Unsaturated hydrocarbons

An unsaturated hydrocarbon is a type of hydrocarbon which has at least one double bond, triple bond, or ring in its carbon chain. These molecules will therefore have fewer hydrogen atoms than the maximum the carbon chain could possibly hold (if all bonds were single bonds). Molecules that have only single bonds and contain the maximum amount of hydrogen are saturated hydrocarbons. [1]

Saturated hydrocarbons have the molecular formula CnH2n+2, where n refers to the number of carbon atoms, and can be any natural number. For example: ethane ([math]\ce{C_2H_6}[/math]). By contrast, an unsaturated hydrocarbon with n carbon atoms will have fewer than n + 2 hydrogen atoms in its molecular formula. For example: ethene ([math]\ce{C_2H_4}[/math]).

Each ring, or each extra bond added on a single bond (making it a double or triple bond), in an unsaturated hydrocarbon will reduce the number of hydrogen atoms in the molecule by 2, which is referred to as adding one degree of unsaturation.[1] You can see this pattern in the molecular formulas for these related hydrocarbons that all have 2 carbon atoms:

  • Ethane - [math]\ce{C_2H_6}[/math] - saturated
  • Ethene - [math]\ce{C_2H_4}[/math] - unsaturated (one degree of unsaturation)
  • Ethyne - [math]\ce{C_2H_2}[/math] - unsaturated (two degrees of unsaturation)

Combustion Properties

The general chemical equation for combustion of a hydrocarbon is:

[math]C_xH_y + (x + \frac{y}{4}) O_2 \rightarrow x CO_2 + \frac{y}{2} H_2O [/math]

Compared to the combustion of saturated hydrocarbons, unsaturated hydrocarbons will release less water per molecule of hydrocarbon combusted, and also consume less oxygen. For example, comparing the combustion of one molecule of ethane ([math]\ce{C_2H_6}[/math] - saturated) to combustion of one molecule of ethene ([math]\ce{C_2H_4}[/math] - unsaturated):

ethane: [math]C_2H_6 + \frac{7}{2} O_2 \rightarrow 2 CO_2 + 3 H_2O [/math]
ethene: [math]C_2H_4 + 3 O_2 \rightarrow 2 CO_2 + 2 H_2O [/math]

For every mol of ethene combusted, 0.5 mol less oxygen is required, and 1 mol less water is produced compared to ethane.

There are fewer high-energy bonds in unsaturated hydrocarbons (compared to saturated hydrocarbons of the same size), unsaturated hydrocarbons will release less energy per molecule when combusted. [2]

Unsaturated Fats

The same meaning of unsaturated used here for hydrocarbons is also used to describe fats and oils, which are hydrocarbon derivatives. Unsaturated fats contain carbon chains that are unsaturated (usually containing at least one double bond). Some fats contain several double bonds, and these are called polyunsaturated fats. The degree of unsaturation affects the melting point of the fat, as well as its health properties. [3]

To learn more about this topic, check out the Chemistry LibreTexts page on alkenes (one type of unsaturated hydrocarbon) or alkynes, another type of unsaturated hydrocarbon.

For Further Reading

References

  1. 1.0 1.1 “Calculating Degree of Unsaturation - Chemistry LibreTexts,” Chemistry LibreTexts, 5-Jun-2019. [Online]. Available: https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Map%3A_Organic_Chemistry_(McMurry)/Chapter_07%3A_Alkenes%3A_Structure_and_Reactivity/7.02_Calculating_Degree_of_Unsaturation. [Accessed: 14-Jun-2019].
  2. "Determination of the Relative Ratio of Unsaturated Fats to Saturated Fats in Nuts Using Bomb Calorimetry", T. Stacey, 2014. [Online]. Available: https://www.mckendree.edu/academics/scholars/issue23/tara-stacey.pdf. [Accessed: 14-Jun-2019].
  3. "Fats and Oils", chem.libretexts.org, 5-Jun-2019. [Online]. Available: https://chem.libretexts.org/Bookshelves/Introductory_Chemistry/Book%3A_The_Basics_of_GOB_Chemistry_(Ball_et_al.)/17%3A_Lipids/17.2%3A_Fats_and_Oils. [Accessed 14-Jun-2019].

Authors and Editors

Amanda Musgrove, Luisa Vargas Suarez, Jason Donev
Last updated: April 28, 2020
Get Citation