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Elucidation of Reaction Mechanisms - Investigation of Stereochemistry

The Structure of the Electrophilic Intermediate in the Addition of Bromine to Alkenes

With the assistance of nucleophilic reagents, an electrophilic intermediate could be captured in the addition of bromine to alkenes. Thus its existence was proved. Nevertheless, the exact structure of this electrophilic intermediate remained unknown.

However, in the investigation of the products' stereochemistry, several conceivable intermediates or transition states may be distinguished. Three possible cases are dicussed below:

  • Hypothesis: the intermediate is a bromocarbenium ion. Due to the free rotation of the carbon-carbon single bond and the planar structure of the carbenium ion, the entry of the bromide anion (nucleophile) should not be preferred from any of the diastereotopic sides of the bromocarbenium ion in this case. In addition, the first bromine atom is capable of attacking the alkene at two different positions - namely the carbons of the double bond. In each case, there is an equal probability of attack from two different diasterotopic sides. Therefore, the reaction would yield the racemate.
Addition of bromine to alkenes with a hypothetical intermediate bromocarbenium ion.

Not all stereochemical variants mentioned in the text are shown.

  • Hypothesis: the intermediate is a bromonium ion. In the bromonium ion, one of the two diastereotopic sides is sterically shielded by the bromine atom. As a result, the attack of the bromide anion can proceed only from the back side. That is, only trans addition would occur. In the addition of bromine to trans-pent-2-ene, for instance, only two of the four conceivable stereoisomers, 2S,3R- and 2R,3S-dibromopentane, could be formed. When cis-pent-2-ene is the starting product, the reaction would yield 2R,3R- and 2S,3S-dibromopentane. Initially, the bromine molecule can attack the alkene from two different diastereotopic sides. The bromide anion can subsequently enter the bromonium ion only from one of the diastereotopic sides, though at two different positions. The end result of this reaction would be the formation of only two of the four conceivable stereoisomers (green framed structures).
Addition of bromine to trans-pent-2-ene with an intermediate bromonium ion.
Hypothetical concerted mechanism with cis addition of bromine in the addition of bromine to alkenes.

•  A concerted mechanism of the addition of bromine to alkenes was already excluded, as the intermediate could have been captured with nucleophilic reagents. A concerted mechanism would result in the cis addition of the bromine molecule to the double bond. When the same starting products are applied, only the two (the double bond can be attacked from two different diastereotopic sides) stereoisomers other than those found in the trans addition with the intermediate bromonium ion would therefore be formed. In the addition of bromine to trans-pent-2-ene, the concerted mechanism, in contrast to the "bromonium route", would yield 2S,3S- and 2R,3R-dibromopentane.

Practically speaking, only stereoisomers that result from a trans addition are obtained!

Therefore, the intermediate product in the mechanism of the addition of bromine to alkenes is obviously the bromonium ion.

The following movie supports the fact that the mechanism of a reaction cannot be elucidated by merely observing the reaction in a practical experiment. Usually, a series of different analytic investigations is required for the elucidation of a reaction mechanism instead.

Movie on the addition of bromine to cyclohexene.
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