Functional Classes II, Reactions - Organic Chemistry Concepts: An EFL Approach - page 111

FIGURE 7.12
Chain building alkynide reaction sequence.
7.5 ALKYL HALIDES
As shown in Figure 7.1 , the highly polarized carbon-halogen bond makes alkyl
halides key compounds in functional group interconversions. This is because
alkyl halides can take part in several different reactions. The polarity of the
carbon-halogen bond leads to heterolytic replacement of the halide by a new
bond. This is usually done by nucleophilic substitution and elimination reac-
tions. Both of these reaction types can occur on the same alkyl halide substrates.
This is an example of competing reactions.
7.5.1 Carbon-Halogen Bond Polarity
The polarization of the carbon-halogen bond lets alkyl halides react with silver
nitrate, AgNO 3 , to give precipitates of silver halides. This is shown in Figure
7.13 , and is a simple visual test for alkyl halides.
FIGURE 7.13
Visual test for alkyl halides.
The formation of Grignard reagents is another important metal-based reaction
which relies on this bond polarity. As Figure 7.14 shows, magnesium is placed
between carbon and halide. This gives another organometallic reagent which is
a useful carbanion. These carbanions react as nucleophiles in reactions to give
new C-C bonds.
FIGURE 7.14
Formation of Grignard reagents.
7.5.2 Nucleophilic Substitution
Nucleophilic substitution is one of the major functional group reactions. It is
simply the replacement of a leaving-group ligand by an incoming nucleophile
ligand. There is no change in the nominal oxidation number at the carbon cen-
ter of interest. There is also no change in the bond order. Compare this with
addition, Section 7.3.1 , and elimination, Section 7.5.3 . The reactions are shown
by the equations in Figure 7.15 . The only difference between these two examples
is whether or not the nucleophile has a formal negative charge.