Weygand/Hilgetag Preparative Organic Chemistry

228 Formation of carbon-halogen bonds
Formation of the desired alkyl halide then follows, this being analogous to the
second stage of the Arbusov reaction (d):
(RO)3P + R'X -^> (RO)3PR / X -^> (RO)2P(+ O)R' + RX (d)
The reaction is particularly advantageous for preparation of alkyl bromides
and iodides from sterically hindered or unsaturated alcohols. For preparative
purpose it is especially useful when R'X is benzyl bromide, methyl iodide,
bromine (Br2), or iodine (I2), but use of benzyl chloride and chlorine (Cl2)
is also significant; R' can usefully be H only when R'X is HCl. When the alcohol
is sensitive to heat, the quasi-phosphonium compound should be prepared
first and then allowed to react with the alcohol. Also, for use with unsaturated
alcohols in cases when R'X is Cl2, Br2, or I2 the crude (C6H5O)3PX2 should
be prepared as a separate stage. In general, however, the three reactantscan
be allowed to react together, as in (a).
Methyltriphenoxyphosphonium iodide: 957 Triphenyl phosphite (31 g) and methyl iodide
(21 g) are heated for 36 h under reflux (exclusion of water!); the temperature of the boiling
mixture rises from 70° to 115°. Addition of dry ether after cooling gives yellowish-brown
needles; these are repeatedly washed with dry ether and dried over phosphorus pentoxide
(42 g, 94%). For the following reactions, which must be carried out with careful exclusion of
water, it is best to use a crude product that has been stored under dry ether. The iodide can,
however, be purified by rapid dissolution in dry acetone and precipitation by dry ether,
then having m.p. 146°.
General procedure for preparation of alkyl iodides from (C6H5O)3PCH3I: 957 The appropriate
alcohol (1 mole) is added cautiously to the crude iodide (1 mole); cooling is necessary
for reactive alcohols. The method of working up is suited to the properties of the alkyl iodide:
(a) if the iodide boils lower than the phenol it can be distilled out directly in a vacuum;
(b) the phenol and alkyl iodide are distilled off together, the distillate is diluted with ether,
and the phenol is extracted with dilute, ice-cold NaOH solution; (c) the reaction mixture is
diluted with ether, the phenol is washed out with ice-cold, dilute NaOH solution, and water,
and the ethereal solution is dried and distilled.
Direct preparation of an alkyl iodide by reaction (a): neopentyl iodide, (CH3)3CCH2I: 957 ' 958
Methyl iodide (1.4 moles), triphenyl phosphite (1 mole), and neopentyl alcohol (1 mole) are
heated in a bath for 24 h, whereby, owing to continuing consumption of methyl iodide, the
internal temperature rises from 75° to 130°. The crude iodide and the phenol are distilled
off in a vacuum and the distillate is washed with ice-cold, dilute NaOH solution, and water
until free from phenol. The product contains about 6% of terf-pentyl iodide, 958 to remove
which it is shaken for 5 h with three times its volume of water which is then discarded. The
organic layer is next shaken with its own volume of O.lN-aqueous AgNO3, washed with
water, dried, and fractionated. This gives a 53-57 % yield of iodide of b.p. 71°/100 mm.
For numerous other examples see Rydon and his co-workers. 957 * 959
Preparation of (C6H5O)3PX2 and its direct reaction with alcohols: 960 Triphenyl phosphite,
b.p. 209-210°/l mm, is prepared by warming PC13 (1 mole) with phenol (3 moles) until no
more HCl is evolved and distilling the product in a vacuum. 961 Halogen (X2) is then added
with careful exclusion of moisture, stirring, and cooling. The alcohol (1 mole) is dropped into
the crude (C6H5O)3PX2 thus formed, with stirring at room temperature, and cooling if
necessary. The alkyl halide is then usually separated by vacuum distillation. Yields are 60 to
98% for saturated alcohols, 75% for allyl bromide.
Direct reaction of triphenyl phosphite, alcohol, and halogen (X2): 960 The halogen (X2)
is added gradually to an equimolar mixture of triphenyl phosphite and an alcohol at 0°.
After reaction for some time at room temperature, the alkyl halide is isolated, e.g., by direct
958 N. Kornblum and D. C. Iffland, /. Amer. Chem. Soc, 77, 6653 (1955).
959 A. Campbell and H. N. Rydon, /. Chem. Soc, 1953, 3004.
960 D. G. Coe, S. R. Landauer, and H. N. Rydon, /. Chem. Soc, 1954, 2281.
961 H. B. Gottlieb, /. Amer. Chem. Soc, 54, 749 (1932).