Table of Contents
Alkenes containing carbon to carbon double bond. Or, Unsaturated hydrocarbons containing carbon to carbon double bonds. Like; ethene, propene, butene ….etc. Alkenes are also called as ‘olefins’. Fatty acids are formed with the help of olefins. If one carbon in alkenes is attached with the help of double bond and the rest of the carbon is forming single bond, then the hybridization of that alkenes is sp2. The general formula for alkenes is CnH2n.
Preparation of alkenes
Preparation of alkenes from alkyl halides
The H-X unit is removed to form alkenes from alkyl halides, where H-X means hydrogen and halogen. The process of removing H-X from alkyl halides is called dehydrohalogenation of alkyl halides. The process of dehydrohalogenation of alkyl halides is carried out in the presence of alcohol K-OH, which removes the H-X unit and forms alkenes.
Preparation of alkenes from Di halogen derivatives
Alkenes are formed from Di halogen derivatives. This process is completed in the presence of zinc dust because zinc trust removes the X-group from Di halogen derivatives, leading to the formation of alkenes.
Preparation of alkenes from alcohol
Alkenes can be easily obtained from alcohols. To obtain alkenes from alcohol, alcohol is dehydrated, which leads to the removal of water. when an alcohol is heated in presence of strong dehydration agents like- concentrated H2SO4, Alumina(Al2O3) and H3PO3. It gives an alkene.
Preparation of alkenes by Kolbe’s electrolysis
We can easily prepare alkanes, alkenes and alkynes with the help of Kolbe’s electrolysis. Carboxylic acid salts are electrolyzed to form alkenes by Kolbe’s electrolysis process, which leads to the formation of alkenes. As explained in the example, when sodium succinate is electrolyzed, ethene is obtained.
Physical property of alkenes
- Physical state and smell:- Ethene, propene and butene, all these compounds are gases, but as their molar mass increases, all these compounds will convert from gases to liquid and from liquid to solid. There is a faint sweet smell of ethene.
- Alkenes with 5 to 18 carbon atoms are liquid. Higher members of Alkenes are solid. Alkenes except ethene are odorless.
- Melting point:- alkenes have higher melting points than alkanes because alkenes have more percentage as character than alkanes and for this reason alkenes also have more electronegativity than alkanes and alkenes are polar because of higher electronegativity whereas alkanes are non-polar.
- Boiling points:- Boiling point of alkenes increases with increase in number of carbon atoms. Amongst geometrical isomeric alkenes Cis-alkenes have higher boiling point than its trans-isomer.
- Dipole moment:- π-electron of double bond are easily polarized. Therefore, their dipole moments are high. Symmetrical trans alkene have zero dipole moment.
- Solubility:- Alkenes are insoluble in water and readily dissolve in organic solvents. Like ether and benzene.
Chemical properties of alkenes
Addition of nitrosyl chloride
Nitrosyl chloride is also called Tilden’s reagent whose molecular formula is NOCl, in which the electrophile of NOCl is NO+. In this reaction markovnikov rule is used, according to markovnikov rule the positive part will be added where the number of hydrogen is more because the carbocation which is secondary is stable.
Oxidation of cold KMnO4 (Hydroxylation)
The addition of hydroxyl groups (OH-) across the double bonds is known as hydroxylation. In this reaction, KMnO4 is used, therefore the color of the purple KMnO4 disappears. This is used as a laboratory test to identify the existence of double bonds known as the Bayer test.
Hydration of alkenes
Addition of a water (H-OH) molecule across the double bond of an alkene is called hydration. In the reaction of hydration of alkene, addition takes place according to markowniff’s rule where carbocataion is formed as intermediate.