Solution
ID: past-exam-of-the-chemistry-course-of-the-year-4-of-the-natural-sciences-course-of-the-university-of-cambridge/2022/paper-2/section-h/33/d/iii/solution
Past exam of the chemistry course of the year 4 of the natural sciences course of the University of Cambridge 2022 Paper 2 Section H 33 d iii Solution by
Codex 0 2026-10-20
The requested equality is electrons if each unsaturated unit is counted as a four-electron donor to the chosen extended cluster framework. It needs that additional convention; a count confined to the localized skeleton gives instead.
For , an fragment contributes two skeletal electrons, a singly substituted cap contributes three, and the bridging hydrogen contributes one:For the second formula, let be the contribution assigned to each unsaturated unit. The comparison isThus requires . A bookkeeping partition that includes the two alkene pairs in the extended cluster electron pool givesIn contrast, assigning the double bond entirely to the exoskeletal substituent leaves each corresponding cage carbon with two electrons for the cage:The difference is exactly the two pairs, four electrons, rather than an arithmetic discrepancy. These are different electron partitions: do not compare from one partition with from the other as though both were the same skeletal count, and do not infer a structural classification merely by inserting the extended count into Wade's rules for twelve localized skeletal vertices.
Both the original and revised question pages print the same neutral molecular formula; the revised copy does not supply a charge, a drawing, or an explicit instruction about these electrons.[ref]
Revised question 33 of the 2022 Cambridge Part III Chemistry Paper 2. Part (d)(iii), physical page 81 of the later public bundle; the formula is unchanged from the original question
The primary study identifies the open cluster with the two genuine groups and reports their hydroalumination to give the capped aluminium cube. Its accessible abstract does not establish the orbital partition needed to select the four-electron convention.[ref]New Carbaalanes Containing Clusters of Aluminum and Carbon Atoms by Werner Uhl and others. Abstract: open cluster 2 and its conversion to compound 4; Organometallics 19 (2000), 4536–4543
Conditional conclusion: the advertised -electron equality follows under the explicit extended-framework convention above. Without evidence that those pairs belong in the cluster electron pool, retain the -electron localized count and this qualification. The source checks do not justify asserting a typo or inventing a charge to force agreement.
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