ELECTROPHILIC ADDITIONS OF ALKENES AS THE
COUNTERPART OF ELIMINATIONS
INTRODUCTION
- Chapter 8 is mostly about alkene reactions. That is, how one can transform alkenes into other functional groups. Most
of these reactons are electrophilic additions, or the addition of electrophiles across the double bond. Several of these reactions amount to
addition of water to the p-bond. The result is the transformation of alkenes into alcohols. We now look at the generalities of this type of reaction
(electrophilic addition) and then we look at each reaction individually.
The C=C p-bond of alkenes is a source of electrons. It is considered a weak base or nucleophile. As such, it can react with strong electrophiles.
We have already learned how to identify electrophiles, but the p-bond requires strong electrophiles to react with. These can be strong proton
acids, or species containing atoms with incomplete octets (Lewis acids). Examples of strong proton acids are HBr and H2SO4. Examples of
Lewis acids are BH3, transition metal salts such as HgSO4, and carbocations.
The addition of strong electrophiles to the C=C p-bond can be viewed as the opposite of the elimination reaction, as illustrated below.
H3C C CH3
CH3
Br
E2
H3C
C
CH2
CH3
+HBr
Elimination results in net loss of HBr to form a new C=C bond.
Addition of HBr (a strong electrophile) across the p-bond
forms a new functional group, in this case an alkyl halide.
H3C
C
CH2
CH3
+HBr Addition
H3C C CH3
CH3
Br
