Only iodine, however, forms positive
ions, as in I
+ NO 3
– .
halon A compound obtained by replacing the hydrogen atoms of a hydrocarbon by bromine along with
other halogen atoms (see halocarbons), for instance halon 1211 is bromochlorodiÛuoromethane (CF 2 BrCl)
and halon 1301 is bromotriÛuoromethane (CF 3 Br). Halons are very stable and unreactive and are widely
used in Üre 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, nonÛammable oily liquid, CHBrClCF 3 ;
b.p. 51°C. It smells like trichloromethane and has a sickly burning
taste. Halothane is widely used as a
general anaesthetic, often administered also with oxygen and dinitrogen oxide.
Hamiltonian Symbol H. A function
used to express the energy of a system in terms of its momentum and
positional coordinates. In simple
cases this is the sum of its kinetic
and potential energies. In Hamiltonian equations, the usual equations
used in mechanics (based on forces)
are replaced by equations expressed
in terms of momenta. This method
of formulating mechanics (Hamiltonian mechanics) was Ürst 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 reactivity of side-chain derivatives of aromatic 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 constants 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 hydrolysis of benzyl chloride. The term
log(K/K 0 ) = σ is called the substituent
constant, since the nature of the substituent affects the strength of the
benzoic acid. If σ is positive, the substituent is electron attracting, while
if σ is negative the substituent is electron donating. ρ is a reaction constant, which is determined for a
given reaction by the slope of a
graph of log(k/k 0 ) against σ. The numerical value of ρ depends on temperature and the type of solvent.
The Hammett equation applies to
meta- and para- substituents (provided that resonance interaction
from the substituents does not occur)
but not to ortho-substituents.
Hantz–Widman system A system
for naming heterocyclic compounds,
independently introduced for naming 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 heteroatoms. It is based on preÜxes denoting the hetero atoms(s) (e.g. aza- for
nitrogen, thia- for sulphur, oxa- for
oxygen), and items denoting ring
size and saturation. For example, 4membered rings have ete for unsaturated rings and etane for saturated;
263
Hantz–Widman system
h
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ions, as in I
+ NO 3
– .
halon A compound obtained by replacing the hydrogen atoms of a hydrocarbon by bromine along with
other halogen atoms (see halocarbons), for instance halon 1211 is bromochlorodiÛuoromethane (CF 2 BrCl)
and halon 1301 is bromotriÛuoromethane (CF 3 Br). Halons are very stable and unreactive and are widely
used in Üre 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, nonÛammable oily liquid, CHBrClCF 3 ;
b.p. 51°C. It smells like trichloromethane and has a sickly burning
taste. Halothane is widely used as a
general anaesthetic, often administered also with oxygen and dinitrogen oxide.
Hamiltonian Symbol H. A function
used to express the energy of a system in terms of its momentum and
positional coordinates. In simple
cases this is the sum of its kinetic
and potential energies. In Hamiltonian equations, the usual equations
used in mechanics (based on forces)
are replaced by equations expressed
in terms of momenta. This method
of formulating mechanics (Hamiltonian mechanics) was Ürst 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 reactivity of side-chain derivatives of aromatic 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 constants 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 hydrolysis of benzyl chloride. The term
log(K/K 0 ) = σ is called the substituent
constant, since the nature of the substituent affects the strength of the
benzoic acid. If σ is positive, the substituent is electron attracting, while
if σ is negative the substituent is electron donating. ρ is a reaction constant, which is determined for a
given reaction by the slope of a
graph of log(k/k 0 ) against σ. The numerical value of ρ depends on temperature and the type of solvent.
The Hammett equation applies to
meta- and para- substituents (provided that resonance interaction
from the substituents does not occur)
but not to ortho-substituents.
Hantz–Widman system A system
for naming heterocyclic compounds,
independently introduced for naming 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 heteroatoms. It is based on preÜxes denoting the hetero atoms(s) (e.g. aza- for
nitrogen, thia- for sulphur, oxa- for
oxygen), and items denoting ring
size and saturation. For example, 4membered rings have ete for unsaturated rings and etane for saturated;
263
Hantz–Widman system
h
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