Alkenes: We are familiar with saturated hydrocarbons-alkanes, but there is another family of hydrocarbons found in crude oils that is unsaturated and is more reactive. In this article, let’s understand everything about unsaturated hydrocarbon having at least one double bond between carbon-carbon atoms, that is Alkenes.

What is Alkene?

Alkenes are unsaturated hydrocarbons having a double bond between the carbon atoms. There is at least one double bond between carbon-carbon atoms. The general formula of alkenes is \( { {\text{C}}_ {\text{n}}} { {\text{H}}_ { {\text{2n}}}}\). Alkenes are often used as a synonym of olefin.

The name olefin is derived from the Greek word olefin gas, which means oil forming. The presence of double bonds in alkenes makes them highly reactive chemically.

Nomenclature of Alkenes

The general formula of alkenes is \( { {\text{C}}_ {\text{n}}} { {\text{H}}_ { {\text{2n}}}}\) where n-\(2,\,3,\,4,\,5,……\) According to the IUPAC system, the names of alkenes have a suffix-ene, and the prefix depends on the number of carbon atoms. Some names are as follows:

No. of Carbon Atoms	Formula	IUPAC Name
\(2\)	\( { {\text{C}}_2} { {\text{H}}_4}\)	Ethene
\(3\)	\( { {\text{C}}_3} { {\text{H}}_6}\)	Propene
\(4\)	\( { {\text{C}}_4} { {\text{H}}_8}\)	Butene
\(5\)	\( { {\text{C}}_5} { {\text{H}}_10}\)	Pentene
\(6\)	\( { {\text{C}}_6} { {\text{H}}_12}\)	Hexene
\(7\)	\( { {\text{C}}_7} { {\text{H}}_14}\)	Heptene
\(8\)	\( { {\text{C}}_8} { {\text{H}}_16}\)	Octene
\(9\)	\( { {\text{C}}_9} { {\text{H}}_18}\)	Nonene
\(10\)	\( { {\text{C}}_10} { {\text{H}}_20}\)	Decene

In the case of the substituted alkenes, the numbering of the parent chain is done from the end, which is near to the double bond or gives the lowest number to the carbon atom carrying the double bond. A few examples are shown below:

Isomerism In Alkene

Alkenes show both structural isomerism and stereoisomerism.

Structural Isomerism

Ethene and propene have only one structure, but alkenes containing four or more carbon atoms show both chain and position isomerism.

a.) In chain isomerism, the position of the double bond is not changed.
Thus, for example, in Butene, the chain isomerism will be as follows:

b.) In position isomerism, the isomers differ for the position of the double bond. For example, two isomers of butene are as follows:

Stereoisomerism or Geometrical Isomerism

Such isomers with the same structural formula differ in the relative spatial arrangement of atoms or groups of atoms around the double bond called geometrical isomers. Thus, the phenomenon is called geometrical isomerism. The isomer in which a similar group of atoms is present on the same side of the double bond is called cis-isomer. On the other hand, the isomer which has similar bonds on the opposite side is called trans-isomerism.

For example,

General Methods of Preparation of Alkenes

The Below list of methods is used to prepare alkenes:

1. By partial reduction of alkynes – Alkenes can be prepared by reducing alkynes with hydrogen in the presence of a specific catalyst palladised charcoal, which is moderately deactivated with the help of quinoline or sulfur compounds.

2. From alkyl halides – Alkyl halides on halides on heating with a strong base such as sodium ethoxide or a concentrated alcoholic solution of potassium hydroxide undergo dehydrogenation to give alkenes.

3. From vicinal halides – Dihalogen derivatives of alkanes in which two halogen atoms are present on the adjacent carbon atoms are called vicinal halides. Alkenes can be prepared by heating the suitable dihalides with zinc dust in methanol or ethanol.

4. From alcohols- Alcohols on heating with mineral acids such as concentrated sulphuric acid eliminate a water molecule to form alkenes.

Laboratory Method of Preparation

Ethylene is prepared in the laboratory by heating ethyl alcohol with conc.
\({{\text{H}}_{\text{2}}}{\text{S}}{{\text{O}}_{\text{4}}}\) at \(440\, {\text{K}}\). These are mixed in the ratio of \(1:2\) by volume and is added from a dropping funnel into a round-bottomed flask containing anhydrous \({\text{A}}{{\text{l}}_{\text{2}}}{\left({{\text{S}}{{\text{O}}_{\text{4}}}} \right)_{\text{3}}}\) and sand in which \({\text{A}}{{\text{l}}_{\text{2}}}{\left({{\text{S}}{{\text{O}}_{\text{4}}}} \right)_{\text{3}}}\) catalyzes the reaction, whereas sand is used to avoid frothing.

The ethene produced contains impurities that can be removed by passing it through a solution of sodium hydroxide. Pure ethene is collected over water.

Physical Properties of Alkenes

1. Physical state- The first three members are colorless gases; the next fourteen \(\left({{{\text{C}}_{\text{5}}}{\text{-}}{{\text{C}}_{{\text{18}}}}} \right)\) members are liquids, while the higher ones are solids.
2. Colour and smell- Except ethene which has a pleasant smell, all other alkenes are colorless and odourless.
3. Solubility- Alkenes are insoluble in water but fairly soluble in non-polar solvents such as benzene, petroleum, ether, etc.
4. Boiling Points- The boiling points increase with an increase in the molecular mass. Straight-chain alkenes have higher boiling points than isomeric branched chain alkenes.

5. Density- All alkenes are lighter than water. The densities of alkenes increase with an increase in the molecular mass till the limiting value of \({\text{0}}{\text{.8}}\,{\text{g}}\,{\text{c}}{{\text{m}}^{{\text{-3}}}}\) is reached.

Chemical Reactions of Alkenes

1. Addition of Hydrogen- Alkenes readily add hydrogen in the presence of either of the catalyst nickel, palladium, platinum at room temperature to form alkanes. For example,

2. Addition of halogens- Alkenes reacts with halogens by mixing in an inert solution like \({\text{CC}}{{\text{l}}_4}\) to form halogen derivatives.

3. Addition of halogen halides- Alkenes react with halogen halides \(\left( {{\rm{HCl, HBr, HI}}} \right)\) to form mono haloalkanes called alkyl halides.

Uses of Alkenes

1. Alkenes and substituted alkenes upon polymerization form many useful compounds like polythene, PVC, Teflon, orlon, etc.
2. Its lower members are used as fuels and illuminants.
3. It is used for the artificial ripening of fruits.
4. Ethylene is used in the manufacture of ethyl alcohol and ethylene glycol.
5. It is used for making anti-knock for car engines.
6. Ethylene is used in making oxygen-ethylene flame for cutting and welding.

Summary

This article concludes that alkenes are unsaturated hydrocarbons with at least one double bond between carbon atoms. Also, we learned its preparation, properties, decolourization and isomerism.

FAQs on Alkenes

Q.1. What is the function of alkenes?
Ans: Alkenes are useful in many ways as they are used for the artificial ripening of fruits, making anti-knock for car engines, in the manufacture of ethyl alcohol and ethylene glycol, and many useful compounds like polythene, PVC, Teflon, and orlon.

Q.2. What are the first ten alkenes?
Ans: The following is a list of the first ten alkenes:
Ethene \(\left({{{\text{C}}_{\text{2}}}{{\text{H}}_{\text{4}}}} \right)\)
Propene \(\left({{{\text{C}}_{\text{3}}}{{\text{H}}_{\text{6}}}} \right)\)
Butene \(\left({{{\text{C}}_{\text{4}}}{{\text{H}}_{\text{8}}}} \right)\)
Pentene \(\left({{{\text{C}}_{\text{5}}}{{\text{H}}_{\text{10}}}} \right)\)
Hexene \(\left({{{\text{C}}_{\text{6}}}{{\text{H}}_{\text{12}}}} \right)\)
Heptene \(\left({{{\text{C}}_{\text{7}}}{{\text{H}}_{\text{14}}}} \right)\)
Octene \(\left({{{\text{C}}_{\text{8}}}{{\text{H}}_{\text{16}}}} \right)\)
Nonene \(\left({{{\text{C}}_{\text{9}}}{{\text{H}}_{\text{18}}}} \right)\)
Decene \(\left({{{\text{C}}_{\text{10}}}{{\text{H}}_{\text{20}}}} \right)\)

Q.3. What are the characteristics of alkenes?
Ans: The first three members are colorless gases; the next fourteen \(\left({{{\text{C}}_5}{\text{-}}{{\text{C}}_{18}}} \right)\) members are liquids, while the higher ones are solids, except ethene which has a pleasant smell, all other alkenes are colorless and odorless, alkenes are insoluble in water but fairly soluble in non-polar solvents such as benzene, petroleum, ether, etc., the boiling points increase with an increase in the molecular mass. Straight-chain alkenes have higher boiling points than isomeric branched chain alkenes. 

Q.4. What is an alkene in chemistry? 
Ans: Alkenes are unsaturated hydrocarbons having a double bond between the carbon atoms. There is at least one double bond between carbon-carbon atoms. The general formula of alkenes is \({\text{C}}_{\text{n}} {\text{H}}_{\text{2n}}\). Alkenes are often used as a synonym of olefin. The name olefin is derived from the Greek word olefin gas, which means oil forming. The presence of double bonds in alkenes makes them highly reactive chemically.

Q.5. What are the 2 uses of alkenes?
Ans: The two uses of alkenes are-
Alkenes and substituted alkenes upon polymerization from many useful compounds like polythene, PVC, Teflon, orlon, etc.
Alkenes are used for the artificial ripening of fruits.

Q.6. How do you test for an alkene?
Ans: Alkenes can be tested by using bromine water. Alkenes decolorise the solution.

Q.7. What are the 3 properties of alkenes?
Ans: The three properties of alkenes are as follows-
They are colourless gases.
They are insoluble in water.
They are lighter than water.

Q.8. What is the general formula for alkenes?
Ans: The general formula of alkenes is \({\text{C}}_{\text{n}} {\text{H}}_{\text{2n}}\).