117
chapter seven
Gases
7.1 Introduction
Many organic reactions require the use of gases, either inert gases that are
used to protect a reaction or reagent gases that participate in the reaction.
Special experimental techniques are required for handling gases, and this
chapter contains a summary of methods for the preparation, handling,
and measurement of the most commonly encountered gases.
It must be emphasized at the outset that many gases are very haz-
ardous, either because they are toxic or because they are supplied in
cylinders that contain compressed gas at very high pressures, and, as a
result, when manipulating gases particular attention must be paid to
safe practice as outlined in this chapter.
7.2 Use of gas cylinders
A large number of gases are commercially available, and details about
individual gases are provided in Sections 7.5 and 7.6. The purpose of this
section is to describe recommended methods for handling the containers
in which these gases are supplied.
1,2
Most gases are supplied in pressurized metal cylinders of sizes rang-
ing from 50 cm × 350 cm lecture bottles to 0.25 m × 1.5 m gas cylinders.
Some gases with higher boiling points are supplied at lower pressures
in relatively light metal cylinders. The ttings on the cylinders may vary
depending on the supplier and the gas; hence, it is essential to comply
with the supplier’s instructions regarding ttings and accessories.
Most cylinders are tted with a head unit containing an on-off valve,
and an outlet screw tting where a pressure regulator must be attached
(Figure 7.1). This head unit should never be tampered with. It is a weak
point in the cylinder and can be dislodged or damaged if the cylinder is
dropped. Apart from releasing a potentially dangerous gas, any damage
to the head unit can cause the highly pressurized gas to vent uncontrol-
lably, converting the cylinder into an extremely dangerous missile. For
this reason, cylinders and lecture bottles should never be allowed to
stand unsupported. They should always be securely clamped to a bench
or a wall. If they must be moved frequently, they should be supported in
118 Advanced practical organic chemistry
a sturdy metal frame or in a trolley designed for this purpose. Cylinders
should only be moved in purpose-designed trolleys and should always be
treated with great care.
Cylinders are generally pressurized to 175–200 atm, and the on-off
valve provides no more control than its name suggests. So cylinders must
always be tted with a pressure regulator before use. A two-stage pressure
regulator (Figure 7.2) is normally used with large gas cylinders. Pressure
regulators provide a constant outlet pressure that can be adjusted to suit a
particular application. If you need precise control over outlet pressure, it is
a good idea to t an additional needle valve to the regulator outlet. Avery
useful accessory is a multiway needle valve outlet (Figure 7.2) thatallows
more than one apparatus to be connected to the cylinder at one time.
7.2.1 Fitting and using a pressure regulator on a gas cylinder
To t a pressure regulator to a gas cylinder, you need to do the following:
1. Ensure that you have the correct regulator for the gas cylinder in
question. Different gas cylinders have different ttings, and it is
essential to ensure that the gas does not come in contact with incom-
patible materials. So the appropriate regulator, tted with the cor-
rect threaded connector, must be used. Gas suppliers should provide
detailed information on which regulators are compatible with which
cylinder. For safety reasons, it is best to choose a regulator that has
a fairly low delivery pressure (0–50 psi) unless a high output pres-
sure is specically required (1 atm = 1.01 bar = 14.5 psi [lb·f/in.
2
] =
1.03 kg·f/cm
2
= 760 mmHg).
2. Remove the protective plastic cap from the cylinder tting and
ensure that the tting is clean and dry. Never use grease or PTFE
tape on cylinder ttings.
Gas cylinder
Cylinder valve
Screw fitting for regulator
Figure 7.1 Gas cylinder head unit.

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