Regioselectivity Reversal
Regioselectivity Reversal - Related Videos
02:35
Regioselectivity of Electrophilic Additions-Peroxide Effect
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2023
In the presence of organic peroxides, the addition of hydrogen bromide to an alkene yields the isomer that is not predicted by Markovnikov’s rule. For example, the addition of hydrogen bromide to 2-methylpropene in the presence of peroxides gives 1-bromo-2-methylpropane. This addition reaction proceeds via a free radical mechanism, which reverses the regioselectivity. The free radical reaction mechanism involves three stages: initiation, propagation, and termination. In the first initiation...
02:36
Regioselectivity and Stereochemistry of Hydroboration
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2023
A significant aspect of hydroboration–oxidation is the regio- and stereochemical outcome of the reaction. Hydroboration proceeds in a concerted fashion with the attack of borane on the π bond, giving a cyclic four-centered transition state. The –BH2 group is bonded to the less substituted carbon and –H to the more substituted carbon. The concerted nature requires the simultaneous addition of –H and –BH2 across the same face of the alkene giving syn stereochemistry. The observed preference in...
01:33
Regioselective Formation of Enolates
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2023
As depicted in the figure below, the unsymmetrical ketones can form two possible enolates: less substituted or more substituted enolates. Usually, the thermodynamic enolates are formed from the more substituted α-carbon atom, while the kinetic enolates are formed faster by deprotonation from the less substituted position. The thermodynamic enolates have lower energy, so they are more stable. But the energy required to form kinetic enolates is less. This regioselectivity in enolate formation...
02:34
Regioselectivity and Stereochemistry of Acid-Catalyzed Hydration
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2025
The rate of acid-catalyzed hydration of alkenes depends on the alkene's structure, as the presence of alkyl substituents at the double bond can significantly influence the rate. The reaction proceeds with the slow protonation of an alkene by a hydronium ion to form a carbocation, which is the rate-determining step. The reaction involving a tertiary carbocation intermediate is faster than a reaction proceeding through a secondary or primary carbocation. This can be justified by comparing their...
04:38
Preparation of Contiguous Bisaziridines for Regioselective Ring-Opening Reactions
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2022
Contiguous bisaziridines containing non-activated and activated aziridines were synthesized by asymmetric organocatalytic aziridinations and then subjected to chemoselective ring-opening reactions under acidic or basic conditions. The non-activated aziridine ring opens with less reactive nucleophiles under acidic conditions, whereas the activated aziridine ring opens with more reactive nucleophiles under basic conditions.
FAQs
At the mechanistic level, the reaction can proceed through more than one pathway, with each pathway favoring bond formation or bond cleavage at a different site. Regioselectivity reversal occurs when altered conditions change the relative energies of those pathways. The newly favored route then directs formation of a different constitutional isomer, even though the transformation remains the same or closely related.
Reagents, catalysts, solvents, temperature, and substrate activation can each alter the balance between competing pathways. These factors may change how readily particular bonds form or break, thereby changing the relative favorability of reaction sites. Comparing outcomes under different conditions helps identify which variables control the selectivity and whether a reversal is mechanistically plausible.
A changed product ratio provides evidence that the reaction does not follow one unchanging preference under all conditions. Instead, the result reflects condition-dependent competition between pathways with different relative energies. Studying this behavior connects observed constitutional isomers to mechanistic changes and helps chemists distinguish a genuine shift in reaction control from an apparently contrasting outcome.
A practical investigation begins by identifying the unsymmetrical substrate and the reaction sites that could lead to different constitutional isomers. Chemists then compare the transformation under deliberately varied reagents, catalysts, solvents, temperatures, or activation conditions. Evaluating which product is favored in each set of conditions reveals whether changing the pathway alters the preferred site.
The principle is useful when conventional conditions favor a regioisomer that is poorly suited to the desired synthesis. By selecting conditions that shift the relative energies of competing pathways, chemists can target an alternative site and access a different constitutional isomer. This flexibility supports reaction planning and can improve the construction of complex molecules.
Contrasting products show that the reaction outcome depends on more than the substrate’s initial structure. They indicate that reagents, catalysts, solvent, temperature, or substrate activation may have redirected the preferred pathway. In chemistry, this comparison links product identity to condition-dependent mechanism and provides a basis for choosing conditions that favor the required regioisomer.