Chemical Properties of Alkenes:
The reactions of alkenes can be divided into three categories.
Addition Reactions:
Alkenes being unsaturated compounds mostly show addition reactions and the addition is electrophilic addition.
Mechanism:
A polar or temporarily polarized molecule approaches to alkene and +ve end of the molecule pulls its π-electrons as a result reagent molecule breaks up and its +ve ion (electrophile) forms a temporary bond with two multiple bonded carbons.
The elecrophile makes permanent bond with one of the carbon and leaving +ve charge on other carbon.
The –ve ion (nucleophile) attacks carbonium ion and offers its electrons to positively charged carbon and makes bond with it.
Some addition reactions of alkenes are following.
Hydrogenation:
Addition of hydrogen is called hydrogenation. Molecular hydrogen cannot break π-bond however atomic hydrogen can break it. Therefore to dissociate molecular hydrogen into atomic hydrogen either heating (from 240-300oC) is carried out in presence of Nickel catalyst. If platinum catalyst is used then without heating hydrogen dissociates.
Hydrogenation is an exothermic process.
Halogenation:
Addition of halogens is called halogenation. It is carried out in presence of inert solvent like CCl4. Halogenation of alkenes produces vicinal dihaloalkanes.
Halogenation of alkenes with chlorine and bromine is carried out but not with fluorine because it is too reactive to control while iodine does not react.
Bromine water (bromine dissolved in water) is used to detect presence of carbon-carbon multiple bond (i.e double or triple bond). Bromine adds to multiple bonded carbons thus reddish colour of solution disappears. This test is called bromine water test.
Mechanism:
When halogen molecule approaches to alkene it is polarized. The +ve end of the halogen molecule pulls π-electrons and as a result halogen molecule breaks up and +ve halogen ion (chloronium or bromonium acts as electrophile) forms a temporary bond with two multiple bonded carbons.
The electrophile (Bromonium) makes permanent bond with one of the carbon and leaving +ve charge on other carbon.
The –ve ion (nucleophile i.e bromide ion) attacks the carbonium ion and offers its electrons to positively charged carbon and makes a bond with it.
Hydrohalogenation:
The addition of hydrogen and halogen is called hydrohalogenation. When alkenes are treated with halogen acids hydrohalogenation takes place.
The order of reactivity of halogen acids towards this reaction is
HI > HBr > HCl > HF
If alkene is unsymmetrical then addition takes place according to Markovnikov’s rule. If alkene is symmetric then only one addition product is possible.
Both possibilities gave same product.
But if alkene is asymmetric then two addition products are possible.
But actually only 2-haloalkane will be obtained as major product. This shows that when alkene is unsymmetrical then addition take place according to Markovnikov’s rule.
Markovnikov’s rule:
“When an unsymmetrical reagent adds to an unsymmetrical alkene or alkyne, the positive part of the reagent becomes attached to the double or triple bonded carbon which bears the greatest number of hydrogen atoms and vice versa.”
“When an asymmetric molecule is added to an asymmetric alkene or alkyne then the negative part of the molecule to be added goes to that multiple bonded carbon which has less number of hydrogen atoms and vice versa.”
In above mentioned later case (propene) addition took place according to Markovnikov’s rule. In case-I primary carbonium ion will be formed while in later case secondary carbonium ion will be formed. The stability of carbonium ions is
Ter-carbonium ion > Sec-carbonium ion > Pri-carbonium ion
As in case-II stable carbonium ion is formed so addition takes place according to Markownikoff’s rule.
Addition of Hypohalous Acid or Halo hydroxylation:
When alkenes are treated with aqueous solution of hypohalous acids HOX addition reaction take place to give halohydrins (haloalcohols).
The order of reactivity of halogen acids towards this reaction is
HOCl > HOBr > HOI
Hypoclorous acid Hypobromous acid Hypoiodous acid
Hypoiodous acid is nonreactive in this reaction and will not show this reaction.
If alkene is unsymmetrical then addition takes place according to Markownikoff’s rule. If alkene is symmetric then only one addition product is possible.
But actually, only 1- Chloro-2-propanol will be obtained as a major product. This shows that when alkene is unsymmetrical then addition takes place according to Markownikoff’s rule.
Addition of Sulphuric Acid:
When alkenes are treated with cold concentrated H2SO4 addition of acid take place.
In asymmetric alkenes addition takes place according to Markownikoff’s rule.
The alkyl hydrogen sulphates on dilution with water and boiling give alcohols.
As acid is replaced by water so actually addition of water has taken place, therefore, the reaction is called Hydration of alkenes.
Reaction with dilute Sulphuric acid (Hydration):
When ethene is treated with dilute (10%) H2SO4 in presence of mercuric sulphate hydration occurs.
Oxidation Reactions:
The π-bond in alkenes is a weak bond and π.-electrons are loosely bound to carbon nuclei and hence are susceptible to the attack of oxidizing agents. Therefore oxidizing agents can easily oxidize alkenes.
Some oxidation reactions of alkenes are following.
Combustion:
Alkenes on burning produce CO2 and H2O vapours along with the liberation of heat.
This is an oxidation reaction.
Ozonolysis:
Ozone is highly reactive so it reacts vigorously with alkenes and produce corresponding alkylene ozonide. This is oxidation reaction. Alkylene ozonide then on reaction with Zinc dust in presence of water reduces to some aldehyde or ketone or mixture of the two.
As water is recovered so it is acting as catalyst.
This reaction is used to locate position of the double bond in alkenes and also to identify alkenes e.g. if ozonolysis of an alkene ‘X’ followed by reduction with zinc produces actetaldehyde (ethanal) and 2-butanone then alkene is 3-methyl-2-pentene.
Epoxide Formation:
When a mixture of alkene and air is passed over a heated silver oxide catalyst, an atom of oxygen adds to the alkene molecule and alkylene epoxide is formed. This is an oxidation reaction.
Hydroxylation (Baeyer’s test):
When alkenes are treated with dilute alkaline KMnO4 solution hydroxylation of alkenes occurs and as a result, vicinal glycols (Vicinal dihydroxy alcohols) are produced. This is also an oxidation reaction. This reaction could be used as an identification test for the presence of carbon-carbon multiple bonds and is known as Baeyer’s test.
Baeyer’s test: The test which indicates the presence of a carbon-carbon double or triple bond by the decolourization of the pink colour of an alkaline aqueous solution of KMnO4 is called Baeyer’s test.
Polymerization:
When ethene is subjected to a pressure of 1000 atm it liquefies. Then on heating liquid ethane to a temperature of 100-300oC it polymerizes to polyethylene or polythene. If catalysts like aluminium triethyl Al(C2H5)3 or titanium tetrachloride TiCl4 are used good quality polythene is obtained.
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