Introduction to Alkenes

 Alkenes

Introduction

Alkenes are hydrocarbons that contain at least one carbon-carbon double bond. They follow the general formula C_nH_2n. A double bond makes them unsaturated and more reactive than alkanes.

Olefins is another name for alkenes, derived from "olefiant gas" (oil-forming gas). This term historically referred to ethene () because it reacts with chlorine to form an oily liquid.

Properties of Alkenes

1.      Physical Properties:

o    Insoluble in water.

o    Soluble in organic solvents.

2.      Structure:

o    Contains a carbon-carbon double bond.

o    Exhibits sp² hybridization at the double-bonded carbons.

Like alkanes, alkenes are also subject to only weak van der Waals attractive forces. The physical properties of alkenes are therefore essentially similar to those of the corresponding alkanes. At room temperature, the lower members of the family (alkenes containing upto 4 carbon atoms) are gases, the members containing 5 to 17 carbon atoms are volatile liquids and those containing 18 or more carbon atoms are solids.

The boiling points of alkenes are almost the same as those of the corresponding alkanes, though slightly on the lower side. Although alkenes are also less denser than water, their densities are always slightly higher than those of the corresponding alkanes. The isomeric alkenes have very close boiling points. It is difficult to separate them except with a very efficient fractionating column, the straight chain 1-alkenes have boiling points a few degrees lower than the straight chain alkenes with an internal double bond.

Classification of Alkenes

Alkenes are classified based on the number of double bonds they contain:

1. Monoenes:
• Alkenes with a single double bond.
• Example: Ethene (C₂H₄).
2. Dienes:
• Alkenes with two double bonds.
• Types of dienes:
• Conjugated Dienes: Double bonds separated by a single bond (e.g., 1,3-butadiene).

                                                     

• Cumulated Dienes: Adjacent double bonds (e.g., allene).

                                                   

• Isolated Dienes: Double bonds separated by two or more single bonds.