alkyl group of the alcohol. The two substituents are related by the symmetry of the ring.
The proton assignments are:
| chemical shift / ppm | Assignment | Reason |
|---|---|---|
| Two aromatic H, at positions 3 and 6 | symmetry makes them equivalent; neither has an adjacent aromatic H to give a resolved ordinary ortho splitting. | |
| Two methoxy groups, six H | Equivalent groups give a NMR singlet. | |
| Two ethyl groups, four H | Each couples to a terminal three-proton methyl group, giving a NMR quartet. | |
| Four methyl groups attached to the two quaternary benzylic carbons, twelve H | These methyl groups have no neighboring methine group proton and give a NMR singlet. | |
| Two terminal ethyl methyl groups, six H | Each couples to two methylene group protons, giving a NMR triplet with the same coupling constant as the NMR quartet. |
The integrals add to 30 H. The unusually shielded terminal ethyl methyl signal is compatible with an aryl-containing environment; its NMR quartet–NMR triplet partner and integral are the decisive assignment evidence. The molecular formula and degree of unsaturation, , fit a benzene ring with saturated side chains.
Original structures of the rearranged aromatic alkylation product and dialkylated beta-ketoester
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Complete constitution: cinnamoyloxymethyl at cyclohexene position 1 and a tertiary-alcohol substituent at position 4
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The constitution is [4-(2-hydroxypropan-2-yl)cyclohex-1-en-1-yl]methyl (E)-3-phenylprop-2-enoate, with absolute configuration at the saturated substituted ring carbon unspecified.
The OCH side chain is attached to the unprotonated alkene carbon 4: F correlates through the ring junction to the alkene carbons and an allylic methylene, while its correlation to 1 fixes the ester linkage. The side chain is attached to methine carbon 12, as shown by the methyl-to-12 HMBC correlations and the contiguous ring COSY network.
The formula check gives 19 carbons and 24 hydrogens including OH. Its degree of unsaturation is eight: four from phenyl, one ester carbonyl, one cinnamate C=C, one cyclohexene C=C and one ring. No additional double bond or hidden second carbonyl is needed.