Functional Classes II, Reactions - Organic Chemistry Concepts: An EFL Approach - page 108

Table 7.1 Common Alkene Addition Sequences
Reagent Process Product

HCl, HBr, HI Hydrohalogenation Alkyl halides

H 2 O (as H 3 O + ) Hydration Alcohols

HCN Hydrocyanation Alkyl cyanides (nitriles)

Cl 2 , Br 2 , I 2 Halogenation Alkyl halides

The halogenation of alkenes is an important laboratory and industrial process.
As shown in Figure 7.7 , the addition of bromine acts as a fast visual test for the
presence of unsaturation.

FIGURE 7.7
Visual chemical test for unsaturation.

The mechanisms in Figures 7.4 and 7.6 both have polar reagents in which it
is easy to pick out the electrophile. In the halogenation reaction in Figure 7.7 ,
the symmetrical diatomic reagent does not have an electrophilic end to react
in the stepwise addition. The model in Figure 7.8 explains how the initial
halogen molecule is first polarized by the π-bond electrons. The positive end
of this temporary dipole is the electrophile in the first step of the addition
reaction.

FIGURE 7.8
Temporary polarization during halogenation.

7.3.2 Reduction
The addition of hydrogen was defined as reduction in Chapter 5. With alkenes,
this addition is done with the help of metal catalysts. The most common cata-
lysts are palladium on carbon (Pd-C) and platinum oxide (PtO 2 ).
This reduction process is called hydrogenation and gives a saturated product
from an unsaturated one as shown in Figure 7.9 . The mechanism of this reaction
shows both hydrogen atoms adding to the same face of the double bond. This
means there may be a specific stereochemical result when the new sp 3 -hybrid
carbons are formed. Although this is not needed at this point, further detail is
given in Appendix 8.