Physical Properties
The first three members of this series are ethene, propene and butene gases. After this the members up to C16H32 are liquid and the members higher than this are solid.
They are insoluble in water but soluble in carbonic solvents such as alcohol, benzene and ether.
Their relative density, melting point and boiling point increase with increase in molecular weight.
All alkenes burn with a light-containing flame in air.
Anesthetic properties are found in the lower members of this category.
Chemical Properties
Due to the presence of double bonds, these compounds are highly reactive and often exhibit such reactions in which the pi bond of the double bond is broken. Following are their major chemical reactions –
Addition reactions :- Due to presence of double bond in alkenes, these compounds show addition reactions. In these reactions, the pi bond of the double bond is broken, the reagent splits into two parts, one part of the reagent gets attached to a carbon atom forming the double bond and the other part to another atom.
Following are some important examples of addition reactions of alkenes:
Addition of hydrogen : Alkene combines with hydrogen at 250-300°C temperature to form alkane in the presence of nickel powder.
CH2 = CH2 + H2 → CH3 – CH3
CH3 – CH = CH2 + H2 → CH3 – CH2 – CH3
The addition reaction of alkenes and hydrogen in the presence of nickel is called Sebastian and Senderens Reaction. This reaction takes place at high temperature. In the presence of palladium or paletium catalyst, alkene and hydrogen react at ordinary temperature to form alkene.
Addition of halogens:- Alkene combines with halogens to form dihalogen compound. The order of reactivity of halogens in this reaction is Cl2 > Br2 > I2. This reaction is carried out in a non-polar solvent such as CCl4 and in the presence of sunlight or in a polar solvent such as water.
Example :
CH2 = CH2 + Cl2 → CH2Cl – CH2Cl
CH3 – CH = CH2 + Br2 → CH3 – CHBr – CH2Br
Addition of Hydrogen Halides: One molecule of any alkene combines with one molecule of any hydrogen halide to form an additive compound.
Example :-
CH2 = CH2 + H – Cl → CH3 – CH2 – Cl
The order of reactivity of hydrogen halides in the above reaction is as follows –
H – I > H – Br > H – Cl > H – F
If hydrogen halide combines with an unsymmetrical alkene, then two types of additive products can be formed –
The main product in the above reaction is 2-bromo propane. In this regard, Markownikoff presented the following rule on the basis of many experiments, which is called Markownikoff law. according to this rule-
When an alkene alkene combines with a dissimilar molecule, the positive side of the joining molecule joins the carbon atom forming double bonds, on which more hydrogen atoms are present.
Thus, in the above reaction, the positive part of HBr i.e. H+ is combined with carbon atom No 1 because more hydrogen atom is present on carbon atom No 1 than carbon atom No 2.
Khrash and his colleagues found by experiments in 1933 that the addition of HBr to propene and other dissimilar alkenes in the presence of light (hv) and a peroxide such as benzoyl peroxide is contrary to the Markownikoff law.
Example :-
CH3 – CH = CH2 + HBr → CH3 – CH2 – CH2Br
This abnormal behavior of alkenes in the presence of light and peroxide is called the peroxide effect.
Addition of Hypochlorous acid :- Like hydrogen halides, hypochlorous acid also reacts with alkenes to form compounds.
example :
CH2 = CH2 + HOCl → CH2Cl–CH2OH
In hypochlorous acid (HOCl) there is a partial negative charge on the hydraxyl group and a partial positive charge on the chlorin atom. Hence, the chlorine atom is the positive part of hypochlorous acid.
According to Markownikoff’s law, the hypochlorous acid chlorin atom is double bonded to the carbon atom which has more number of hydrogen atoms.
example :
CH3 – CH = CH2 + HOCl → CH3 – CH(OH) – CH2Cl
Addition of sulfuric acid : Concentrated sulfuric acid absorbs alkenes to form alkyl hydrogen sulfate.
example :