equation, pV = nRT, where n is the
amount of gas in the specimen and R
is the *gas constant. The gas laws
wereÜrst established experimentally
for real gases, although they are
obeyed by real gases to only a limited
extent; they are obeyed best at high
temperatures and low pressures. See
also equation of state.
gasoholA mixture of petrol (gaso-
line) and alcohol (i.e. typically
ethanol at 10%, or methanol at 3%),
used as an alternative fuel for cars
and other vehicles in many coun-
tries. The ethanol is obtained as a
*biofuel by fermentation of agricul-
tural crops or crop residues, for ex-
ample sugar cane waste. Many cars
can also use a mixture of 85% ethanol
and 15% petrol, called E85. Ethanol-
based gasohol has a higher octane
rating and burns more completely
than conventional petrol, thus lower-
ing some emissions. However, the
ethanol can damage certain engine
components, such as rubber seals.
Methanol-based gasohol is more toxic
and corrosive, and its emissions in-
clude formaldehyde, a known car-
cinogen.
gas oilA high-density petroleum
fraction (between kerosene and lubri-
cating oil), whose molecules have up
to 25 carbon atoms. It is used as a do-
mestic and industrial heating fuel.
gasoline See petroleum.
gas-phase electrophoresis See
ion-mobility spectrometry.
gas thermometerA device for
measuring temperature in which the
workingÛuid is a gas. It provides the
most accurate method of measuring
temperatures in the range 2.5 to
1337 K. Using aÜxed mass of gas a
constant-volume thermometer meas-
ures the pressure of aÜxed volume
of gas at relevant temperatures, usu-
ally by means of a mercury manome-
ter and a barometer.Gattermann–Koch reaction A re-
action of the type C 6 H 6 →C 6 H 5 CHO,
used to introduce aldehyde groups
onto benzene rings. It is used in the
industrial manufacture of benzalde-
hyde. A mixture of hydrogen chlo-
ride and carbon monoxide is passed
through benzene using a Lewis acid
such as aluminium chloride as a cata-
lyst. An intermediate H–COC+is
formed and electrophilic substitution
takes place on the benzene ring.
There is a variation of the reaction,
sometimes simply called the Gatter-
mann reaction, in which the –CHO
group is substituted onto the ben-
zene ring of a phenol. In this reac-
tion hydrogen cyanide is used rather
than carbon monoxide. A mixture of
zinc cyanide and hydrochloric acid
produces the hydrogen cyanide along
with zinc chloride to act as the cata-
lyst. The electrophile is HCNH+
which produces an imine intermedi-
ate
C 6 H 5 OH →HOC 6 H 4 CH=NH
This then hydrolyses to the aldehyde
HOC 6 H 4 CHO. If an organic cyanide
(nitrite) RCN is used, a ketone is pro-
duced. The reaction was discovered
by Ludwig Gattermann and J.C. Koch
in 1897. The use of hydrogen cyanide
was reported by Gattermann in 1907.Gattermann reaction A variation
of the *Sandmeyer reaction for
preparing chloro- or bromoarenes by
reaction of the diazonium com-
pound. In the Gattermann reaction
the aromatic amine is added to
sodium nitrite and the halogen acid
(10°C), then fresh copper powder
(e.g. from Zn + CuSO 4 ) is added and
the solution warmed. The diazonium
salt then forms the haloarene, e.g.
C 6 H 5 N 2 +Cl–→C 6 H 5 Cl + N 2
The copper acts as a catalyst. The re-243 Gattermann reaction
g