Organic Chemistry: Alkanes

Organic Chemistry: Alkanes

Basic Properties

  • Alkanes are a class of organic compounds that only contain carbon and hydrogen atoms.
  • They are known as saturated hydrocarbons because they contain single bonds only, meaning they hold as many hydrogen atoms as possible.
  • The general formula for alkanes is CnH2n+2.
  • Alkanes are generally non-reactive except in the presence of a catalyst or under extreme conditions.

Examples of Alkanes

  • The simplest alkane is methane (CH4), which consists of a single carbon atom surrounded by four hydrogen atoms.
  • Ethane (C2H6), propane (C3H8), and butane (C4H10) are all examples of alkanes.
  • In these examples, the n in the general formula represents the number of carbon atoms. So, for propane, n is 3, which also shows that there are 8 hydrogen atoms when plugged into the formula.

Nomenclature of Alkanes

  • The name of an alkane is derived from the number of carbon atoms it contains.
  • Methane, ethane, propane, butane, pentane, hexane, heptane, octane, nonane, and decane are the names of the first 10 alkanes.
  • For larger alkanes, the prefix signifies how many carbon atoms the alkane has: ‘pent-‘ stands for 5, ‘hex-‘ stands for 6, and so forth.

Structure of Alkanes

  • Alkanes can either be straight-chain or branched. Straight-chain alkanes have carbon atoms connected in a line while branched ones have chains of carbon atoms off the main chain.
  • The carbon atoms in alkanes are sp3 hybridised, meaning each carbon forms four sigma bonds with the surrounding atoms.
  • Alkanes have a tetrahedral geometry due to the sp3 hybridisation.

Reactions of Alkanes

  • The most common reaction involving alkanes is combustion, where they react with oxygen to produce carbon dioxide and water. This is an exothermic reaction.
  • In the presence of a suitable catalyst, alkanes can also undergo substitution reactions with other atoms or groups of atoms, like halogens.
  • In industry, alkanes are also ‘cracked’ to form smaller hydrocarbons, often with the help of heat or a catalyst. This process is known as cracking.

Uses of Alkanes

  • Alkanes are primarily used as fuels. For example, methane is the main component of natural gas, and propane is often used in camping stoves.
  • They are also used in the production of polymers and as solvents in the chemical industry.
  • Cracking of long-chain alkanes obtains alkene which are important starting materials in the chemical industry.