What are alkanes?

Alkanes are compounds formed exclusively by carbon and hydrogen (hydrocarbons), which only contain simple carbon-carbon bonds.

types of alkanes

Alkanes are classified as linear, branched, cyclic, and polycyclic.

Alkanes nomenclature

Alkanes are named by ending in -ane the prefix that indicates the number of carbons in the molecule (methane, ethane, propane...)

Physical properties of alkanes

The melting and boiling points of alkanes are low and increase as the number of carbons increases due to interactions between molecules by London forces. Linear alkanes have higher boiling points than their branched isomers.

conformational isomers

Alkanes are not rigid due to the spin around the CC bond. The multiple shapes created by these rotations are called conformations.

 Newman projection

The energy of the different conformations can be seen in the Newman projections. Thus, in the case of ethane, the eclipsed conformation is the one with the highest energy, due to the repulsions between hydrogens.

potential energy diagrams

The different conformations of alkanes can be represented in a potential energy diagram, where we can see which conformation is more stable (minimum energy) and the energy needed to go from one conformation to another.

combustion of alkanes

Given their low reactivity, alkanes are also called paraffins. The most important reactions of this group of compounds are radical halogenations and combustion. Combustion is the combination of hydrocarbon with oxygen to form carbon dioxide and water.

Hydrocarbons are compounds that contain only carbon and hydrogen.

They are divided into two classes: aliphatic and aromatic hydrocarbons. Aliphatic hydrocarbons include three classes of compounds: alkanes, alkenes, and alkynes.

ethane Alkanes are hydrocarbons that contain only carbon-carbon single bonds, alkenes contain carbon-carbon double bonds, and alkynes are hydrocarbons that contain one triple bond.

Molecular model of the ethane molecule. Ethane has a tetrahedral geometry, with carbon at the center of the tetrahedron and hydrogens oriented towards its vertices. Note the spatial representation with wedges and dashed lines .

Alkanes are compounds made up of carbon and hydrogen that only contain carbon-carbon single bonds. They comply with the general formula C n H 2n+2 , where n is the number of carbons in the molecule.

Alkanes in which the carbons are linked continuously (without branches) are called straight-chain alkanes .

butane molecular model

Butane molecule. The first drawing shows the form of union of the atoms, in the second the zig-zag shape of the molecule and finally its molecular model, where the spatial arrangement of the atoms is observed.

barbituric acid 01

Barbituric acid, discovered by Adolf von Baeyer, in 1864

In the origins of chemistry, organic compounds were named by their discoverers.
Urea gets its name from being isolated from urine. Barbituric acid was discovered by the German chemist Adolf von Baeyer in 1864. It is speculated that he gave it this name in honor of a friend named Barbara.

Chemical science was advancing and the large number of organic compounds discovered made it essential to use a systematic nomenclature.

alkanes nomenclature 01

[1] isobutane (common name); methylpropane (IUPAC name)

[2] isopentane (common name); methylbutane (IUPAC name)

In the IUPAC system of nomenclature, a name is made up of three parts: prefixes, main, and suffixes; The prefixes indicate the substituents of the molecule; the suffix indicates the functional group of the molecule; and the main part the number of carbons it has.  

Alkanes can be named following seven stages:

Rule 1.- Determine the number of carbons in the longest chain, called the main chain of the alkane. Observe in the figures that it is not always the horizontal chain.

alkanes nomenclature 02

[3] 3-methyloctane

[4] 4-methylheptane

Rule 1. The name of a bicyclic compound is built with the word Bicyclo, followed by a bracket indicating the number of carbons in each of the three chains that start from the bridgehead carbons. The name ends in that of the alkane with the same number of carbons.

bike nomenclature 01

[1] Bicyclo[2.2.1]heptane

[2] Bicyclo[3.2.1]octane

[xxx] The bridgehead carbons of the bicyclic compound are drawn with a blue dot. The number of carbons in each of these chains (numbered in red) is indicated within the bracket, without counting the bridgehead carbons. Note that the numbers are ordered from highest to lowest and are separated by periods (not commas).

The name of the bicyclic compound ends in that of the alkane with the same number of carbons.

Methane (CH 4 ), ethane (C 2 H 6 ) and propane (C 3 H 8 ) are alkanes with only one possible structure. However, there are two alkanes with the formula C 4 H 10 ; butane and 2-methylpropane. These alkanes with the same formula but different structures are called isomers.

alkanes isomers 01

[1] Butane

[2] 2-Methylpropane

 n-Butane and isobutane have the same formula but differ in the way their atoms are joined -they are structural isomers-. Their different structure makes them have different properties, thus, they differ by about 20ºC in their melting point and about 10ºC in their boiling point.

Alkanes are sp 3 on all carbons.

The four substituents that start from each carbon are arranged towards the vertices of a tetrahedron. Bond distances and angles are shown on the following models.

Physical properties of alkanes

Newman projectionThe Newman projection is obtained by looking at the molecule along the CC axis. The front carbon is represented by a point, from which the three bonds that join it to the substituents start. The carbon behind is represented by a circle, and the bonds leaving this carbon are drawn from this circle.

Anti Hydrogen Peroxide ConformationSingle bonds between atoms have cylindrical symmetry and allow rotation of the groups of atoms attached to them. The different spatial arrangements that the atoms adopt as a result of rotation around the bond are called conformations. A particular conformation of many possible ones is called a conformer.

Ethane is the simplest alkane that can exist in different conformations. Of the infinite conformations that can be obtained by turning the carbon-carbon bond, the alternated and eclipsed conformations stand out for their importance.

Let's consider the C2-C3 bond of butane. The 60º turns around this link will generate the possible conformations of butane. There are three conformations of special importance called; butane without , butane anti and butane gauche that we represent in the following models.