This descriptive article attempts to answer the question ‘what are the properties of alkenes’ completely, and thus enabling the reader to understand alkenes structure, bonding patterns, physical and chemical properties in detail. Alkene is a member in the hydrocarbon family. Alkenes are unsaturated and they involve in the chemical reaction more than the alkanes. The Carbon-Carbon double bond is the characteristic feature in alkenes and facilitates electrophilic addition reaction in the molecule. Alkenes show isomerism and this causes the change in the properties of the molecules, which have identical molecular formula.
What are Alkenes
Alkenes are the hydrocarbons, containing a Carbon-Carbon (C=C) double bond. Olefins is an old name used to name the alkene family. The smallest member in this family is ethane (C2H4). It was called olefiant gas (In Latin: ‘oleum’ means ‘oil’ + ‘facere’ means ‘to make’) in early days because the reaction between C2H4 and Chlorine gives C2H2Cl2, which is an oil.
General Molecular Formula of Alkenes
Alkenes have the general chemical formula CnH2n. Alkanes are said to be unsaturated hydrocarbons because they do not contain the maximum number of Hydrogen atoms that a molecule can possess.
Molecular Structure of Alkenes
In alkenes, two Carbon atoms corresponding to the double bond have the SP2 hybridization. Each double bonded Carbon of an alkene has three SP2 orbitals that lie in a plane with angles of 120°C. In this double bond, one is a sigma bond and the other is a pi-bond. The reaming two p-orbitals in the two Carbon atoms form the pi-bond.
What are the Chemical properties of Alkenes
Most of the reactions in alkenes are electrophilic addition reactions. The Carbon–Carbon double bond (C=C) acts as the electrophilic Centre for the reactions to occur.
Alkenes react with Hydrogen in the presence of finely divided metal catalyst to form the corresponding alkane. Without a catalyst, the rate of the reaction is very low.
Catalytic hydrogenation is used in the food industry to convert liquid vegetable oils to semi-solid fat as in the making of margarine and solid cooking fat. This is a reduction reaction in Alkenes.
Alkenes have both geometric and structural isomers. Isomers are the molecules that have the same chemical formula with different chemical structures. Example: C4H8
Structural Isomers (Constitutional Isomers): There are 2-positions for the double bond. Variation in the position of the C=C double bond creates different structures for the molecule.
Geometric Isomers: Cis-trans Isomerism
Some alkenes are polar and some are non-polar. Polar molecules that have a resultant dipole moment have an increase in boiling point.
In a symmetrical transalkene, the net dipole moments are equal to zero, but for cisalkenes there is a resultant dipole moment.
What are the Physical Properties of Alkenes
Alkanes show the similar physical properties of corresponding Alkane. Alkenes whose have lower molecular weights (C2H4 toC4H8)are gases at room temperature and atmospheric pressure and those whose have higher molecular weight are solids.
Alkenes are relatively polar molecules due to the C=C bond. Therefore, they are soluble in non-polar solvents or solvents of low polarity . Water is a polar molecule, and alkenes are slightly soluble in water.
The densities of Alkenes are lower than the density of water.
Boiling points of alkenes are similar to the corresponding alkanes. There is a small number of degrees lower than that value.
Properties of Alkenes – SUMMARY
Alkenes are a member of the hydrocarbon family who have the general structural formula as CnH2n. Each alkene possesses a C=C double bond. These carbon atoms have the SP2 hybridization. Alkenes do not contain the maximum number of Hydrogen atoms it can be bonded to; therefore, they are said to be unsaturated. Alkenes can be polar or non-polar depending on the molecular arrangement. They are slightly soluble in water and react with many chemical reagents under controlled conditions. Some of the alkenes possess a permanent dipole moment. Most physical properties of alkenes are similar to that of the corresponding alkane.