4.1 Alkanes, Alkenes, Alkynes and Aromatic Hydrocarbons

An industrial site with multiple tall towers set in front of mountains.
Anacortes, Washington Refinery

Hydrocarbons are used in a wide variety of applications, and on an enormous scale. They are sources from natural gas and crude oil, the fossil fuel substances taken from underground. These unrefined products are mixtures which are refined in industrial facilities to generate the raw materials used to produce everything from gasoline, to plastics, paints, and cosmetics.

Hydrocarbons are the simplest organic compounds, but they have interesting physiological effects. These effects depend on the size of the hydrocarbon molecules and the part of the body which is exposed to them.

Alkanes of low molar mass—those with from 1 to approximately 10 or so carbon atoms—are gases or light liquids that act as anesthetics. Inhaling these hydrocarbons in gasoline or aerosol propellants for their intoxicating effect is a major health problem that can lead to liver, kidney, or brain damage or to immediate death by asphyxiation by excluding oxygen. Accidental exposure to these fumes is also a danger in certain workplaces.

Swallowed, liquid alkanes do little harm while in the stomach. In the lungs, however, they cause “chemical” pneumonia by dissolving fatlike molecules from cell membranes in the tiny air sacs (alveoli). The lungs become unable to expel fluids, just as in pneumonia caused by bacteria or viruses. People who swallow gasoline or other liquid alkane mixtures should not be made to vomit, as this increases the chance of getting alkanes into the lungs. (There is no home-treatment antidote for gasoline poisoning; call a poison control center.)

Photo shows tubes of hand cream.
Cream cosmetics often contain larger-molecule hydrocarbons. Clairechkim, CC BY-SA 4.0 via Wikimedia Commons

On skin, liquid alkanes with approximately 5–16 carbon atoms per molecule wash away natural skin oils and cause drying and chapping, while heavier liquid alkanes (those with approximately 17 or more carbon atoms per molecule) act as emollients (skin softeners). Such alkane mixtures as mineral oil and petroleum jelly can be applied as a protective barrier. Water and aqueous solutions such as urine will not dissolve such a film, which explains why petroleum jelly is used in diaper ointments. A similar protective barrier is set up in anti-chafing products marketed to athletes.

Alkenes, Alkynes and Aromatic hydrocarbons are a bit more reactive and have more complex chemical and physiological profiles. Their physical properties are quite similar to alkanes, however, since they are nonpolar due to the roughly equivalent electronegativities of carbon and hydrogen.


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Introductory Organic Chemistry Copyright © 2021 by Carol Higginbotham is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.

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