Only iodine, however, forms positive
ions, as in I+NO 3 –.
halonA compound obtained by re-
placing the hydrogen atoms of a hy-
drocarbon by bromine along with
other halogen atoms (see halocar-
bons), for instance halon 1211 is bro-
mochlorodiÛuoromethane (CF 2 BrCl)
and halon 1301 is bromotriÛuoro-
methane (CF 3 Br). Halons are very sta-
ble and unreactive and are widely
used inÜre extinguishers. There is
concern that they are being broken
down in the atmosphere to bromine,
which reacts with ozone, leading to
depletion of ozone layer, and their
use is being curtailed. Although more
chloroÛuorocarbons are present in
the atmosphere, halons are between
three and ten times more destructive
of ozone.
halothane(1-chloro-1-bromo-2,2,2,-
triÛuoroethane) A colourless, non-
Ûammable oily liquid, CHBrClCF 3 ;
b.p. 51°C. It smells like trichloro-
methane and has a sickly burning
taste. Halothane is widely used as a
general anaesthetic, often adminis-
tered also with oxygen and dinitro-
gen oxide.
Hamiltonian Symbol H. A function
used to express the energy of a sys-
tem in terms of its momentum and
positional coordinates. In simple
cases this is the sum of its kinetic
and potential energies. In Hamilton-
ian equations, the usual equations
used in mechanics (based on forces)
are replaced by equations expressed
in terms of momenta. This method
of formulating mechanics (Hamilton-
ian mechanics) wasÜrst introduced
by Sir William Rowan Hamilton
(1805–65). The Hamiltonian operator
is used in quantum mechanics in the
*Schrödinger equation.
Hammett equation An equation
relating the structure to the reactiv-
ity of side-chain derivatives of aro-
matic compounds. It arises from a
comparison between rate constants
for various reactions with the rate of
hydrolysis of benzyl chloride on the
one hand and a comparison between
equilibrium constants (such as the
dissociation constant of benzoic acid)
on the other hand. The Hammett
equation can be written in the form
log(k/k 0 ) = ρlog(K/K 0 ), where log(K/K 0 )
refers to comparing dissociation con-
stants to the dissociation constant,
K 0 , of benzoic acid in water at 25°C,
and log(k/k 0 ) refers to comparing
rates of reaction to the rate, k 0 , of hy-
drolysis of benzyl chloride. The term
log(K/K 0 ) = σis called the substituent
constant, since the nature of the sub-
stituent affects the strength of the
benzoic acid. If σis positive, the sub-
stituent is electron attracting, while
if σis negative the substituent is elec-
tron donating. ρis a reaction con-
stant, which is determined for a
given reaction by the slope of a
graph of log(k/k 0 ) against σ. The nu-
merical value of ρdepends on tem-
perature and the type of solvent.
The Hammett equation applies to
meta- and para- substituents (pro-
vided that resonance interaction
from the substituents does not occur)
but not to ortho-substituents.Hantz–Widman systemA system
for naming heterocyclic compounds,
independently introduced for nam-
ing 5- and 6- membered nitrogen
monocyclic compounds by A.
Hantzsch (1887) and O. Widman
(1888). Subsequently it was extended
to other ring sizes and other hetero-
atoms. It is based on preÜxes denot-
ing the hetero atoms(s) (e.g. aza- for
nitrogen, thia- for sulphur, oxa- for
oxygen), and items denoting ring
size and saturation. For example, 4-
membered rings have etefor unsatu-
rated rings and etanefor saturated;263 Hantz–Widman system
h