Laboratory Methods of Organic Chemistry - Sciencemadness Dot Org

96 EBPLACBMBNT OF HYDEOXYL BY HALOGEN
iodide, pour the latter into the distilling flask through a funnel
plugged with asbestos or glass wool. Ethyl iodide boils at 72°.
Yield, about 50 g. What percentage of the theoretical is this ?
Use in the synthesis of ethyl diethylmalonate and for Grignard
reactions.
Methyl iodide is prepared in a completely analogous manner.
Boiling point 44°. Used like ethyl iodide.
Remarks on 1 and 2.—These two reactions are special cases of a
general change, namely, the substitution of an alcoholic hydroxyl
group by a halogen atom. This substitution can be carried out in
two ways ; first, as in the preparation of ethyl bromide, 1, by acting on
alcohols with hydrogen halides, e.g.
C2H5.| OH + H|Br = H2O + C2H5.Br,
(HC1, HI)
or, second, as in the preparation of ethyl iodide, 2, by treating alcohols
with halogen compounds of phosphorus, e.g.
3C2H5.OH + PI3 = 3C2H5.I + PO3H3
(PCI3, PBr3).
The first reaction proceeds most easily with hydrogen iodide since
in many cases mere saturation with the gaseous acid suffices to bring
it about. Hydrogen bromide reacts with greater difficulty, and in its
case it is frequently necessary to heat the alcohol saturated with this
acid in a sealed tube. The preparation of ethyl bromide described
above, in which, the HBr is liberated from the potassium bromide by
means of concentrated sulphuric acid, constitutes a very smooth
example of this reaction.
Hydrogen chloride reacts most difficultly, and here it is necessary
(as, for example, in the preparation of methyl and ethyl chloride)
to use a dehydrating agent, preferably zinc chloride, or, as in the case
of the higher alcohols, to heat under pressure in a sealed vessel.
Aromatic alcohols, e.g. benzyl alcohol, are thus esterified by concentrated
halogen acids more easily than the aliphatic.
The reaction cannot be extended to phenols, however, but it can
be applied to di- and polyhydric alcohols. Here the number of hydroxyl
groups replaced depends on the experimental conditions, e.g. quantity of
halogen hydride, temperature, and so on, e.g.
CH2.OH CH2.Br
I +HBr = I +H2O,
CH2.OH CH2.OH
Ethylene glycol Ethylene bromohydrin