Organic Chemistry: Alkenes

Organic Chemistry: Alkenes

Basic Properties

  • Alkenes are a class of organic compounds containing carbon and hydrogen atoms.
  • These compounds are unsaturated hydrocarbons, containing at least one carbon-carbon double bond.
  • The general formula for the members of the homologous series of alkenes is CnH2n.
  • Alkenes are much more reactive than alkanes due to the presence of the carbon-carbon double bond.

Examples of Alkenes

  • The simplest alkene is ethylene, also known as ethene (C2H4), which consists of two carbon atoms and four hydrogen atoms.
  • Other examples of alkenes include propene (C3H6), and butene (C4H8).
  • In these examples, the n in the general formula represents the number of carbon atoms. So, for propene, n is 3, which also shows that there are 6 hydrogen atoms when using the formula.

Nomenclature of Alkenes

  • The names of alkenes are similar to alkanes but with an ‘-ene’ ending.
  • The first four members of the series are ethene, propene, butene, and pentene.
  • For larger alkenes, similar to alkanes, the prefix signifies the number of carbon atoms it has: ‘pent-‘ stands for 5, ‘hex-‘ stands for 6, and so forth.

Structure of Alkenes

  • Alkenes can have either straight or branched chains, much like alkanes.
  • Each carbon atom in an alkene forms three sigma bonds and one pi bond due to the double bond.
  • The carbon atoms in alkenes are sp2 hybridised and have a trigonal planar geometry.
  • Due to the presence of the double bond, rotation around the bond is restricted, which can lead to cis-trans isomerism.

Reactions of Alkenes

  • Alkenes undergo addition reactions due to the presence of the double bond.
  • During a typical reaction, the double bond ‘breaks’ open to allow other atoms or groups of atoms to add across the bond.
  • Common addition reactions include hydrogenation, halogenation and hydration.
  • An important reaction of alkenes is polymerisation, where many alkene molecules join together to form long-chain polymers.

Uses of Alkenes

  • Alkenes are used in the production of polymers like polyethylene and polypropylene.
  • They are also used in the synthesis of organic compounds in the chemical and pharmaceutical industry.
  • Ethene is used in the ripening of fruits.
  • Some alkenes are used as monomers for the production of plastic materials.