Functional Classes II, Reactions - Organic Chemistry Concepts: An EFL Approach - page 114

carbon to complete the reaction. The rate is dependent only on the substrate
concentration and not on the base concentration.
To avoid competition between elimination and substitution, elimination is usu-
ally done under E2 conditions. These conditions prevent any competition from
E1 and S N 1, both of which pass through the same carbocation intermediate. For
this reason, the elimination of alkyl halides uses strong bases to make sure the
E2 mechanism as shown in Figure 7.17 is followed.
FIGURE 7.17
Examples of E2 elimination reactions.
7.6 ALCOHOLS AND ETHERS
The alcohol functional group has different bonds which can react in various
ways. In Chapter 6 we discussed the acidity of the O-H bond. The elimination
of the components of water is called dehydration . If this elimination occurs
between adjacent carbon atoms, it gives an alkene. The replacement of the OH
group gives substitution. Finally, removal of H 2 across the alcohol C-O bond is
oxidation and gives a carbonyl C ] O product.
7.6.1 Making and Using Alkoxides
As discussed in Chapter 6, alkoxides are formed by the deprotonation of alco-
hols. Figure 7.18 shows one way to do this by reaction of an alkali metal, usually
Na, with an alcohol. This reaction gives off H 2 and is a visual test for the pres-
ence of an OH group.
FIGURE 7.18
Visual test for alcohols, alkoxide formation.
As Figure 7.19 shows, these oxyanions can have different purposes. They are
often used as bases for the elimination reactions in Section 7.5.3 . Alternatively,
they can be nucleophiles for the substitutions in Section 7.5.2 .
In Figure 7.19 , the substitution reaction shown is an example of a general reac-
tion which is called Williamson ether synthesis. In this reaction, an alkoxide
nucleophile is used to give either symmetrical or unsymmetrical ethers.