4.1 Introduction

Hydrocarbons that contain carbon-carbon double bond are called Alkenes (also called as Olefins ). Many alkenes are found in plant and animals. It has three sp² orbitals that lie in a plane with angles of 120°. One of the carbon–carbon bonds in a double bond is σ-bond, formed by the overlap of a sp² orbital of one carbon with a sp² orbital of the other carbon. The second carbon–carbon bond in the double bond is formed from side-to-side overlap of the remaining p -orbitals of the carbons. These two p -orbitals must be parallel to each other to achieve maximum orbital-orbital overlap. Therefore, all six atoms of the double-bond system are in the same plane (Figure 1). Since there is maximum side-to-side overlap, rotation about a double bond does not occur.

Figure 1

Each C-H σ -bond is formed by overlap of a sp² hybrid orbital of carbon atom with the 1 s orbital of a hydrogen atom. The C-H bond length in ethylene is slightly shorter than the C-H bond in ethane because the sp² orbital in ethylene has more s character that attracts the electrons even more strongly. The C=C bond in ethylene is much shorter than the C-C bond in ethane, partly because the σ -bond of ethylene is formed from sp² orbitals and partly because both σ- and π- bonds are attracting the atoms together (Figure 2).

Figure 2

Alkenes are said to be unsaturated because they are capable of adding hydrogen in the presence of a catalyst. An alkane is called as saturated because it cannot react with any more hydrogen.

4.2 Nomenclature of Alkenes

• Find the longest continuous chain of carbon atoms that includes the double bond and change the - ane ending of the parent alkane to - ene . The chain is numbered as the double bond having the lower possible numbers.

• If a chain has more than one substituent, the substituents are cited in alphabetical order. The prefixes di, tri, sec , and tert are not considered in alphabetizing, but iso, neo, and cyclo are considered. It should also contain the lowest substituent number.

• The double bond should be inbetween carbon 1 and 2, while numbering the ring.

• Lack of rotation of carbon-carbon double bond gives rise to cis - trans isomerism, also called geometrical isomerism. In cis -isomer, two similar groups bonded on the same side of the double bond. If the similar groups are on opposite sides of the double bond, then the alkene is a trans- isomer.