Chemical Properties

Addition of hydrogen

Hydrogenation is the addition of hydrogen to an alkene. Although this reaction is exothermic, it is very slow. The addition of a metal catalyst, such as platinum, palladium, nickel, or rhodium, greatly increases the reaction rate. Although this reaction seems simple, it is a highly complex addition. 

An example of an alkene addition reaction is a process called hydrogenation.

Addition of halogens

A halogen addition reaction is a simple organic reaction where a halogen molecule is added to the carbon–carbon double bond of an alkene functional group.

The general chemical formula of the halogen addition reaction is:

C=C + X₂ → X−C−C−X

(X represents the halogens bromine or chlorine, and in this case, a solvent could be CH₂Cl₂ or CCl₄). The product is a vicinal dihalide.

This type of reaction is a halogenation and an electrophilic addition.

The halogens Br₂ and Cl₂ add to alkenes. This may be surprising because it is not immediately apparent that an electrophile—which is necessary to start an electrophilic addition reaction—is present. 

Addition of hydrogen halides

Hydrogen halides (HCl, HBr, HI) add up to alkenes to form alkyl halides. The order of reactivity of the hydrogen halides is HI > HBr > HCl. Like addition of halogens to alkenes, addition of hydrogen halides is also an example of electrophilic addition reaction.

Addition reaction of HBr to symmetrical alkenes

Addition reactions of HBr to symmetrical alkenes (similar groups attached to double bond) take place by electrophilic addition mechanism.

Addition reaction of HBr to unsymmetrical alkenes (Markovnikov Rule)

How will H – Br add to propene? The two possible products are I and II.

Markovnikov, a Russian chemist made a generalisation in 1869 after studying such reactions in detail. These generalisations led Markovnikov to frame a rule called Markovnikov rule.

The rule states that negative part of the addendum (adding molecule) gets attached to that carbon atom which possesses lesser number of hydrogen atoms.

Thus according to this rule, product I i.e., 2-bromopropane is expected. In actual practice, this is the principal product of the reaction. This generalisation of Markovnikov rule can be better understood in terms of mechanism of the reaction.

Mechanism

Hydrogen bromide provides an electrophile, H⁺, which attacks the double bond to form carbocation as shown below :

(i) The secondary carbocation (b) is more stable than the primary carbocation (a), therefore, the former predominates because it is formed at a faster rate.

(ii) The carbocation (b) is attacked by Br– ion to form the product as follows :

Anti Markovnikov addition or peroxide effect or Kharash effect

In the presence of peroxide, addition of HBr to unsymmetrical alkenes like propene takes place contrary to the Markovnikov rule. This happens only with HBr but not with HCl and Hl. This addition reaction was observed by M.S. Kharash and F.R. Mayo in 1933 at the University of Chicago. This reaction is known as peroxide or Kharash effect or addition reaction anti to Markovnikov rule.

Mechanism:

Peroxide effect proceeds via free radical chain mechanism as given below:

The secondary free radical obtained in the above mechanism (step iii) is more stable than the primary. This explains the formation of 1-bromopropane as the major product. It may be noted that the peroxide effect is not observed in addition of HCl and HI. This may be due to the fact that the H–Cl bond being stronger (430.5 kJ mol^–1) than H–Br bond (363.7 kJ mol^–1), is not cleaved by the free radical, whereas the H–I bond is weaker (296.8 kJ mol^–1) and iodine free radicals combine to form iodine molecules instead of adding to the double bond.

Oxidation:

Alkenes on reaction with cold, dilute, aqueous solution of potassium permanganate (Baeyer’s reagent) produce vicinal glycols. Decolorisation of KMnO₄ solution is used as a test for unsaturation.

Acidic potassium permanganate or acidic potassium dichromate oxidises alkenes to ketones and/or acids depending upon the nature of the alkene and the experimental conditions.

Ozonolysis:

Ozonolysis of alkenes involves the addition of ozone molecule to alkene to form ozonide, and then cleavage of the ozonide by Zn-H₂O to smaller molecules. This reaction is highly useful in detecting the position of the double bond in alkenes or other unsaturated compounds.