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Gasoline as we know it is a blend of combustible hydrocarbons and has a characteristic odor. The natural gas we use to heat our homes and (for some of us) cook our food has its own odor, except...

Natural gas as piped to our homes consists of methane, which is odorless, plus an odorant so that we can be aware of its presence for safety reasons. So the odor we think of as "gas" isn't the fuel at all, but an odorant.

So this leads to my question: is the familiar odor we attribute to gasoline that of one or more of the components which are the fuel itself (heptane, octane, etc.) or an additive for the purpose of giving it an odor, again for safety reasons?

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  • $\begingroup$ @Jacob That video comes as a total surprise. I have always found the odor of gasoline to be rather noxious, which is why I wondered about the presence of an odorant. $\endgroup$ – Anthony X Feb 4 '18 at 16:18
  • $\begingroup$ A roomful of Amoco PHD chemists and chem engineers did not know which compounds caused the odor of gasoline in the '70s. They started a very expensive research project to determine the causes with the intention of improving /eliminating the odor.The Arab oil embargo came and it turned out customers did not care what gasoline smelled like, they only cared about the price. So the project was cancelled so I don't have an answer. $\endgroup$ – blacksmith37 Apr 6 '18 at 20:10
  • $\begingroup$ @blacksmith37 so... short answer, not an odorant but one or more of the fuel components $\endgroup$ – Anthony X Apr 6 '18 at 23:44
  • $\begingroup$ Yes. Very likely "more". $\endgroup$ – blacksmith37 Apr 8 '18 at 21:04
  • $\begingroup$ @Jacob That video is pretty good proof that youtube is a very bad source for high quality scientific evidence. $\endgroup$ – matt_black Sep 2 at 15:29
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Many of the components of gasoline (petrol) have notable odours

A simple starting point for understanding the odour of gasoline is the typical composition of the product. That composition varies a lot and always has done as different refineries use different processing techniques and different crude oil sources have different components to start with. And regulators have changed the rules over time (for example to reduce the amount of benzene, a carcinogen). All that matters, in practice, is the octane rating which is what engines care about and this is achievable in a variety of ways by varying the composition and the additives that modify the combustion characteristics.

This (old) paper gives a crude overview of a typical composition.

The typical composition of gasoline hydrocarbons (% volume) is as follows: 4-8% alkanes; 2-5% alkenes; 25-40% isoalkanes; 3-7% cycloalkanes; l-4% cycloalkenes; and 20-50% total aromatics (0.5-2.5% benzene).

Modern blends will now have far less benzene (but will still have a lot of other aromatics). Without getting too specific on the individual chemicals we can already tell some of the sources of the distinctive odour of the mixture. It is pretty meaningless to try to pin it to a single component.

Aromatics as a class are actually named because of their distinctive smell. even if we substitute other aromatics like toluene or xylene for the benzene, there will be a distinctive "aromatic" component in the product. But plenty of the other hydrocarbons also have notable odours. Cycloakenes (maybe 7% of the mixture) often have sharp smells (cycopentene is even said to have "a petrol-like odour"). Even linear and branched alkanes have some smell (though it is far less intense than the unsaturated components of gasoline).

In short, trying to pin down the typical smell of gasoline to a single compound is a little silly when there are often scores of components with well-known and strong odours and the specific composition varies a lot. aromatics are a big part with other unsaturated compounds providing some other odiferous components with a smaller but notable contribution even from unsaturated hydrocarbons.

Also, gasoline is not "odourised" as it doesn't need to be. But it sometimes has specific ingredients added to make tracing the source easier. But this is done with complex instrumentation not the human nose.

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Gasoline has it's distinctive odor because of benzene, which is found naturally in crude oil, which gasoline is derived from. Benzene is a carcinogenic, highly flammable liquid with a high octane number, but because of it's toxicity is not really used for non industrial uses.

Source: https://en.wikipedia.org/wiki/Benzene

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  • $\begingroup$ The United States Environmental Protection Agency introduced new regulations in 2011 that lowered the benzene content in gasoline to 0.62%. (Wikipedia) $\endgroup$ – James Gaidis Feb 5 '18 at 0:02
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    $\begingroup$ Since gasoline has hundreds of components, assigning the smell to one doesn't sound like a high quality answer. $\endgroup$ – matt_black Sep 2 at 15:25
  • $\begingroup$ @matt_black, if it makes you happy, benzene is responsible for the main odor of gasoline. Benzene is extremely pungent, and sticks out from among the other compounds. If you ever smell benzene (and you probably shouldn't) you'll notice its odor smells distinctly like gasoline. I'm not editing my answer to reflect this, because it should be pretty self-evident that that was my intent. $\endgroup$ – Rafael Sep 2 at 22:19
  • $\begingroup$ @Rafael having worked with benzene, toluene, xylene and a wide variety of other compounds, I disagree. Not least because the higher volume aromatics are also pungent. $\endgroup$ – matt_black Sep 2 at 22:26
  • $\begingroup$ @matt_black, I agree that other aromatics are pungent; However gasoline's distinct odor is from benzene. $\endgroup$ – Rafael Sep 2 at 22:38
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LPG has its smell because of a smelling agent called ethyl mercaptan (ethanethiol). It is added for safety purpose around the household or industrial area

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  • $\begingroup$ Natural gas is also orderized . Methyl mercaptan may also be used. $\endgroup$ – blacksmith37 Apr 6 '18 at 20:12
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    $\begingroup$ Knowing what makes LPG smelly doesn't help understand what makes gasoline smelly. $\endgroup$ – matt_black Sep 2 at 15:24

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