Alkenes are unsaturated hydrocarbons. These contain a C=C. They contain two hydrogens less than corresponding alkanes. Double bonded carbon undergoes hybridisation. These are otherwise known as Olefins.
General formula of alkenes is CnH2n. Bond angle is 120°. The C=C bond length in alkenes is less than the C-C single bond distance of in alkanes.
Due to the presence of a double bond alkenes are more reactive than alkanes. The unsaturation (or) pi bond between carbon and carbon is identified by following reagents:
- Br2 water (reddish brown colour)
- Alkaline KMnO4 (Pink colour of Baeyer's reagent)
Unsaturated hydrocarbons decolourise above reagents.
Alkenes exhibit chain, position, functional (ring chain isomerism) and cis-trans isomerism.
The IUPAC name is derived from the IUPAC name of alkane by replacing ending "ane" by "ene" along with the position of double bonds. Example: But-2-ene
In case of two double (or) three double bonds, the ending "ene" of alkanes is suitably replaced by diene or triene. Example: Penta-1,2-diene
Residual part left after the removal of one H-atom from alkene is known as alkenyl group. According to nomenclature, these groups are named by replacing terminal "e" of alkene by "yl".
Dehydration of ethyl alcohol
Dehydrohalogenation of alkyl halide
Controlled hydrogenation of acetylene (Lindlar's catalyst)
Industrial method (ethane obtained from natural gas)
Ethylene is a colourless gas, and dissolves in non polar solvents like benzene, ether.
First three alkenes are gases, next fourteen alkenes are liquids and higher alkenes are solids.
Boiling point increases with the molecular weight.
Ethene undergoes addition reactions. In addition reaction. Pi bond cleavage takes place and as a result two new sigma bonds are formed in the product.
Addition of Hydrogen halide (HX)
Addition of water
Addition of sulphuric acid
Ozonolysis: It is a test for unsaturation in molecules.