Laboratory Methods of Organic Chemistry - Sciencemadness Dot Org

340 THE GKIGNAKD KBACTION
The simplest example of this type is decomposition by water :
HgC.Mg.I + HOH >• CH4 + HO.Mg.I.
Hence: in all experiments with Grignard reagents moisture must be
completely excluded. Alcohols, phenols, carboxylic acids, primary and
secondary amines, oximes, acetylene, etc., react in the same way as
water.
Since one reactive hydrogen atom always liberates one molecule of
hydrocarbon, the use of methyl magnesium iodide provides a serviceable
method for determining quantitatively the proportion of active
hydrogen by measuring the volume of methane liberated by a weighed
amount of a substance under investigation (Zerevitinofl). The process
is of considerable value for the determination of constitution. On the
method of carrying it out see p. 84.
In virtue of their great capacity for forming addition compounds
the Grignard reagents are primarily used for synthetic purposes. What
was formerly attained with the rather unmanageable zinc alkyls is
nowadays accomplished in a much wider domain by means of the
easily made Grignard reagents (K. Ziegler has recently also used organic
lithium compounds).
Quite generally addition to unsaturated systems takes place, e.g. to
>C=O, >C=N—, —C=N, — N=O, but >C=C< and—CsC—do
not react. The mechanism of the addition is as follows : the Grignard
reagent is added on in the form of its two components R and MgHal;
in the case of the C=O-double bonds the Mg-containing constituent
always unites with the oxygen, R always with the carbon.
If the action of methyl magnesium bromide on acetaldehyde be
taken as example, the following equation holds :
pTT
CH3.CO + CH3.Mg.Br > CH3.c/
H H M)—MgBr
The addition product is decomposed by water, thus :
/ 3 / 3
CH3.C< +H2O > CH3.C< +HO.Mg.Br.
H M)—MgBr H M)H
As a result, therefore, acetaldehyde is converted into isopropyl
alcohol. Quite generally it may be said that the Grignard reaction consists
in the addition to the unsaturated linkage—as H and R—of the
hydrocarbon corresponding to the halide used. In effect a " synthesising
hydrogenation " occurs.
The course of the following Grignard syntheses can thus be understood
without further comment: