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Halogen Chemistry 217

If an extensively unsaturated molecule is highly halo- Since the halogen atoms consumed in the second step [Eq.
genated, the electron-withdrawing effect of the halogens (63)] is regenerated in the third step [Eq. (64)], the process
may remove so much charge from the system that all of is called a chain reaction. In principle, one chlorine atom
the carbons become quite positive and susceptible to nu- could induce the chlorination of an infinite number of hy-
cleophilic attack. Both polychlorinated olefins and poly- drocarbon molecules through a cycle of reactions (63) and
chlorinated aromatics readily undergo nucleophilic fluo- (64). In practice, several chain-termination reactions also
rination by potassium fluoride, the reaction producing a occur that result in the destruction of the free radicals:
whole range of polyfluorinated compounds in good yields.
·
CH + Cl → CH 3 Cl; (65)
·
3
B. Saturated Monohalides Cl + Cl → Cl 2 ; (66)
·
·
There are three common methods for preparing com- CH + CH → CH 3 CH 3 . (67)
·
·
3
3
pounds containing only one halogen atom: (1) addition
of hydrogen halides, HX, to the C C double bond in an Because the halogen atoms are very reactive, more than
alkene(olefin),(2)replacementofhydroxylgroups( OH) one substitution product is usually formed, with the extent
in alcohols, and (3) direct halogenation of saturated hy- of substitution depending on the halogen–hydrocarbon ra-
drocarbons. The reactivity of HX for double bond addi- tio. If monohalogenation is desired, a large excess of hy-
tion decreases in the order HI > HBr > HCl > HF. With drocarbon is advantageous.
an unsymmetrical olefin, the halogen adds selectively to When alkyl halides react, the order of reactivity
the carbon with the smaller number of hydrogen atoms can generally be predicted by the relative ease with
(Markownikoff’s rule): which the respective carbon–halogen bonds are broken:
CH 3 CH CH 2 + HI → CH 3 CHICH 3 . (59) iodides > bromides > chlorides > fluorides, and tertiary
(R 3 C X) > secondary (R 2 CH X) > primary (RCH 2
Exceptions to Markownikoff’s rule occur, as when HBr X). One typical reaction is nucleophilic substitution of
reacts in the presence of oxygen. the halogen by electron-rich groups such as ammonia
Replacement of hydroxyl groups in alcohols is typically (NH 3 , to form amines), cyanide (CN ), hydroxide ion
−
carried out using anhydrous or concentrated aqueous so- [to form alcohols, Eq. (54)], alcohols (to form ethers),
lutions of hydrogen halides, phosphorus halides, thionyl hydride ion (H ), or even another halogen. Substitution
−
chloride (SOCl 2 ), or thionyl bromide: of one halogen for another provides a convenient route
R OH + SOCl 2 → R Cl + SO 2 + HCl; (60) for alkyl halide synthesis. Alkyl iodides are produced by
iodide replacement of chlorine or bromine using sodium
R OH + HX → R X + H 2 O. (61) or potassium iodide as the reagent. Alkyl fluorides can
In these equations, R symbolizes the alkyl group (the be made by the exchange of F for Br or Cl using reagents
carbon chain with all of its attached hydrogen atoms). such as antimony trifluoride or mercuric fluoride.
For reactions using hydrogen halides, the relative rates A second reaction typical of alkyl halides is elimina-
of reaction are HI > HBr > HCl > HF for the hydrogen tion of the halogen to regenerate the C C double bond.
halides and tertiary > secondary > primary for the alco- Halogen elimination occurs to some extent whenever alkyl
−
hol. Tertiary alcohols have three alkyl groups attached halides react with base ( − OH ), but tends to become the
to the hydroxyl carbon (R 3 C OH), secondary have two main reaction if the alkyl halide is tertiary:
(R 2 CH OH), and primary have one (RCH 2 OH). Var-
−
−
ious catalysts such as sulfuric acid or zinc chloride are (CH 3 ) 3 C I + OH → (CH 3 ) 2 C CH 2 + I + H 2 O.
sometimes added to increase the reaction rate. (68)
Direct halogenation of saturated hydrocarbons or alka- A third reaction typical of alkyl halides involves the
nes (compounds with no double or triple bonds) works formation of free radicals in the presence of very active
satisfactorily only with elemental chlorine and bromine. metals such as those of Groups 1 and 2. Subsequent re-
The reaction proceeds via the formation of halogen atoms action of the free radicals can lead to the formation of
[Eq. (62)], which are highly reactive free radicals (species organometallic compounds (an organic compound with a
containing an unpaired electron): metal substituent),
light
·
·
Cl 2 −→ 2Cl ; (62) R I + Mg → R + MgI → R Mg I. (69)
·
CH 4 + Cl −→ CH + HCl; (63)
·
·
3 or coupling of the alkyl radicals, R , to produce longer
·
·
·
CH + Cl 2 −→ CH 3 Cl + Cl . (64) carbon chains, R R.
3