Chemistry Reference
In-Depth Information
Scheme 7.1 Addition of bromine to ethylene.
Scheme 7.2 Molecular bromine reacts with double bonds, generating a bromonium ion and a
bromide anion.
of this species, a bromide anion is displaced. The initial reaction between bromine and
ethylene is illustrated in Scheme 7.2 using arrow pushing.
Once the bromide anion becomes liberated from its parent molecular bromine, it is free
to act as a nucleophile. Due to the positive charge residing on the bridged bromonium ion,
the adjacent carbon atoms now possess partial positive charges. This is due to the positively
charged bromine pulling electron density from the carbon atoms. The electrophilic nature
of the adjacent carbon atoms is illustrated in Scheme 7.3 using resonance structures.
Because the carbon atoms are now electrophilic, they are susceptible to reaction with
the bromide anion that has dissociated as shown in Scheme 7.2. As illustrated in
Scheme 7.4, using arrow pushing, this sequence of events leads to the formation of
1,2-dibromoethane.
Scheme 7.3 Bromonium ions possess electrophilic carbon atoms.
Scheme 7.4 Nucleophilic reaction between a bromide anion and a bromonium ion generates 1,2-
dibromoalkanes.
