Ethyl mercaptan (ethanethiol, C₂H₅SH) is a sulfur compound with a very strong smell. It is added in tiny amounts to LPG, the propane and butane used in camping stoves, barbecues and bottled or tank gas, so that a leak can be smelled before it becomes dangerous. Without it, those fuels would have almost no smell at all.
This article explains what ethyl mercaptan is, how its molecule is built, why gas smells and what is really added to the gas in British homes. It then sets out the official UK steps to take if you smell gas, and finishes with a quiz and answers to common questions.
What is ethyl mercaptan (ethanethiol)?
Ethyl mercaptan is the common name of a colourless, highly volatile liquid. Its systematic (IUPAC) name is ethanethiol, and you will see both names used for the same substance. Its molecular formula is C₂H₆S, usually written C₂H₅SH or CH₃CH₂SH, and its relative formula mass is about 62.1. Its CAS registry number, used to identify chemicals in databases, is 75-08-1.
Ethanethiol belongs to a family of organosulfur compounds called thiols. Every thiol contains the –SH group, known as the thiol or sulfhydryl group. You can think of ethanethiol as the sulfur twin of ethanol: ethanol is C₂H₅OH, and ethanethiol is what you get when the oxygen atom in the –OH group is swapped for sulfur. The prefix "thio" simply means "sulfur".
Compare: C₂H₅OH (ethanol) → swap O for S → C₂H₅SH (ethanethiol)
The smell is the reason ethyl mercaptan matters. The US National Institute for Occupational Safety and Health describes it as having a "strong, skunk-like odor". Others, including the UK gas company SGN, describe the smell as organic, like decaying vegetation. Either way, it is hard to ignore, and that is exactly the point.
Ethanethiol structure and formula
A single formula can be written in several ways, and each one tells you something different. Exam questions often ask you to move between them, so it is worth being clear about all three.
Molecular, structural and condensed formula
The molecular formula, C₂H₆S, counts the atoms: two carbon, six hydrogen and one sulfur. It does not show how they are joined. The condensed formula, CH₃CH₂SH, groups each carbon with its hydrogens so you can see the chain: a CH₃ group, a CH₂ group, then the SH group. Chemists often write C₂H₅SH instead, which keeps the –SH separate so the functional group stands out. If you need a refresher on writing and reading formulae, see our guide on how to write a chemical formula.
The structural formula shows every atom and bond: H₃C–CH₂–S–H, with each hydrogen drawn on its carbon. All the bonds in ethanethiol are single covalent bonds.
Ethanethiol Lewis structure
A Lewis structure shows the outer-shell electrons as dots or lines. In ethanethiol, atoms share pairs of electrons in covalent bonds. Carbon forms four bonds and hydrogen forms one. Count the bonds and you get eight single bonds in total: five C–H bonds, one C–C bond, one C–S bond and one S–H bond.
Sulfur sits in group 6 of the periodic table, so it has six outer-shell (valence) electrons. In ethanethiol it uses two of them to form two single bonds, one to carbon and one to hydrogen. The other four electrons stay on the sulfur as two lone pairs. If you want to know more about the element itself, read about sulfur.
Why is it called a mercaptan?
The word "mercaptan" was coined by the Danish chemist William Christopher Zeise in the 1830s. It comes from the Latin mercurium captans, meaning "capturing mercury", because these compounds react with mercury to form insoluble mercury salts. Zeise also prepared ethanethiol in 1834, which is the first thiol synthesis on record. Modern chemical names use the ending "-thiol" instead, which is why the systematic name is ethanethiol, but the older word is still common in the gas industry.
Physical properties of ethyl mercaptan
Ethyl mercaptan has a very low boiling point for a liquid, so it turns to vapour easily. That is a useful property for an odorant, because it spreads quickly through the air. The table lists the key values.
| Property | Value |
|---|---|
| Appearance | Colourless liquid |
| Odour | Strong, skunk-like |
| Boiling point | 35 °C (it is a gas above this temperature) |
| Melting point | About −148 °C |
| Flash point | About −48 °C (highly flammable) |
| Flammable range in air | 2.8 % to 18.0 % by volume |
| Odour detection threshold | About 0.00035 ppm (0.35 parts per billion) |
A boiling point of 35 °C is about the temperature of a very hot summer day, so at room temperature the liquid evaporates quickly and its molecules move into the air. The flash point of about −48 °C tells you that the pure liquid gives off enough vapour to ignite even in freezing conditions, so it is handled with great care in industry.
The odour threshold is the lowest concentration that a typical nose can detect. For ethanethiol that is about 0.00035 parts per million, which is 0.35 parts per billion. Another reference, the Merck Index, quotes an even lower figure of around 1 part in 50 billion parts of air. Sources differ because people differ and because tests are hard to standardise, but all agree it is astonishingly low. We look at what that means in practice in the next section.
Ethanethiol boils at 35 °C. Explain why a small spill quickly fills a room with its smell. (2 marks)
Its low boiling point means it evaporates easily at room temperature, so a lot of its molecules enter the air as vapour. These particles then spread out by diffusion until they reach the nose.
Why does gas smell? How gas odorants work
The natural gas that reaches many homes is mainly methane, and once it has been processed it has no smell. The propane and butane in LPG are also odourless. The US Energy Information Administration explains that companies therefore add a strong-smelling substance, a mercaptan, to natural gas so that leaks can be detected. The UK gas network SGN says the same about LPG: an odorant is added "so that any gas leaks can be smelled and reported".
The smell is a safety feature that is added on purpose. The fuel itself gives no warning, so the odorant does the job that your nose would otherwise be unable to do.
Why does such a small amount work? Ethanethiol can be detected at about 0.00035 parts per million, which is 0.35 parts per billion. Its molecules evaporate easily, then spread through the room by diffusion, so even a faint leak sends a few molecules to your nose. A trace is therefore enough.
Ethyl mercaptan in LPG
LPG stands for liquefied petroleum gas, a mixture of propane and butane that is stored as a liquid under pressure in cylinders and tanks. It is used for camping stoves, barbecues, caravans and homes that are not connected to the mains gas network. Because LPG is odourless, an odorant is added, and the usual choice is ethyl mercaptan. A Calor Gas (Ireland) safety data sheet for propane lists "small quantities (<0.1%) ethyl mercaptan" as the olfactory indicator, so the fuel is still almost entirely propane.
The smell is also not the same as the one many people expect. SGN describes the smell of ethyl mercaptan as "more organic and likened to the smell of decaying vegetation", and says it is the same smell as portable camping and barbecue gas cylinders. SGN made this change on its piped LPG network in Stornoway from 28 October 2019. Before that, the odorant there was tetrahydrothiophene, which SGN describes as having an "oily or tarry smell".
LPG is heavier than air
There is one more property of LPG worth knowing. Its vapour is heavier than air: butane vapour is about twice as heavy as air, and propane about one and a half times. Calor Gas advises that a leak "will sink to the lowest point and may remain for some time". So unlike a lighter gas, LPG does not just drift upwards and disappear.
A leak may also be spotted by other signs besides smell, including noise or frost at the source. Soapy water or a gas detector can also be used to identify a leak.
What is added to natural gas in the UK?
Here is where many web pages go wrong. They state that ethyl mercaptan is added to natural gas, but that is not what is added to mains gas in Great Britain. The law says that gas conveyed must have "been treated with a suitable stenching agent to ensure that it has a distinctive and characteristic odour" (Gas Safety (Management) Regulations 1996, Schedule 3). The only exception is for gas above 7 bar gauge pressure, such as in the high-pressure transmission system.
In practice, the stenching agent is a two-part odorant called Odorant NB ("New Blend"). It is mostly tert-butyl mercaptan (TBM, also called 2-methyl-2-propanethiol), about 80 %, with about 20 % dimethyl sulfide (DMS). So GB mains gas relies on a different thiol, not ethanethiol. Some countries do use ethyl mercaptan in natural gas, and it is widely used in LPG. To see why gas safety matters so much, you can read about the disadvantages of natural gas.
| Gas | Main substance | Odorant used in the UK | Smell |
|---|---|---|---|
| Mains natural gas | Methane | Odorant NB: tert-butyl mercaptan + dimethyl sulfide | Strong sulfur smell (often called “rotten egg” in the US) |
| LPG (camping, barbecue and other cylinders) | Propane / butane | Ethyl mercaptan | Decaying vegetation |
| Stornoway piped LPG, until 2019 | Propane | Tetrahydrothiophene | Oily or tarry |
The practical lesson is simple. Whatever the exact chemical, a sulfur-type smell in a place where gas is used should be treated as a warning. The next section sets out what to do.
Natural gas and LPG are odourless. Explain why an odorant is added and why only a tiny amount is needed. (3 marks)
An odorant is added so that leaks can be detected by smell before gas builds up. Odorants can be detected at very low concentrations (ethanethiol at about 0.00035 ppm), so a trace, less than 0.1 % in LPG, is enough. The fuel is still almost all propane or butane.
- I can name the odorant used in LPG (ethyl mercaptan).
- I can name the odorant used in GB mains gas (Odorant NB).
- I know why an odorant is needed (the fuel has no smell).
What to do if you smell gas (UK)
The smell of ethyl mercaptan or any other gas odorant is a warning, so act straight away. This section follows the official advice from the UK gas network (SGN), the Gas Safe Register and the Health and Safety Executive (HSE) for mains gas, and a supplier's advice (Calor) for LPG. It is educational information. In a real emergency, follow the instructions of your gas supplier and the emergency services.
Mains gas in Great Britain
If you smell gas from a mains supply, the official advice is as follows.
Do:
- Open windows and doors to ventilate the area.
- Turn off the gas supply at the meter control valve as well as gas appliances.
- Call the National Gas Emergency number, 0800 111 999. The call is free and the line is open 24 hours a day.
Do not:
- Smoke or use any naked flames.
- Touch any electrical switches. As SGN puts it, "turning a switch on or off could ignite a gas leak".
- Enter a cellar, even if your gas meter is in the cellar.
If you use Relay UK, use the Relay UK app with the prefix 18001 before 0800 111 999. Callers in other parts of the British Isles should use the numbers given by the Gas Safe Register: Northern Ireland 0800 002 001, Isle of Man 0808 1624 444, Guernsey 01481 749 000 and Jersey 01534 755 555.
Do not use a gas appliance that you suspect is unsafe. Leave it off until a Gas Safe registered engineer has checked it. The HSE states: "It is illegal for anyone to use a gas appliance if they suspect it is unsafe."
- Open windows and doors
- Turn off the gas at the meter control valve and turn off gas appliances
- Do not smoke or use naked flames
- Do not touch electrical switches
- Do not go into a cellar
- Call 0800 111 999 (free, 24 hours)
Bottled or tank LPG
LPG is not supplied through the mains network, so the Gas Safe Register's advice is to contact your LPG supplier directly. As one example of a supplier's advice, Calor Gas gives the following steps:
- Never search for leaks with a naked flame.
- Do not use electrical switches.
- Isolate the leak by turning off all valves on the tank or cylinders, and the emergency control valve outside the house.
- If the cylinder is portable, move it to a safe place if possible.
- Open all doors and windows.
- Do not use mobile phones in the affected area.
- Ring your supplier's emergency line (from outside the affected area). For Calor customers this is 03457 444 999.
- If there is a fire, call 999.
Remember that LPG vapour is heavier than air, as covered earlier, so it collects low down.
Carbon monoxide has no smell
The gas odorant only marks the fuel. It does not warn you about carbon monoxide, a poisonous gas. The NHS says carbon monoxide is "colourless and does not smell, so you cannot tell if it is around you". SGN says to call the same number, 0800 111 999, if you suspect carbon monoxide.
The NHS advice if you think you have carbon monoxide poisoning is to stop using the appliance if you can, go outside, and get medical advice as soon as possible. Do not go back in until you have got advice. This article cannot diagnose anything; if you or someone else feels unwell, contact NHS 111, or call 999 if it is serious.
Is ethyl mercaptan harmful?
There are two separate questions here, and it helps to keep them apart.
The odorant itself
Pure ethyl mercaptan is a chemical that workplaces control carefully. NIOSH recommends a ceiling exposure limit of 0.5 ppm (over 15 minutes). The US OSHA limit is a ceiling of 10 ppm, and the level classed as immediately dangerous to life or health is 500 ppm. Exposure can irritate the mucous membranes and cause headache and nausea.
Now compare those numbers with the odour threshold of 0.00035 ppm. Divide the workplace ceiling by the smell threshold:
0.5 ppm ÷ 0.00035 ppm ≈ 1,400
You can smell ethyl mercaptan at a concentration roughly 1,400 times lower than the NIOSH workplace ceiling. In other words, the nose gives a warning long before the level that the limit is designed to guard against. The amounts used as an odorant in fuel are tiny, and as the previous section showed, below 0.1 % of LPG.
The real danger is the fuel
The more serious danger in a gas leak is the fuel itself. Gas that has leaked into a room can form a flammable mixture with air, and a spark can ignite it. A switch, a lighter or a cigarette can each be enough, which is why the official advice bans all of them. To see what happens when fuels burn, read our complete lesson on combustion.
Ethanethiol is itself flammable. Its flash point is about −48 °C, and its vapour is flammable in air between 2.8 % and 18.0 % by volume. That is why the pure chemical is handled with care in industry.
A householder smells gas. Give two things they should not do and one thing they should do. (3 marks)
Any two of: do not smoke or use naked flames; do not touch electrical switches; do not enter a cellar. They should open the windows and doors, or turn off the gas at the meter control valve, or call 0800 111 999 (any one of these).
Uses of ethanethiol beyond gas
Most people meet ethyl mercaptan only as a smell, but chemists use ethanethiol for much more than odourising LPG. It is an intermediate, meaning a starting material that is converted into other products, in the making of plastics, insecticides and antioxidants. It also serves as an intermediate for acaricides (chemicals that kill mites), defoliants, pharmaceuticals and adhesives. It is listed as a flavouring agent in the food-flavour database kept by JECFA, the joint FAO/WHO expert committee on food additives.
How ethanethiol is made
One industrial route combines ethylene with hydrogen sulfide in the gas phase over an aluminium oxide catalyst:
C₂H₄ + H₂S → C₂H₅SH
The equation balances. Each side has 2 carbon atoms, 6 hydrogen atoms (4 + 2 on the left, 5 + 1 on the right) and 1 sulfur atom. Ethanethiol can also be made by reacting ethanol with hydrogen sulfide over a catalyst:
C₂H₅OH + H₂S → C₂H₅SH + H₂O
Check the atoms again: both sides have 2 C, 8 H, 1 O and 1 S. In this reaction the oxygen of the alcohol is replaced by sulfur, and the oxygen leaves as water. This is the swap between ethanol and ethanethiol that we met earlier.
Thiols and other smelly sulfur compounds
Ethanethiol is far from the only sulfur compound that you can smell. Thiols and related sulfur compounds are behind several of the most noticeable smells in everyday life.
- Skunk spray. Two thiols, (E)-2-butene-1-thiol and 3-methyl-1-butanethiol, are responsible for the repellent odour. The spray also contains thioacetates, which turn into more potent thiols when they meet water. That is why a wet dog can smell of skunk again.
- Garlic. Chopping or crushing a clove releases allicin, which forms from alliin and the enzyme alliinase. Allicin breaks down into smelly sulfur compounds, and the most persistent is allyl methyl sulfide, which causes garlic breath. See our guide to garlic for more.
- Overcooked cabbage. Long cooking releases sulfur compounds such as hydrogen sulfide.
What these smells have in common is that we can detect them at very low concentrations. Ethanethiol is a good example, at about 0.00035 ppm. To learn more about how the sense of smell works, take a look at aromatherapy and olfactory exercises.
Smell it or not? Flip the cards
Frequently asked questions about ethyl mercaptan
What is ethyl mercaptan?
Ethyl mercaptan, also called ethanethiol, is a colourless, flammable liquid with the formula C₂H₅SH. It belongs to the thiols, which contain an –SH group. It has a very strong smell, which is why a tiny amount is added to odourless fuels such as LPG so that leaks can be noticed.
What does ethyl mercaptan smell like?
The US National Institute for Occupational Safety and Health describes the smell as strong and skunk-like. SGN, the UK gas network, describes it as more organic and likened to decaying vegetation. It is the smell of camping and barbecue gas cylinders, and it can be detected at about 0.00035 parts per million.
Why is ethyl mercaptan added to gas?
Propane and butane (LPG), and natural gas once processed, have no smell of their own. Ethyl mercaptan is added on purpose so that a leak can be smelled and reported. Only a trace is needed because the human nose can detect it at extremely low concentrations, so the fuel is still almost entirely propane or butane.
What is the chemical formula of ethyl mercaptan?
The molecular formula of ethyl mercaptan is C₂H₆S, usually written C₂H₅SH or CH₃CH₂SH to show the –SH group. Its relative formula mass is about 62.1. Do not confuse it with CH₃SH, which is methanethiol (methyl mercaptan) and has only one carbon atom instead of two.
Does UK natural gas contain ethyl mercaptan?
Not normally. Mains natural gas in Great Britain is odourised with Odorant NB, a blend of roughly 80 % tert-butyl mercaptan and 20 % dimethyl sulfide. Ethyl mercaptan is the usual odorant in LPG, such as camping gas and barbecue cylinders. Some other countries do use it in natural gas.
Is ethyl mercaptan harmful to breathe?
At the trace levels used in fuel you smell it long before it is harmful. For the pure chemical, workplace limits exist: NIOSH recommends a ceiling of 0.5 ppm, and exposure can irritate the mucous membranes and cause headache and nausea. If anyone feels unwell, get medical advice from NHS 111, or call 999 if serious.
What should I do if I smell gas in the UK?
For mains gas in Great Britain, open windows and doors, turn off the gas at the meter control valve, and call the free 24-hour number 0800 111 999. Do not smoke, use naked flames, touch electrical switches or go into a cellar. For LPG, contact your supplier.
Can you smell carbon monoxide?
No. The NHS says carbon monoxide is colourless and does not smell, so you cannot tell if it is around you. The odorant added to gas only marks the fuel itself. If you suspect carbon monoxide, SGN says to call 0800 111 999, and the NHS says to go outside and get medical advice.
Is LPG heavier than air?
Yes. LPG vapour is heavier than air: butane vapour is about twice as heavy as air and propane about one and a half times. Calor Gas says a leak will sink to the lowest point and may remain for some time. Leaks can also show up as noise or frost at the source.
Why is it called a mercaptan?
The Danish chemist William Christopher Zeise coined the word "mercaptan" in the 1830s. It comes from the Latin mercurium captans, meaning "capturing mercury", because these sulfur compounds form insoluble mercury salts. Modern chemical names use the ending "-thiol", so ethyl mercaptan is systematically named ethanethiol.
