Science and Technology for UPSC Examinations [Chemistry] Part 16- Alkenes

An alkene, olefin, or olefine is an unsaturated chemical compound containing at least one carbon-to-carbon double bond.. Thus, Alkenes are the simplest of the unsaturated hydrocarbons, hydrocarbons which will react with hydrogen. Alkenes contain one or more reactive double bonds between carbon atoms, and are easily indicated by the symbol =. Since a double bond requires two carbon atoms, the simplest alkene is C₂H₄ or H₂C=CH₂, ethene. The presence of a double bond is indicated by the change of ending from -ane to -ene. An older form of the name, ethylene, is used in chemical industry. The general formula is C_nH_2n. All alkenes have at least one double bond.

Alkenes and Cycloalkanes have the same general formula, CnH2n.   Therefore, the general formula does not identify the structure as an alkene nor a cycloalkane.  To further become problematic there are alkenes which contain more than one double bond.  Those with two double bonds have the formula, CnH2n-2.

The physical properties of alkenes are comparable with those of alkanes. The main differences between the two are that the acidity levels of alkenes are much higher than the ones in alkanes. The physical state depends on molecular mass. The simplest alkenes, ethene, propene and butene are gases. Linear alkenes of approximately five to sixteen carbons are liquids, and higher alkenes are waxy solids.

Distinugusing Alkanes with Alkenes

An alkene may be distinguished from an alkane by shaking the hydrocarbon with bromine water. Bromine water is reddish-brown, and will decolourise with an alkene but not with an alkane. Bromine adds across the double bond of an alkene to form a colourless dibromo alkane. This is an example of an addition reaction. An addition reaction occurs when two or more reactants join together to form a single product.

Some Common reactions related to Alkenes

Alkenes are relatively stable compounds, but are more reactive than alkanes. Some common reactions related to alkenes are as follows:

Hydrogenation

Hydrogenation of alkenes produces the corresponding alkanes. The reaction is carried out under pressure at a temperature of 200 °C in the presence of a metallic catalyst. Common industrial catalysts are based on platinum, nickel or palladium.

Oxidation

Alkenes are oxidized with a large number of oxidizing agents. In the presence of oxygen, alkenes burn with a bright flame to produce carbon dioxide and water.

Ozonolysis

Ozonolysis is the cleavage of an alkene or alkyne with ozone to form organic compounds in which the multiple carbon–carbon bond has been replaced by a double bond to oxygen. The outcome of the reaction depends on the type of multiple bond being oxidized and the workup conditions. Alkenes can be oxidized with ozone to form alcohols, aldehydes or ketones, or carboxylic acids.

Notable Points about Alkenes

• Alkenes are unsaturated hydrocarbons with one double bond (R-C=C-R). They are from a homologous series with the general formula CnH2n. The 1st member to possess this functional group is ethane, C2H4. Ethene is an important raw material for the petrochemical industry.
• Alkenes are extremely important in the manufacture of plastics. All plastics are in some way related to alkenes. The names of some plastics (Polythene or Poly Ethene, Polypropene), relate to their alkene partners. Plastics are used for all kinds of tasks, from packaging and wrapping, to clothing and outdoor apparel.
• Lower alkenes are used as fuel and illuminant. These may be obtained by the cracking of kerosene or petrol.
• For the manufacture of a wide variety of polymers, e.g., polyethene, polyvinylchloride (PVC) and teflon etc.
• As raw materials for the manufacture of industrial Chemicals such as alcohols, aldehydes, and etc.
• Besides, alkenes also used for artificial ripening of fruits, as a general anesthetic, for making poisonous mustard gas (War gas) and ethylene-oxygen flame.