Coupon Accepted Successfully!


Physical Properties of Alkenes

Alkenes containing one to four carbon atoms are gases. Those having 5 to 15 carbon atoms are liquids, while higher members are solids.

In general, alkenes have a low density and are non polar in nature. The melting and boiling points of alkenes show similar trends to alkanes. However, alkenes have higher boiling points than the corresponding saturated hydrocarbons (alkanes).

Chemical Properties of Alkenes

Alkenes contain two types of bonds, namely σ and π -bonds. The π -electrons are loosely held between carbon atoms and are quite mobile. Hence, electron-deficient reagents are attached by π -electrons. Therefore, such reagents readily react with alkenes to give addition products. The presence of mobile (or loosely held π -electrons) are responsible for the reactive nature of alkenes.

The addition reaction is a characteristic of unsaturated compounds (say containing double or triple bonds). It is the most common type of reaction of alkenes. Other important common types of reactions of alkenes are oxidation and polymerisation.

Addition Reactions

Alkenes readily undergo addition reactions with a number of reagents and to give a variety of compounds due to addition to the double bond.

Addition reactions are those in which the product is formed by the combination of two or more reacting substances. For example, the addition of chlorine to ethylene is an addition reaction.
  1. Hydrogen
    The addition of hydrogen is called hydrogenation. Alkenes add onto hydrogen to form alkanes. Thus, the double bond in alkenes gets saturated.
    Alkenes react with hydrogen to give alkanes when heated under pressure in the presence of a suitable catalyst such as finely divided nickel, platinum or palladium.

    In the absence of a suitable catalyst, the hydrogenation reaction is extremely slow.
    Hydrogenation of vegetable oils (which contain compounds having double bonds) yields a solid fat (vanaspati ghee)
  2. Halogens
    Halogens react with alkenes to give dihalogen compounds. The order of reactivity is fluorine > chlorine > bromine > iodine.

    For example,
  3. Halogen Acids
    (HCl, HBr and HI) add to alkenes to give alkyl halides. The order of reactivity of halogen acids to alkenes is HI>HBr>HCl. In this case HI is the most reactive
Markownikoff's Rule
It helps to predict the nature of the addition product formed in the reaction of an alkene and molecules, such as halogen acids (HX), water, H2SO4, etc.
In the case of a symmetrical molecule, such as ethene, on the addition of molecules, such as HX, etc., only one product is formed. In the case of unsymmetrical alkenes, such as propene, two products are expected, depending upon how the addition takes place.

However, it has been found that isopropyl bromide is the major product of the reaction between HBr and propene. Markownikoff studied a number of such reactions using different alkenes and the molecules, which add to alkenes. He proposed a rule known as Markownikoff's rule, to predict the product formed in such addition.

According to Markownikoff's rule, the negative part of the molecule being added gets attached to the carbon atom that has the least number of hydrogen atoms.

In the case of HBr, the bromide ion (Br-) is the negative part of the adding molecule. Therefore, it gets attached to the carbon atom numbered 2 because it has only one hydrogen and not to carbon atom numbered 1 that is attached to two hydrogen atoms.
  1. Water
    Some of the reactive alkenes directly react with water containing a small quantity of acid to give alcohol. This reaction is called hydration of alkene. This reaction follows Markownikoff's rule.
  2. Sulphuric Acid
    When alkene is bubbled through concentrated sulphuric acid, alkyl hydrogen sulphate is formed. A molecule of sulphuric acid adds onto the double bond.

    The addition of sulphuric acid to alkenes is similar to other addition reactions.

    4. Ozonolysis is a method of locating unsaturation in a hydrocarbon. Ozone reacts with alkenes (also with alkynes) to form an ozonide. On reduction of ozonide with zinc water, the products formed are ketones or aldehydes.
    By identifying the products, it is possible to fix the position of the double bond in an alkene.
    For example, different isomers of pentene can be identified by examining the products formed on ozonolysis.


Alkenes undergo polymerization to form several useful polymers. Polymers are molecules having very high molecular masses. They are made up by the linking of several smaller molecules called monomers.

For example, ethene polymerizes to give polyethylene.

The relative molecular mass of most polymers is between 10,000 (104) and 10,000,000(107).
Polymers are of great use in our daily life. Plastics, synthetic fibres, etc., are examples of polymers.

Test Your Skills Now!
Take a Quiz now
Reviewer Name