Organic Reaction Mechanisms.
1. Electrophilic addition to alkenes.
a. Addition of X2.
1. Mechanism.

Return to ORM Index. | More on the mechanism.

Mechanism Overview

(The red arrows are curved or curly arrows showing electron movement.)

Reaction involves two steps.

  • Step 1: (slow)

    The alkene pi (p) electrons react with a source of electron deficient Br.
    A cyclic bromonium ion is formed in which the positive charge is distributed over the three atoms in the ring. The most substituted carbon is the more positive of the two carbons, illustrated by the ofsetting of the + symbol.

    Note: although Br2 is non-polar, it effectively becomes polarized near the electron rich C=C bond.

  • Step 2:

    The electron deficient bromonium ion reacts with an electron rich Br- ion.

    Because of the cyclic nature of the cation, the bromide ion effectively substitutes at the most electropositive carbon (the more substituted) coming in from the opposite side of the molecule to the bromine already attached. (This attack from the back during a substitution process is very typical.) As a consequence the two bromines add to opposite sides of the double bond, a so-called anti addition process. (See examples below.)

More on the mechanism. | Animated mechanism: 1. Br2 and trans-but-2-ene. | Animated mechanism: 2. Br2 and cyclopentene.


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Reagent(s) and conditions
(yield: %)
(fraction formed: %)

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Note the bromines are trans to each other.



Note the chlorines are trans to each other.

Date created: 2005 06 24.