halogen. This occurs with alkanes,
where the reaction involves free radi-
cals and requires high temperature,
ultraviolet radiation, or a chemical
initiator; e.g.
C 2 H 6 + Br 2 →C 2 H 5 Br + HBr
The halogenation of aromatic com-
pounds can be effected by elec-
trophilic substitution using an
aluminium chloride catalyst:
C 6 H 6 + Cl 2 →C 6 H 5 Cl + HCl
Halogenation can also be carried out
using compounds, such as phospho-
rus halides (e.g. PCl 3 ) or sulphur di-
halide oxides (e.g. SOCl 2 ), which react
with –OH groups. Such compounds
are called halogenating agents. Addi-
tion reactions are also referred to as
halogenations; e.g.
C 2 H 4 + Br 2 →CH 2 BrCH 2 Brhalogens(group 17 elements) A
group of elements in the *periodic
table (formerly group VIIB):Ûuorine
(F), chlorine (Cl), bromine (Br), iodine
(I), and astatine (At). All have a char-
acteristic electron conÜguration of
noble gases but with outer ns^2 np^5
electrons. The outer shell is thus one
electron short of a noble-gas conÜgu-
ration. Consequently, the halogens
are typical nonmetals; they have
high electronegativities – high elec-
tron afÜnities and high ionization en-
ergies. They form compounds by
gaining an electron to complete the
stable conÜguration; i.e. they are
good oxidizing agents. Alternatively,
they share their outer electrons to
form covalent compounds, with sin-
gle bonds.
All are reactive elements with the
reactivity decreasing down the
group. The electron afÜnity decreases
down the group and other properties
also show a change fromÛuorine to
astatine. Thus, the melting and boil-
ing points increase; at 20°C,Ûuorine
and chlorine are gases, bromine a liq-uid, and iodine and astatine are
solids. All exist as diatomic mol-
ecules.
The name ‘halogen’ comes from
the Greek ‘salt-producer’, and the el-
ements react with metals to form
ionic halide salts. They also combine
with nonmetals, the activity decreas-
ing down the group:Ûuorine reacts
with all nonmetals except nitrogen
and the noble gases helium, neon,
and argon; iodine does not react with
any noble gas, nor with carbon, ni-
trogen, oxygen, or sulphur. The el-
ementsÛuorine to iodine all react
with hydrogen to give the acid, with
the activity being greatest for
Ûuorine, which reacts explosively.
Chlorine and hydrogen react slowly
at room temperature in the dark
(sunlight causes a free-radical chain
reaction). Bromine and hydrogen
react if heated in the presence of a
catalyst. Iodine and hydrogen react
only slowly and the reaction is not
complete. There is a decrease in oxi-
dizing ability down the group from
Ûuorine to iodine. As a consequence,
each halogen will displace any halo-
gen below it from a solution of its
salt, for example:
Cl 2 + 2Br–→Br 2 + 2Cl–
The halogens also form a wide vari-
ety of organic compounds in which
the halogen atom is linked to carbon.
In general, the aryl compounds are
more stable than the alkyl com-
pounds and there is decreasing resis-
tance to chemical attack down the
group from theÛuoride to the iodide.
Fluorine has only a valency of 1, al-
though the other halogens can have
higher oxidation states using their
vacant d-electron levels. There is also
evidence for increasing metallic be-
haviour down the group. Chlorine
and bromine form compounds with
oxygen in which the halogen atom is
assigned a positive oxidation state.halogens 262h