Question

# Identify the chemical which is used to detect leakage of LPG:                  (A) Methyl mercaptan                  (B) Propyl mercaptan                  (C) Ethyl mercaptan                  (D) n-butyl mercaptan

Hint: This gas has a very distinct odour for easy detection; it has a thiol (SH) group in its formula making it more volatile than alcohol (due to more H bonding) and can be toxic at high concentrations. It is also used in the mining industry as ‘stench gas’.

Complete answer: -LPG (Liquefied petroleum gas) is a mixture of hydrocarbon gases which is flammable and is used as a fuel in vehicles and cooking equipment. It is a non- renewable source of energy and is extracted from crude oil and natural gases.
The hydrocarbon gases which are constituents of LPG have 3 to 4 carbon atoms, which means that normal constituents of LPG are propane (${C_3}{H_8}$) and butane (${C_4}{H_{10}}$). Other hydrocarbons may also be present in small concentrations.
-Being flammable it burns readily in air and the amount of energy it produces is similar to that of petrol and twice of what is produced by natural gas.
-LPG is usually stored in liquid form under very high pressures. The boiling point of LPG is below room temperature and so it rapidly evaporates at normal temperatures and pressures, thus it is transported in pressurized steel vessels. The contained liquid undergoes thermal expansion and so the vessel is filled up to only 80-85% of its capacity.
-A very powerful odorant ethyl mercaptan also known as ethanethiol and ‘stench’ is also added to LPG so that any leaks can be easily detected. It is a liquid organosulfur compound having a very distinct odour and the chemical formula of: $C{H_3}C{H_2}SH$. It has an ethyl group attached to a thiol group (SH) and its structure is similar to that of ethanol with just S atom replacing O atom. Ethyl mercaptan has a distinguished ability to form H bonds which makes it more volatile than ethanol. It can be toxic also at high concentrations.
-The odour of ethyl mercaptan is distinct enough for us to detect easily even at small concentrations. Its threshold or smallest concentration at which it can be detected by us is around 2.8 billion parts of air (0.36 parts per billion).
-It is known as stench gas because of its use in the underground mining industry. During any kind of emergency this gas is released into mine ventilation ducts to alert the workers inside.

Note: If the level of LPG and air is within explosive limits and there is a source of ignition, LPG can cause a large explosion LPG in air displaces the oxygen present leading to decrease in oxygen level, which makes it toxic. Also, LPG cannot be substituted for natural gas because natural gas is lighter than it due to presence of methane and ethane also LPG has a higher calorific value than natural gas. But it can be mixed with air to produce synthetic natural gas.