The Chemistry of Alkanes

Alkanes are a group of aliphatic hydrocarbons that contain only single covalent bonds (σ) bonds between carbon atoms. The hybridization of all the carbon atoms in an alkane is sp³. Since there are only single bonds in alkanes, they are also known as saturated hydrocarbons.

• The empirical formula of alkanes is C_nH_2n+2.

Carbon atoms bond linearly to form acyclic alkanes. Sometimes there are branches in the carbon backbone. Cyclic alkanes can also be found when carbon atoms bonded as cycles.

Properties of alkanes

Alkanes are nonpolar compounds. Therefore, they have only Wan-der Waals interactions between molecules as secondary interaction forces. When the number of carbon atoms increases, the molecule gets bigger, so it gets a higher surface area.

Thus, the Wan-der Waals forces also increase. Alkanes that contain 1 - 4 carbon atoms exist as gases at room temperature. When the number of carbon atoms are increasing it could be a liquid or a solid at room temperature.

Alkanes are insoluble in water. But they are soluble in organic solvents. When alkanes are burnt in open air it results in carbon dioxide and water as products. The combustion of alkane is highly exothermic. Therefore, alkanes are used as fuels.

• E.g methane (CH₄), Petrol, Diesel, LP gas, etc

General nomenclature of alkanes

Linear alkanes without any pendent groups are named according to their number of carbon atoms in the carbon backbone. So when naming an alkane, the “ane” suffix is added to the stem name.

IUPAC nomenclature of alkanes

There can be different structural formulas for one empirical formula. As an example, the C₄H₁₀ empirical formula has two different structural formulas as follows.

IUPAC nomenclature is introduced considering the structural formula as well as the empirical formula.

The carbon backbone in an alkane can be attached to other hydrocarbon chains. These groups are known as substituent groups or pendent groups. A pendent group can be an alkyl group which is formed by removing a Hydrogen atom from an alkane (parent hydrocarbon).

Alkyl groups are named according to the number of carbon atoms in them. When naming an alkyl group, the “yl” suffix is added to the stem name. An alkyl group is commonly indicated as "-R".

Steps to name an alkane

1. Identify the longest hydrocarbon chain with the maximum number of substituent groups.
2. Number the hydrocarbon chain where the carbon atoms with substituent groups get the lowest possible number,
3. Name the substituent groups according to the alphabetical order. When naming a substituent group, write the number of the carbon atom, then put a dash, and then write the name of the alkyl group.
4. Finally, write the stem name of the hydrocarbon chain and add the suffix “ane” at the end.

• All the letters should be simple letters and the name must be a single word.
• If there are similar alkyl groups in different carbon atoms they are named as follows.

Hybridization of carbon in alkane

All the carbon atoms in an alkane possess sp³ hybridization. At ground state, carbon has four electrons in its valence shell. The electron configuration of carbon at the ground state is 1s² 2s² 2p². When it is supplied energy, an electron in 2s orbital moves 2p. Thus, four unpaired electrons are obtained. It can be represented in an energy diagram as follows.

It forms four sp³ hybridized orbitals out of 2s and 2p orbitals. Before hybridization, there is an energy gap between 2s and 2p orbitals. But after hybridization, it forms 4 sp³ hybridized orbitals which are similar in energy, size, and shape. These sp³ hybridized orbitals remain tetrahedral geometry and 109.5⁰ angles to each other.

In an alkane, the C-C bond is formed by the linear overlapping of sp³ hybridized orbitals. A C-H bond is formed by the linear overlapping of the sp³ hybridized orbital and un-hybridized s orbital of hydrogen.

Preparation of alkanes

Alkene or alkyne is treated with a hot air current of hydrogen gas. In the presence of Nickel (Ni), Platinum (Pt), or palladium (Pd) catalysts, Hydrogen will be added to the alkenes and alkynes and it will result in alkanes.

Here, the pi bonds are removed and hydrogen is added instead. It is added 1 mol of H₂ for 1 mole of double bond (In alkenes) and 2 mols of H₂ for 1 mol of triple bond (in alkynes). The hybridization of the carbon atom also changes. If there was an alkyne the hybridization changes sp² to sp³. If there was an alkyne the hybridization changes sp to sp³.

The carbonyl carbon in aldehydes or ketones can be reduced by using zinc amalgam and concentrated hydrochloric acid (Zn(Hg).conc HCl). This reaction is known as Clemmensen reduction.

Carboxylic acids are treated with “Soda lime” reagent to prepare alkanes. Soda lime is a mixture of sodium hydroxide (NaOH) and calcium oxide (CaO). In this action, the carboxyl group (COOH) is substituted by a Hydrogen atom and results in an alkane.

When Grignard reagent is treated with dilute acid or water, it results in an alkane. Grignard reagents can be prepared by treating alkyl halides with magnesium (Mg) in the medium of dry ether.