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Halogen Chemistry 219

RC CNa + I 2 → RC Cl + NaI, (73) There are three main methods for introducing halogens
directly onto an aromatic ring. The first involves elec-
or by (2) elimination of X 2 or HX from halogenated alka- trophilic substitution of hydrogen by a halogen (Cl, Br, or
nes or alkenes. The products of these reactions tend to be I) using elemental halogen, hypohalous acids, halogenated
unstable, and the lower molecular weight compounds are amides, or hypohalites as the halogenating agent:
prone to explosion. In general, stability of the haloalkynes
increasesintheorderF < Cl < Br < I.Theparticularinsta- 5C 6 H 6 + 2I 2 + HIO 3 → 5C 6 H 5 I + 3H 2 O; (76)
bility of the fluoroalkynes has been attributed to electron-
FeCl 3
pair repulsions between the unshared pairs on the fluorine C 6 H 6 + Br 2 −−−→ C 6 H 5 Br + HBr. (77)
atom and the adjacent triple bond.
Due to resonance effects similar to those shown in Ferric chloride, aluminum chloride, or boron trifluoride
Eq. (55), a halogenated alkyne is more resistant to nu- may be used as a catalyst in chlorination and bromination.
cleophilic substitution than halogenated alkanes. Conse- Nitric acid is added in iodination.
quently, many nucleophiles react with haloalkynes first All four halogens can be introduced into the ring via
by addition to the triple bond and then by elimination of a diazonium salt, a method involving replacement of an
hydrogen halide: amino group ( NH 2 ). The aromatic amine (Ar NH 2 ,
where Ar stands for any aromatic ring system) is converted
ClC CCl + NH 3 → ClCH C(NH 2 )Cl
to a diazonium salt (Ar N + NX ), and the diazonium
−
group is then replaced with halogen using cuprous chlo-
→ HCl + ClC CNH 2
ride, cuprous bromide, or potassium iodide. Fluoride sub-
→ CH 2 ClC N. (74)
stitution is produced by heating the diazonium salt in the
In Eq. (74), ammonia (the nucleophile) adds to dichloro- presence of fluoroborate ion, BF . Very pure products are
−
4
acetylene.Theinitialproduct,1,2-dichloroaminoethylene, produced because substitution of the halogen occurs only
then loses HCl to give an alkyne, which isomerizes to at the position originally occupied by the amino group.
give chloroacetonitrile as the final product. A third method involves loss of HX or X 2 from poly-
In principle, nucleophilic attack can occur at several halogenated cyclohexanes or cyclohexenes in which the
different locations in a haloalkyne: at the halogenated car- six-membered alkane or alkene ring is transformed into a
bon in the triple bond [Eq. (75b)], at the halogen itself six-membered aromatic benzene ring. Heating in aqueous
[Eq. (75a)], and at the second carbon in the triple bond alcohol and base leads to elimination of HX and heat-
[Eq. (75c)]. All three of these possibilities are observed ing with zinc or iron in ethanol removes X 2 to produce a
in reactions with haloalkynes and sodium methoxide in mixture of halogenated aromatic products.
methanol. The mechanisms of some of these substitution As discussed in Section IX.A, the presence of one Cl,
reactions are not clear, and acetylide ion intermediates, Br, or I decreases the reactivity of the benzene ring and
−
RC C: may be involved. directs further substitution into the ortho and para posi-
Haloalkynes also undergo spontaneous polymerization tions. Aryl halides are much more resistant to nucleophilic
reactions. In some cases, these reactions can be regulated attack than are alkyl halides, unless the halogen on the ring
so that trimerization to halogenated benzenes or benzene is activated by the presence of other electron-withdrawing
derivatives occurs. groups in the molecule (e.g., nitro, NO 2 , ortho or para to
the halogen) or the reaction conditions are especially se-
vere (high temperatures and the presence of strong base).
E. Aromatic Halides
The presence of fluorine also directs further substitution
Halogenated aromatic compounds are derived from exten- to the ortho and para positions, but does not deactivate the
sively unsaturated benzene-type (benzenoid) systems and ring.
are classified into two groups: aryl halides in which the The residual hydrogens in aryl halides are generally
halogen is attached directly to a ring carbon, and halides susceptible to electrophilic attack and can be halogen-
in which the halogen forms part of an alkyl group attached ated, nitrated (substitution by NO 2 ), or sulfonated (sub-
to the ring (an alkyl side chain). The preparation and reac- stitution by SO 2 ). In highly halogenated ring sys-
tivity of the two groups of compounds are quite different. tems, residual hydrogens become acidic because of the

C 6 H 5 C CH (75a)

− + (75b)
C 6 H 5 C CX + CH 3 O: Na → C 6 H 5 C COCH 3 + C 6 H 5 CH CXOCH 3

C 6 H 5 C(OCH 3 ) CHX (75c)