Weygand/Hilgetag Preparative Organic Chemistry

216 Formation of carbon-halogen bonds
water, drying it over K2CO3, and distilling it through a column at 135 mm Hg (b.p. 41.8 to
42.57135 mm). Cold distilled water hydrolyses only the ter/-butyl bromide, and the amount
of this in the original mixture can be determined by Volhard titration of the aqueous layers. 870
The secondary halides, which are relatively stable to cold water, are, however, unlike unbranched
primary halides, cleaved with loss of HX when shaken with alkali carbonate
solution, and again this reaction can be used for their approximate determination. Gas
chromatography is now available for exact analysis. 871
When an OH group in an allylic position is replaced by X, the possibility of
an allylic rearrangement (a) must not be overlooked. It depends on the reaction
medium and on the natures of R and R' whether reaction (a) or (b) predominates.
RCH—CH=CHR / nx RCH—CH=CHR / ^ RCH=CH—CHR'
(b) | (a)
X OH X
(c)
Allylic rearrangement of the halide products is also possible and further rearrangements
may occur in subsequent reactions, so that prediction of the structure
of the ultimate products becomes complicated. Allyl chlorides are more
stable than the corresponding bromides. Primary and secondary allyl chlorides
show little tendency to isomerize during the usual operations of purification. Allyl
halides are considerably more reactive than other alkyl halides, the iodides being
particularly reactive and unstable. For further information about the reaction
mechanism and numerous references see deWolfe and Young; 872 it must
suffice here to mention that the allylic rearrangement can be rendered complete
or entirely avoided by maintaining specific conditions. For instance, in the
following reactions allylic rearrangement can be achieved to the extent of 100%
by using SOC12 in dilute ethereal solution. 873 The last of these reactions is
particularly noteworthy since in general maximum conjugation is retained
in exchange reactions. 199 * 874
CH3—CH=CH—CH2OH > CH3—CHC1—CH=CH2
CH3—CHOH—CH=CH2
> CH3—CH=CH—CH2C1
C6H5—CH=CH—CH2OH > C6H5—CHC1—CH=CH2
If R, or R and R', in allyl alcohols of type RCHOH—CH-CHR' denote
C=C or C^C groups in suitable positions, the longest chain of conjugation
is formed with complete allylic rearrangement. 875 " 877
870 A. Michael and H. Leupold, Ann. Chem., 379, 263 (1911).
871 R. Kaiser, " Gas-Chromatographie," Akademische Verlagsgesellschaft Geest &
Portig K.-G., Leipzig, 2 nd ed., 1962.
872 R. H. DeWolfe and W. G. Young, Chem. Rev., 56, 801 (1956).
873 F. F. Caserio, G. E. Dennis, R. H. DeWolfe, and W. G. Young, /. Amer. Chem.
Soc, 77, 4182 (1955).
874 L. Claisen and E. Tietze, Ber. Deut. Chem. Ges., 58, 279 (1925).
875 A. Roedig and H.-J. Becker, Chem. Ber., 89, 1726 (1956).
876 C. Prevost and F. Bidon, Bull. Soc. Chim. France, 1955, 1408.
877 I. M. Heilbron and co-workers, /. Chem. Soc, 1945, 77.