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Alkanes are composed of \(\ce{C}\) and \(\ce{H}\) and non-polar \(\ce{C-C}\) and \(\ce{C-H}\) \(\sigma\) bonds. They may be linear chains, branched chains, or cyclic chains of carbons. The atoms in alkanes may acquire different arrangement in space, called conformations, due to free rotation around \(\sigma\)-bonds. The conformational analysis of alkanes, conformational analysis and stabilities of cycloalkanes and the IUPAC nomenclature of alkanes have been described earlier in this book.
Alkanes are raw materials for the synthesis of other organic compounds. Primary sources of alkanes are 1) natural gas that contains ~70% methane \(\ce{CH4}\), ~10% ethane \(\ce{CH3CH3}\), ~15% propane \(\ce{CH3CH2CH3}\), and small amount of other hydrocarbons and impurities, and 2) petroleum which is a mixture of hydrocarbons. The figure on the right margin shows a breakdown of a typical barrel of US oil, in which dark grey represents fuels and light grey represents other products.
Physical properties of alkanes
Physical and chemical properties of alkanes are dictated by the non-polar nature of \(\sigma\)-bonds in them. For example, the physical properties of alkanes are based on transient dipole-induced dipole interactions (London dispersion forces), which gradually increase with an increase in molecular weight.
Solubilities and densities of alkanes
Being non-polar, alkanes are insoluble in polar solvents like water and soluble in non-polar solvents like toluene. Alkanes have a density of ~0.7 g/ml, lighter than water ~1 g/ml, which makes oil spills float on the surface of water. Figure on the left margin of a major oil spill in the Gulf of Mexico in 2010 (Copyright: NOAA Satellites, Public domain, via Wikimedia Commons).
Melting points and boiling points of alkanes
Melting points and boiling points of alkanes gradually increase with an increase in the number of carbons in the molecule, as shown in Figure \(\PageIndex{1}\).
This is because transient dipole-induced dipole interactions also increase in the same order. Boiling points increase regularly by 20\(^{\circ}C\) to 30\(^{\circ}C\) upon addition of a \(\ce{-CH2{-}}\) to the molecule. Melting point increases, but not as smoothly as the boiling point. This is because freezing of a substance involves stacking up of molecules that happens more efficiently for alkanes with an even number of \(\ce{Cs}\) than for odd numbers of \(\ce{Cs}\) in a linear chain.
MP (red) and BP (blue) y-axis vs # of \(\ce{C's} in alkane
Branched-chain alkanes have lower melting and boiling points than their linear isomers -the more branching, the lower the melting and boiling point.
The transient dipole induces a dipole in the neighboring molecule through surface-to-surface contact. Therefore, linear chain organic molecules having a cylindrical shape with a larger surface area have higher melting and boiling points than branched molecules of the same molecular weight having a globular shape that can make less area of contact with the neighboring molecules, as illustrated in Figure \(\PageIndex{1}\) (left).
Petroleum refining
Petroleum is a mixture of hydrocarbons. Raw petroleum, also called crude oil, is refined in petroleum refineries by distillation, which separates the components based on their boiling points. The fractions, separated in the petroleum refineries, have different names and different uses, as illustrated in the Figure \(\PageIndex{2}\).
This page titled 5.1: Physical propertes of alkanes was last modified on Sun, 24 Aug 2025 22:37:55 GMT and is shared under a Public Domain license and was authored, remixed, and/or curated by Muhammad Arif Malik.
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