Markovnikov Regioselectivity
Markovnikov Regioselectivity - Related Videos
02:20
Regioselectivity of Electrophilic Additions to Alkenes: Markovnikov's Rule
0 Views •
2026
If a set of reactants can yield multiple constitutional isomers, but one of the isomers is obtained as the major product, the reaction is said to be regioselective. In such reactions, bond formation or breaking is favored at one reaction site over others. The hydrohalogenation of an unsymmetrical alkene can yield two haloalkane products, depending on which vinylic carbon takes up the halogen. However, one product usually predominates, where hydrogen adds to the vinylic carbon bearing the...
02:35
Regioselectivity of Electrophilic Additions-Peroxide Effect
0 Views •
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...
01:17
Radical Anti-Markovnikov Addition to Alkenes: Mechanism
0 Views •
2025
The reaction of hydrogen bromide with alkenes in the presence of hydroperoxides or peroxides proceeds via anti-Markovnikov addition. The radical chain reaction comprises initiation, propagation, and termination steps. The mechanism starts with chain initiation, which involves two steps. In the first chain initiation step, a weak peroxide bond is homolytically cleaved upon mild heating to form two alkoxy radicals. In the second initiation step, a hydrogen atom is abstracted by the alkoxy radical...
02:36
Regioselectivity and Stereochemistry of Hydroboration
0 Views •
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
0 Views •
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...
FAQs
The deciding step is formation of the carbocation intermediate during protonation. The orientation that places the positive charge on the more stable, more substituted carbon is favored, so that pathway leads to the major constitutional isomer. This intermediate-based reasoning explains why product prediction depends on comparing possible carbocations rather than simply counting the alkene’s carbon atoms.
The number of hydrogens already attached to each alkene carbon determines where protonation is favored. Hydrogen adds to the carbon that already bears more hydrogen atoms, leaving the developing carbocation at the more substituted position. A nucleophile can then bond at that position, establishing the preferred regiochemical arrangement of the product.
Its two alkene carbons are not equivalent, so the reagent can add in two different orientations. These alternatives place hydrogen and the incoming nucleophile on opposite carbon frameworks and therefore create different constitutional isomers. Markovnikov regioselectivity identifies which arrangement is favored under typical ionic conditions by evaluating the resulting carbocation stability.
First, identify the two carbons of the unsymmetrical alkene and compare how many hydrogens each bears. Place hydrogen on the carbon with more hydrogens, then place the halide or hydroxyl-containing nucleophile on the more substituted carbon. Finally, compare the resulting structure with the possible constitutional isomer to select the favored product.
The principle is especially useful for hydrohalogenation and acid-catalyzed hydration of unsymmetrical alkenes. In hydrohalogenation, the halide favors the more substituted carbon; in acid-catalyzed hydration, hydroxyl occupies that position. Recognizing the reaction class tells you which incoming group to track after determining the preferred orientation of addition.
It helps chemists anticipate product identity before carrying out a reaction, particularly when an unsymmetrical alkene could produce more than one constitutional isomer. The predicted placement of a halide or hydroxyl group also supports comparison with anti-Markovnikov pathways. This distinction is useful when selecting or interpreting an addition reaction for a desired structure.