Chemistry of Alkenes

Alkenes are a type of aliphatic hydrocarbons, that contain one or more double bonds in the hydrocarbon chain. The hybridization of the carbon,  that has been included in double bonds, is sp². Since there are double bonds in alkenes, they are also a type of unsaturated hydrocarbons.

So, they could undergo electrophilic addition reactions from the cleavage of the pi bond. Therefore, alkenes are used in the preparation of polymers like polythene, polypropylene, etc.

The empirical formula of alkenes that has only one double bond is C_nH_2n. Carbon atoms bond linearly to form acyclic alkenes. Sometimes there are branches in the carbon backbone. Cyclic alkenes can also be found when carbon atoms bonded as cycles. In this article, we discuss linear alkenes that only have one double bond.

Properties of alkenes

Alkenes are nonpolar compounds. Therefore, they have only Wan-der Waals interactions between molecules as secondary interaction forces. When the number of carbon atoms increases, the molecule gets bigger, so it gets a higher surface area. Thus, the Wan-der Waals forces also increase. When the number of carbon atoms are increasing it could be a liquid or a solid at room temperature.

Due to the double bond, alkenes could form diastereoisomers as cis and trans structures. The isomerism also affects the melting and boiling point of the alkenes. As an example, butane forms three different isomers 1-butene, trans-2-butene, and cis-2-butene. Although the number of carbon atoms is the same in each structure, they have different melting and boiling points.

Usually, when an alkene has a linear structure it gets a higher surface area. Therefore, the wander Waals forces are high. Thus, the melting points and the boiling points of linear alkenes are high compared to the branched alkenes with a similar number of carbon atoms.

Since alkenes are nonpolar compounds, they do not dissolve in polar solvents like water. But they are soluble in organic solvents.

General nomenclature of alkenes

Linear alkenes without any pendent groups and diastereoisomers are named according to their number of carbon atoms in the carbon backbone. The stem name is added “ene” suffix to name an alkane.

IUPAC nomenclature of alkenes

There can be different structural formulas for one empirical formula. As an example, C₄H₈ empirical formula has two different structural formulas as follows.

IUPAC nomenclature is introduced considering the structural formula as well as the empirical formula.

Steps to name an alkane

1. Identify the longest hydrocarbon chain that includes the double bond and has a maximum number of substituent groups.
2. Number the hydrocarbon chain where the carbon atoms that are included in the double bond get the lowest possible number. If it takes the same number from either side, choose the way the substitute groups get the Lowes numbers also.
3. Name the substituent groups according to the alphabetical order. When naming a substituent group, it writes the number of the carbon atom and then puts a dash and then writes the name of the alkyl group.
4. Then it writes the stem name of the hydrocarbon chain and then puts a dash, writes the number of the carbon atom that has a double bond, then puts a dash again. Finally, add the suffix “ene” at the end.

• All the letters should be simple letters and the name must be a single word.
• If there are similar alkyl groups in different carbon atoms they are named as follows.

Hybridization of carbon in alkane

In an alkene, there are two types of bonds which are pi bonds and sigma bonds. Carbon atoms that form double bonds are sp² hybridized. Carbon atoms that only form sigma bonds are sp3 hybridized.

At ground state, carbon has four electrons in its valence shell. The electron configuration of carbon at the ground state is 1s² 2s² 2p². When it is supplied energy, an electron in 2s orbital moves 2p. Thus, four unpaired electrons are obtained. It can be represented in an energy diagram as follows (figure 05).

Two of the 2p atomic orbitals and 2s orbital are hybridized and form three sp² hybridized orbitals. After hybridization, it remains one un-hybridized 2p orbital that contains one lone electron. The three sp2 hybridized orbitals are similar in size, shape, and energy. They have lower energy than the un-hybridized 2p orbital and higher energy than the 2s orbital.

These sp² hybridized orbitals remain trigonal planar geometry and 120° angle to each other. The un-hybridized 2p orbital remains perpendicular to the trigonal planar. In alkenes C=C double bond is formed by the linear overlapping of sp² hybridized orbitals. A C-H bond is formed by the linear overlapping of the sp² hybridized orbital and un-hybridized s orbital of hydrogen.

Preparation of alkenes

If there is hydrogen on the carbon next to the -OH bonded carbon in an alcohol, such alcohols are treated with anhydrous aluminum oxide and heated up to 350 ⁰C to obtain alkenes.

A similar result can be obtained using concentrated sulfuric acid and heating up to 170 ^oC. Here, the -OH group and the hydrogen in the neighboring carbon will be eliminated as a water molecule. If there are two neighboring carbon atoms that have hydrogen, hydrogen will be eliminated from the carbon with the least hydrogen.

If there is hydrogen on the carbon next to the halogen-bonded carbon in an alkyl halide, such alkyl halides are treated with alcoholic KOH. Here, the halogen and the hydrogen in the neighboring carbon will be eliminated as a hydrogen halide molecule.

If there are two neighboring carbon atoms that have hydrogen, hydrogen will be eliminated from the carbon with the least hydrogen.