has been used as a possible treat-
ment for a number of medical condi-
tions. It is used illegally in the form
of cannabis, which is a class C drug
in the UK. Tetrahydrocannabinol can
be detected by the Duquenois–
Levine test.
tetrahydrofuran (THF)A colour-
less volatile liquid, C 4 H 8 O; r.d. 0.89;
m.p. –65°C; b.p. 67°C. It is made by
the acid hydrolysis of polysaccha-
rides in oat husks, and is widely used
as a solvent.
tetrahydroxomonoxodiboric(III)
acidSee boric acid.
tetraoxophosphoric(V) acidSee
phosphoric(v) acid.
tetraoxosulphuric(VI) acidSee
sulphuric acid.
tetravalent(quadrivalent)Having
a valency of four.
texaphyrin A synthetic molecule
similar to a porphyrin but containing
Üve central nitrogen atoms rather
than four, thus increasing the size of
the central ‘hole’ and enabling larger
cations, such as cadmium, to be
bound stably. See also supramolecu-
lar chemistry.
thallium Symbol Tl. A greyish
metallic element belonging to
*group 13 (formerly IIIB) of the peri-
odic table; a.n. 81; r.a.m. 204.39; r.d.
11.85 (20°C); m.p. 303.5°C; b.p.
1457 ± 10 °C. It occurs in zinc blende
and some iron ores and is recovered
in small quantities from lead and
zinc concentrates. The naturally oc-
curring isotopes are thallium–203
and thallium–205; eleven radioiso-
topes have been identiÜed. It has few
uses – experimental alloys for special
purposes and some minor uses in
electronics. The sulphate has been
used as a rodenticide. Thallium(I)
compounds resemble those of the al-
kali metals. Thallium(III) compounds
are easily reduced to the thallium(I)
state and are therefore strong oxidiz-
ing agents. The element was discov-
ered by Sir William *Crookes in
1861.
A- Information from the WebElements site
theobromine (3,7-dimethylxan-
thine)An alkaloid, C 7 H 8 N 4 O 2 , struc-
turally related to caffeine. It is found
in small quantities in tea and larger
quantities in the cacao tree, hence its
presence in cocoa and chocolate. Like
theophylline it is used in treating cer-
tain respiratory conditions. Note that
the compound contains no bromine.
The name comes from the name of
the cacao tree, Theobroma cacao. See
also methylxanthines.
theophylline(1,3-dimethylxan-
thine)An alkaloid, C 7 H 8 N 4 O 2 ,
structurally related to caffein. It is
found in small quantities in tea and
is used medicinally for certain respi-
ratory conditions. See also methylxan-
thines.theorySee laws, theories, and hy-
potheses.thermal analysisA technique for
chemical analysis and the investiga-
tion of the products formed by heat-
ing a substance. In differential
thermal analysis (DTA) a sample is
heated, usually in an inert atmos-
phere, and a plot of weight against
temperature made. In differential
scanning calorimetry (DSC) heat is
added to or removed from a sample
electrically as the temperature is in-
creased, thus allowing the enthalpy
changes due to thermal decomposi-
tion to be studied.thermal capacitySee heat capac-
ity.thermal equilibriumSee equilib-
rium.523 thermal equilibrium
t