Weygand/Hilgetag Preparative Organic Chemistry

106 Formation of carbon-halogen bonds
ethers. Addition of X~ as, e.g., NaCl or NaBr 17 depresses the formation of
halohydrins, but removal of X~ by, e.g., AgNO3 favors it.
\ /
I
/ \
(3)
Some olefins, for instance, tetraphenylethylene, polyhaloolefins, and maleic
anhydride, add halogen with difficulty or not at all. Completely dry ethylene
does not add bromide at all in the dark. For further information on the theory
of halogen addition and substitution see Gould 18 and de la Mare. 19
Addition of bromine to C=C bonds does not begin with electrophilic
addition of Br + ; nucleophilic mechanisms are possible. Thus, for instance,
addition of bromine in glacial acetic acid to maleic or fumaric acid, stilbene,
cinnamic acid, or tetrachloroethylene, which is normally slow, is catalysed by
LiBr or KBr. 20
3. Addition of halogen to simple unsaturated hydrocarbons
Straight-chain olefins of low molecular weight, namely, ethylene, propene,
«-butenes, and «-pentenes, undergo addition of Cl2 (a) on reaction therewith
at room temperature; however, olefins whose chains branch at the double
bond are substituted (bx) even at room temperature, the double bond being
retained as such. The former class can be substitutively chlorinated only at
higher temperatures (400-500°) (b2). 21
CH3—CH=CH2 -^> H3C—CHC1—CH2C1 (a)
H3C\ ch C1H2C\
>C=CH2 > >O=CH2 + HCI (bx)
H3C / H3C/
CH3—CH=CH2 — 2 -> C1H2C—CH=CH2 + HCI (b2)
In the liquid phase addition of the resulting HCI occurs as a competing reaction
to (bi), but this addition is much suppressed in the gas phase. The large amount
of energy liberated on addition of chlorine (about 40 kcal/mole) can induce
substitution by chlorine of the primary chlorine-addition products unless strong
17 E. M. Terry and L. Eichelberger, /. Amer. Chem. Soc, 47, 1067 (1925).
18 E. S. Gould, " Mechanismus und Struktur in der organischen Chemie," Verlag Chemie,
Weinheim/BergstraBe, 2nd ed, 1964, pp. (a) 617, (b) 622, (c) 634, (d) 715, (e) 889.
19 P. B. D. de la Mare, Quart. Jlev. (London), 3, 126 (1949).
20 R. A. Ogg, /. Amer. Chem. Soc, 57, 2728 (1935).
21 H. P. A. Groll and G. Hearne, Ind. Eng. Chem., 31, 1530 (1939).