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218 Halogen Chemistry

C. Saturated Polyhalides either HX or X 2 is lost with the resulting formation of
alkenes or alkynes.
There are a number of methods for introducing more than
one halogen into an organic molecule. Some of these pro-
cedures are selective, while others produce complex mix- D. Unsaturated (Nonaromatic) Halides
tures of products.
Unsaturated halogenated compounds containing double
Reaction of aldehydes or ketones with phosphorus
bonds can be classified into two groups: vinyl halides and
halides or other halogenating agents yields compounds
allyl halides. Vinyl halides (e.g., CH 2 CHCl) are those in
in which two halogens are attached to the same carbon
which the halogen is attached to a carbon atom bearing a
atom:
double bond. Allyl halides (e.g., ClH 2 C CH CH 2 ) con-
PCl 5 + CH 3 C( O)CH 3 → POCl 3 + CH 3 CCl 2 CH 3 . tain a halogen attached to a carbon which is one atom
(70) removed from the double bond. The preparations and
Alternatively, two HX molecules may be added to a chemical reactivities of these two types of compounds
carbon–carbon triple bond in an alkyne. Dihalides in differ because of the different spatial relationships of the
which the two halogens are attached to different carbons halogen atom to the site of unsaturation.
are formed by reaction of the diatomic halogen X 2 or in- Vinyl halides are typically prepared by three methods:
terhalogen XY molecule with double bonds in alkenes, or, (1) addition of HX, X 2 ,orHCX 3 to a triple bond, (2)
less effectively, by halogenation of compounds that con- elimination of HX (dehydrohalogenation) from polyhalo-
tain more than one alcohol group (glycols). Trihalogen genated alkanes, or (3) elimination of X 2 from polyhalo-
compounds, in which all three halogen atoms are on the genated alkanes. Dehydrohalogenation is often accom-
same carbon, can be prepared by addition of haloforms plished by heating with alkali, while elimination of X 2
(CHX 3 ) to alkenes: is carried out by heating with zinc dust in ethanol.
As discussed in Section IX.A, interaction of the un-
CH 2 CH 2 + HCX 3 → CH 3 CH 2 CX 3 . (71)
shared pairs of electrons on the halogen with the C C
Reaction (71) proceeds via the formation of two free radi- double bond makes the C X bond stronger than in or-
cals, CH 2 CH 2 CX 3 and CX 3 . Unless conditions are con- dinary alkyl halides and one of the carbon atoms in the
·
·
trolled carefully, coupling of the radicals may also occur, C C bond electron rich. As a result, vinyl halides do
resultinginamixtureofproductscontainingcarbonchains not undergo nucleophilic substitution reactions unless the
of different lengths. double bond is extensively halogenated. The most com-
Direct halogenation of saturated hydrocarbons using el- mon reactions of vinyl halides are (1) electrophilic ad-
emental chlorine and bromine has been previously men- dition of positive groups to the double bond, (2) free-
tioned in the preparation of monohalogenated alkanes. radical addition to the double bond, and (3) formation
(Direct iodination does not occur.) Analogous reactions of organometallic halides. Halogenated alkenes (such as
using larger halogen–hydrocarbon ratios can also be used vinyl chloride, CH 2 CHCl), which contain only two
to produce highly halogenated molecules. Fluorination is carbon atoms, readily polymerize to form long chains
so vigorous that there is almost no selectivity in replace- (polyvinyl chloride).
ment of the hydrogen atoms. (Some reactions involving Allyl halides are frequently prepared by reaction of HX
fluorine and adamantanes or steroids are very selective.) with allyl alcohols. Allyl halogens are very reactive be-
Bromination tends to occur more readily at tertiary hydro- cause loss of the halogen as X produces a particularly sta-
−
gen atoms, followed by secondary and primary hydrogens ble carbonium ion with partial positive charge distributed
in that order. Halogenation selectivity for chlorine is inter- over two sites:
mediate to that of fluorine and bromine. Improved selec- + +
CH 2 CH CH 2 ↔ CH 2 CH CH 2 . (72)
tivity for fluorination can be achieved by first preparing the
polychlorinated or polybrominated compounds and then Again, the double-headed arrow indicates that the actual
exchanging the chlorine or bromine for fluorine. state of the ion is between the two resonance forms. At-
The reactions of polyhalogenated alkanes are similar to tack by an electron-rich nucleophile may occur at either of
those of monohalogenated alkanes, except that the pres- the two positive carbons, making allyl halides more sus-
ence of more than one halogen on the same carbon re- ceptible to nucleophilic substitution of the halogen than
sults in a marked decrease in reactivity. For example, ordinary alkyl halides.
monofluorinated alkanes are quite unstable, but polyflu- Compounds containing halogen atoms directly joined
orinated alkanes are exceptionally resistant to chemical to triply bonded carbon (haloalkynes) are formed either
attack and thermal decomposition. Highly halogenated by (1) reaction between molecular halogen and a metallic
alkanes mainly undergo elimination reactions in which derivative of the alkyne,