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Regiospecific electrophilic addition to alkenes is a major issue in organic chemistry. The fundamental principle of regiospecific electrophilic addition is Markovnikov's rule. Despite the significance of this principle, the only reported experiment for organic chemistry that demonstrates Markovnikov addition is oxymercuration (1), which generates mercury metal. We have developed a microscale experiment based on the bromohydrin derived from 1-methylcyclohexene; it alleviates the generation of noxious material yet demonstrates unequivocally the fundamental concepts of Markovnikov addition. This experiment can be conducted in a single laboratory period and is simple in both procedure and equipment needed.
Theory
Markovnikov's rule (2) originally stated that the electrophilic addition of HX to an unsymmetrical alkene leads to attachment of the hydrogen to the less substituted carbon. In a more general application, the rule states that in the addition of an unsymmetrical reagent to a multiple bond, the positive portion of the reagent is introduced at the less-substituted carbon. This regioselectivity has been attributed to formation of the more stable, positively charged intermediate.
Bromohydrin formation resulting from addition of HO-Br to an alkene is regiospecific in the absence of steric restrictions (3). The mechanism of bromohydrin formation occurs in two steps as shown in Figure 1. (Figure 1 omitted) Electrophilic addition of the positively charged bromine atom to the alkene results in formation of the bromonium ion intermediate. Nucleophilic addition of water or hydroxide to this intermediate results in bromohydrin formation. In the case of an unsymmetrical bromonium ion, the hydroxyl group is introduced at the more-substituted carbon, which is best able to support positive charge in accordance with Markovnikov's rule.
Experimental Procedure
In a 5.0-mL conical vial...