307 Chemistry Chapter 7 Miscellaneous SN2, SN1, E2, E1 (with some interesting previews) Divergent rates of the E2 reactions of cis- and trans-1-bromo4-tert-butylcyclohexane; one of the isomers reacts significantly faster than the other.
Would you like to venture an educated guess?
Regiochemistry of the E2 reaction
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Regiochemistry of the E2 reaction: the two stereo-isomers, cis- and trans-1-bromo-2-methyl-cyclohexane, generate different products by E2 elimination.
Would you venture an educated guess?
One isomer has only one eligible (anti-trans-) H Factors in the regiochemistry of the E2 reaction
Bulk of Alkoxid e Anions
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Regiochemistry of the E2 reaction with a bulky base
SN2 at two differently substituted centers
Stereochemistry of the E2 reaction the general case
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Stereochemistry of the E2 reaction which of 2 eligible Hs is chosen?
Carbocation rearrangements
Some reactions proceeding via carbocations lead to unexpected products. The "normal" substituted product is obtained in low yield and an unexpected product in higher yield.
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2 carbocations formed by dissociation of I from an iodoalkane may form more stable carbocations by rearrangement. Migration (1,2-shifts) of H or R yield "unexpected" substitution products.
[the symbol "
" signifies rearrangement] Both
carbocations react with methanol but the intramolecular rearrangement is faster than the intermolecular "capture" by the nucleophile. In order to understand mechanism the by which these rearrangements occur, we recall the concept of hyperconjugation stabilizing a free radical or a carbocation (cf., Figure 9.2). Hyperconjugation works by distorting or shifting the electrons of a CH bond in the direction of the electron-poor carbon. For the rearrangement the hydrogen migrates along with the electron pair; this can be described as a hydride shift. The positive charge ends up at the carbon from where the hydride left. Three different types of rearrangement have been documented: (i) 1,2-hydride shift
Rearrangement by 1,2-hydride shift is exemplified by the first reaction in this section.
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The second example shows that the 1,2-hydride shift can be followed by capture (SN1) or by deprotonation (E1). The cyclohexene derivative is a minor product.
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The curly arrow indicates random stereochemistry, as you had expected for a reaction involving a carbocation. (ii) 1,2-alkyl shift In some cases, secondary carbocations undergo rearrangement by 1,2-methyl shift, followed by either nucleophilic capture (SN1) or by deprotonation (E1).
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(iii) ring expansion releasing angle strain Bromoethylcyclobutane generates a secondary carbocation with three neighboring groups that can migrate. The 1,2-hydride shift generates a tertiary carbocation with increased ring strain; shift of either of the two equivalent alkyl groups forms a secondary carbocation with greatly reduced ring strain.
We will revisit all of these reactions in future chapters.
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