47 Preparation
In the Alkenes^1 section, we already covered a few methods for synthesizing alcohols. One
is the hydroboration-oxidation of alkenes and the other is the oxymercuration-reduction of
alkenes. But there are a great many other ways of creating alcohols as well.
A common source for producing alcohols is from carbonyl compounds. The choice of car-
bonyl type (ketone, aldehyde, ester, etc.) and the type of reaction (Grignard addition or
Reduction), will determine the product(s) you will get. Fortunately, there are a number of
variations of carbonyls, leading to a number of choices in product.
There are primarily two types of reactions used to create alcohols from carbonyls: Grignard
Addition reactions and Reduction reactions. We’ll look at each type of reaction for each
type of carbonyl.
47.0.1 Grignard Addition Reactions
As we learned previously, Grignard reagents^2 are created by reacting magnesium metal with
an alkyl halide (aka haloalkanes^3 ). The magnesium atom then gets between the alkyl group
and the halogen atom with the general reaction:
R-X + Mg→R-Mg-X
In our examples, we’ll be using bromine in our Grignard reagents because it’s a common
Grignard halogen and it will keep our examples a little clearer without the need for X.
Figure 131 Mechanism of Grignard reagent reacting with a carbonyl
The general mechanism of a Grignard reagent reacting with a carbonyl (except esters)
involves the creation of a 6-membered ring transition state. The pi bond of the oxygen
attacks a neighboring magnesium bromide which in turn, releases from its R group leaving a
carbocation. At the same time, the magnesium bromide ion from another Grignard molecule
is attacked by the carbocation and has its magnesium bromide ion stolen (restoring it to
its original state as a Grignard reagent). The second molecule’s carbocation is then free
to attack the carbanion resulting from the vacating pi bond, attaching the R group to the
carbonyl.
At this point, there is a magnesium bromide on the oxygen of what was a carbonyl. The
proton from the acidic solvent easily displaces this magnesium bromide ion and protonates
1 Chapter 51 on page 191
2 Chapter45.2on page 177
3 Chapter 42 on page 165