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Reactions of Alkenes (Halogenation)

Stereochemistry of Alkene Halogenation

Stereochemistry
Alkenes react with either bromine or chlorine to form the corresponding dihalides in an anti-addition mode.

The stereochemistry of halogen addition is unequivocally determined by the anti addition and the almost completely restricted rotation of the carbon-carbon bond of the halonium ion. Therefore, the bromination of cis-2-butene yields a racemate of (2R,3R)- and (2S,3S)-dibromobutane, whereas the bromination of trans-2-butene yields the meso compound.

Fig.1
Bromination of cis-butene.
Fig.2
Bromination of trans-butene.
Hint
Using the illustrations below, determine if the products of bromine addition to cis- and trans-butene are enantiomers or identical compounds (meso).
Tab.1
Products of cis-butene bromination
Fig.3
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Fig.4
Fig.5
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Fig.6
Tab.2
Products of trans-butene bromination
Fig.7
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Fig.8
Fig.9
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Fig.10

Addition of bromine to cis-butene produces an achiral bromonium ion. Subsequent attack of the bromide ion following either pathway a or b (Fig.1) gives the same product because the bromonium ion is symmetrically substituted. The product of the reaction is the racemate of the enantiomeric products (2R,3R)-2,3-dibromobutane and (2S,3S)-2,3-dibromobutane. In contrast, the addition of bromine to trans-butene as shown in Fig. 2 yields a chiral intermediate bromonium ion possessing no mirror plane. However, in this special case the products (2S,3R)-2,3-dibromobutane and (2R,3S)-2,3-dibromobutane are meso compounds and therefore identical. Regardless whether the reaction takes pathway a or b, the same product (with a mirror plane) is always formed, meso-2,3-dibromobutane.

Hint
The addition of halogens to alkenes using starting materials with different stereochemistry (cis or trans) always yields products with different stereochemistry.

Exercise

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