Alkanes: Definition, Formula, Structure, List, Examples

– Alkanes are saturated hydrocarbons; that is, they contain only carbon-carbon single bonds.

– In this context, saturated means that each carbon has the maximum number of hydrogens bonded to it.

– We often refer to alkanes as aliphatic hydrocarbons because the physical properties of the higher members of this class resemble those of the long carbon-chain molecules we find in animal fats and plant oils (Greek: aleiphar, fat, or oil).

Classification of Hydrocarbon

– A hydrocarbon that contains one or more carbon-carbon double bond, triple bond, or benzene ring is classified as an unsaturated hydrocarbon.

– Methane (CH₄) and ethane (C₂H₆) are the first two members of the alkane family.

– The shape of methane is tetrahedral and all H-C-H bond angles are 109.5°.

– Each carbon atom in ethane is also tetrahedral, and all bond angles are approximately 109.5°.

– Although the three-dimensional shapes of larger alkanes are more complex than those of methane and ethane, the four bonds about each carbon are still arranged in a tetrahedral manner, and all bond angles are approximately 109.5°.

– The next three alkanes are propane, butane, and pentane.

– In the following representations, these hydrocarbons are drawn first as condensed structural formulas that show all carbons and hydrogens. They are also drawn in an even more abbreviated form called a line-angle formula.

– In a line-angle formula, each vertex and line ending represents a carbon atom.

– Although we do not show hydrogen atoms in line-angle formulas, we assume that they are there in sufficient numbers to give each carbon four bonds.

– We can write structural formulas for alkanes in still another abbreviated form. 

– The structural formula of pentane, for example, contains three CH₂ (methylene) groups in the middle of the chain.

– We can collect them and write the structural formula of pentane as CH₃(CH₂)₃CH₃.

– The table gives the names and molecular formulas of the first 20 alkanes.

– Note that the names of all these alkanes end in -ane.

– We will have more to say about naming alkanes in the following lessons.

General molecular formula of Alkanes

– Alkanes have the general molecular formula C_nH_2n+2.

– Thus, given the number of carbon atoms in an alkane, we can determine the number of hydrogens in the molecule and its molecular formula.

– For example, decane, with 10 carbon atoms, must have ( 2 × 10) + 2 = 22 hydrogen atoms and a molecular formula of C₁₀H₂₂.

Constitutional isomers of alkanes

– Constitutional isomers are compounds that have the same molecular formula but different structural formulas.

– By “different structural formulas,” we mean that constitutional isomers differ in the kinds of bonds they have (single, double, or triple) and/or in the connectivity of their atoms.

– For the molecular formulas CH₄, C₂H₆, and C₃H₈, only one connectivity is possible.

– For the molecular formula C₄H₁₀, two connectivities are possible.

– In one of these, named butane, the four carbons are bonded in a chain; in the other, named 2-methylpropane, three carbons are bonded in a chain with the fourth carbon as a branch on the chain.

– Butane and 2-methylpropane are constitutional isomers; they are different compounds and have different physical and chemical properties.

– Their boiling points, for example, differ by approximately 11°C.

– To determine whether two or more structural formulas represent constitutional isomers (that is, different compounds with the same molecular formula), write the molecular formula of each and then compare them.

– All compounds that have the same molecular formula, but different structural formulas (different connectivities of their atoms), are constitutional isomers.

Solved Problems

Problem (1): Do the condensed formulas in each pair represent the same compound or constitutional isomers?

Solution

(a) The molecules are drawn here as both condensed structural formulas and line angle formulas.

– Each formula has an unbranched chain of six carbons; the two are identical and represent the same compound.

(b) Each formula has a chain of five carbons with two -CH₃ branches.

– Although the branches are identical, they are at different locations on the chains; these formulas represent constitutional isomers

Problem (2): Write line-angle formulas for the five constitutional isomers with the molecular formula C₆H₁₄

Solution

– In solving problems of this type, you should devise a strategy and then follow it. Here is one such strategy:

– First, draw a line-angle formula for the constitutional isomer with all six carbons in an unbranched chain.

– Then, draw line-angle formulas for all constitutional isomers with five carbons in a chain and one carbon as a branch on the chain.

– Finally, draw line-angle formulas for all constitutional isomers with four carbons in a chain and two carbons as branches.

– No constitutional isomers with only three carbons in the longest chain are possible for C₆H₁₄.

Number of constitutional isomers in Alkanes

– The ability of carbon atoms to form strong bonds with other carbon atoms results in a staggering number of constitutional isomers.

– As the table shows, there are 3 constitutional isomers with the molecular formula C₅H₁₂, 75 constitutional isomers with the molecular formula C₁₀H₂₂, and almost 37 million with the molecular formula C₂₅H₅₂.

– For even a small number of carbon and hydrogen atoms, a very large number of constitutional isomers is possible.

– Because constitutional isomers have different chemical properties, a rich diversity of chemistry is possible within these sets.

Reference: Organic chemistry / William H. Brown, Christopher S. Foote, Brent L. Iverson, Eric V. Anslyn, Bruce M. Novak. ( sixth edition) . United States