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Why secondary alkyl groups are more prone to shifting than primary? I attach example from Grossman book [p. 20]:

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When $\ce{R} = \ce{CH2CH2Ph}$, the coordinated $\ce{Al}$ simply transfers a $\ce{Me}$ group to the carbocation $\ce{C}$ (σ bond nucleophile). The $\ce{O}$ atom then coordinates another equivalent of $\ce{AlMe3}$ before the product is obtained upon workup.

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When $\ce{R} = \text{cyclohexyl}$, the $\ce{R}$ group migrates (1,2-alkyl shift) to give a new carbocation. (2° Alkyl groups are more prone to migrate than 1° alkyl groups.) After $\ce{Me}$ transfer to the new carbocation and coordination of another equivalent of $\ce{AlMe3}$, workup gives the product.

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This is quoted from "The Art of Writing Reasonable Organic Reaction Mechanisms", Second Edition by Robert B. Grossman, DOI: 10.1007/b97257.

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    $\begingroup$ I don't know what Grossman book is, and Google finds way too many results. Please add human-readable references and link through a stable URL, preferably a DOI. See here for more information. $\endgroup$ – Martin - マーチン Dec 15 '17 at 6:47

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