Functional Classes II, Reactions - Organic Chemistry Concepts: An EFL Approach - page 115

FIGURE 7.19
Selected applications of metal alkoxides.
7.6.2 Alcohol Dehydration
Alcohols can lose the elements of water to give alkenes. However, the hydrox-
ide group is a good base and nucleophile. Therefore, it is a poor leaving group.
Because of this, elimination is done under acidic conditions. The acid acts as
a catalyst to protonate the alcohol -OH group. This gives a protonated alkyl-
oxonium ion intermediate. The leaving group is now a neutral water molecule.
Figure 7.20 shows this sequence for this E1 reaction.
FIGURE 7.20
Acid-catalyzed alcohol dehydration.
7.6.3 Alcohol Substitution
Figure 7.21 shows one of the most useful examples of alcohol substitution.
This reaction is used in the preparation of alkyl halides. This reaction is the
reverse of the nucleophilic substitution of halide by hydroxide, which we saw in
Section 7.5.2 . The poor leaving group property of -OH is again improved to
allow reaction to occur.
One way to improve the leaving group is to react the alcohol with concentrated
hydrohalous acids (HCl, HBr, HI). As with elimination in Figure 7.20 , protonation
of the alcohol oxygen gives an alkyloxonium ion. Loss of a neutral water molecule
gives the carbocation. This reacts with halide ions to give the alkyl halide product.
This process follows an S N 1 mechanism and needs a relatively stable carbocation.