Laboratory Methods of Organic Chemistry - Sciencemadness Dot Org

EBPLACBMBNT OF HYDKOXYL BY HALOGEN 97
CH2.OH CH2.OH
CH.OH + 2HC1 = CH.C1 +2H2O .
CH2.OH CH2.C1
Glycerol Dichlorohydrin
Hydrogen iodide not only esterifies polyhydric alcohols but also
reduces them. Thus glycerol passes by way of 1 : 2 : 3-triiodopropane
into isopropyl iodide :
CH2OH.CHOH.CH2OH + 3HI = CH2I.CHI.CH2I + 3H2O
CH2I.CHI.CH2I + 2HI = CH3.CHI.CH3 + 2I2.
Similarly the tetrahydric erythritol and the hexahydric mannitol
are converted respectively into 2-iodobutane and 2-iodohexane. Formulate
! Naturally the reaction also takes place with hydroxyacids.
The second reaction proceeds much more energetically than the
first, especially if preformed phosphorus halide is used. This is, however,
not always necessary, at least not in the case of replacements by bromine
and iodine ; in many cases the procedure is rather to produce the halide
only during the reaction, either by dropping bromine from a separating
funnel into a mixture of alcohol and red phosphorus or, as above, by
adding finely powdered iodine. Like the former, this reaction can also
be applied to polyhydric and to substituted alcohols; indeed, it is
possible to replace all the OH groups by halogens, and in particular
also by chlorine.
In many cases the solid, more active and much less volatile pentachloride
is used instead of phosphorus trichloride. Then it is necessary
to use a whole mole of PC1S for each mole of alcohol since the reaction
leads to the formation of the much more 'sluggish phosphorus oxychloride,
e.g.
CH3.CH2OH + PC15 = CH3.CH2C1 + POC13 + HC1.
In recent years thionyl chloride has also been brought into use for
the same reaction ; it has the advantage that its decomposition products
are gaseous and hence do not interfere with the working up of the
reaction mixture.
C4H9.CH2OH + SOC12 = C4H9.CH2C1 + SO2 + HC1.
Amyl alcohol Amyl chloride
The more energetic action of the halide of phosphorus also shows
itself in the fact that even the hydroxyl groups of phenols can be replaced
by this reaction :
C6H6.OH + PC15 = C6H5.C1 + POCI3 + HC1,
Phenol (Br5) (Br) (Br,) (Br) .
H