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1: 1‐(2‐methylbutan‐2‐yl)cyclopentan‐1‐ylium; 2: 1‐tert‐butylcyclopentan‐1‐ylium

(1) should be more stable as the $\ce{CH3}$ group is electron donating. But apparently (2) is more stable. Is there another phenomenon taking place here?

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    $\begingroup$ As formulated one should expect 2 to be more stable. Point is eventually why is the ethyl fragment not stabilising more than a methyl, as usually found and as resumed here chemistry.stackexchange.com/questions/47008/… $\endgroup$
    – Alchimista
    Mar 4, 2019 at 16:15
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    $\begingroup$ This is getting absurd. I quite wonder how detached from reality these textbook exercises would get. For all practical purposes there no difference here. You can argue about "inductive effect" vs "hyperconjugation" however long you wish... $\endgroup$
    – Mithoron
    Mar 4, 2019 at 17:43
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    $\begingroup$ On the same line of @Mithoron though the example might be of interest in a proper contest (e.g. computational approach) it seems to stretched simple rules to the limit. $\endgroup$
    – Alchimista
    Mar 5, 2019 at 8:36
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    $\begingroup$ So does that mean these structures simply cannot be compared? $\endgroup$
    – user638473
    Mar 5, 2019 at 11:58
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    $\begingroup$ No - you YES can. But they should be rather the same and whatever difference should be experimentally seen than explained, not the vice versa. It is the weight of subtle hyperconjugation that might differentiate between. $\endgroup$
    – Alchimista
    Mar 5, 2019 at 16:06

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