CC Brδ Br
+ δ−
C
Br
+ Br −
+ +
CC
Br+
Br −
Bromonium ion
onium ion a positively charged bromine atom is bonded to two
carbon atoms by twwo pairs of electrons: one pair from the π bond of the
alkene, the other pair from the bromine atom (one of its unshared pairs) ⇒ all
the atom onium ion have an octet of electrons.
- In the second step, the bromide ion produced in step 1 attacks the back side of one
ing
the three-membered ring.
bromide ion acts as a nucleophile while the positive bromine of the
bromonium ion acts as a leaving group.
8.7 STEREOCHEMISTRY OF THE ADDITION OF HALOGENS TO
- Anti addition of bromine to cyclopentene:
i) In the brom
s of the brom
of the carbon atoms of the bromonium ion.
1) The nucleophilic attack results in the formation of a vic-dibromide by open
2) The
ALKENES
Br 2
H H CCl^4 + enantiomer
trans-1,2-Dibromocyclopentane
A bromonium ion formed in the first step.
Br H
H Br
1)
2) A bromide ion attacks a carbon atom of the ring from
bromonium ion.
3) Nucleophilic attack by the bromide ion causes inversion of the configuration of
the opposite side of the
