The simplest alkene is ethene having formula . These compounds are also known as olefins (a Latin word meaning oil forming) because first members form oily products when react with halogens.
A few alkenes with their molecular, condensed, structural and cross and dot formulae are given in Table 12.1.
| Name | Molecular formula | Condensed formula | Structural formula | Cross and dot formula |
|---|---|---|---|---|
| (i) Ethylene or ethene | C2H4 | H2C = CH2 | H-C=C-H | H-C=C-H |
| (ii) Propylene or propene | C3H6 | H3C-HC = CH2 | H-C-C=C-H | H-C-C=C-H |
| (iii) Butylene or butene | C4H8 | H3C-H2C-HC=CH2 | H-C-C-C=C-H | H-C-C-C=C-H |
| (iv) Pentene | C5H10 | H3C-CH2-CH2CH=CH2 | H-C-C-C=C-H | H-C-C-C=C-H |
Occurrence
(i) Alkenes being more reactive than alkanes, seldom occur free in nature. (ii) Lower alkenes occur in coal gas in minute quantities. (iii) Ethylene is present in natural gas sometimes to the extent of . (iv) Alkenes are produced in large amounts by cracking of petroleum.
12.2.1.1 Dehydration of Alcohols
Alkenes are prepared by the removal of small atoms (H,OH,X) from the adjacent carbon atoms of the saturated compounds, so as to create a double bond between carbon atoms.
12.2.1.1 Dehydration of Alcohols
Dehydration is removal of water. Ethene is prepared by heating a mixture of ethanol and excess of concentrated sulphuric acid at . In first step, ethyl hydrogen sulphate is formed which decomposes on heating to produce ethene, which is collected over water.
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12.2.1.2 Dehydrohalogenation of Alkyl Halides
On heating, ethyl bromide with alcoholic KOH, ethene is formed. Removal of hydrogen and halogen takes place from adjacent carbon atoms to create a double bond.
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Physical Properties of Alkenes
(i) The first member of the alkenes is ethene. It is a colourless gas with pleasant odour. (ii) Alkenes are nonpolar, therefore, they are insoluble in water but soluble in organic solvents. (iii) The first member of the series ethene is slightly less dense than air. (iv) Alkenes are flammable hydrocarbons. On complete combustion, they form carbon dioxide and water with release of energy. However, their flame is smokier than alkanes having a similar number of carbon atoms. (v) Their melting and boiling points gradually increase with the increase of molecular sizes of the compounds in the series.
12.2.2 Chemical Reactions
Alkenes are reactive compounds because the electrons of the double bond are easily available for reaction. These compounds have the tendency to react readily by adding other atoms, to become saturated compounds. As a result, the double bond is converted into a single bond that is more stable. Thus, addition reactions are characteristic property of unsaturated compounds. These are the reactions in which the products are formed by the addition of some reagents like etc., to an unsaturated organic compound. In the process, one of the bonds of a double bond gets broken and two new single bonds are formed.