Chemical Properties of Alkenes

Alkenes - Member of Hydrocarbon Family

In organic chemistry, we study the various types of compounds formed from the carbon atom. Based on its bonding organic compounds are of three types Alkanes (single bond), Alkenes (double bonds), and Alkynes (triple bonds). When there is a single bond between the carbon-carbon atom then it forms alkanes (C-C). Whenever there is a double bond between the carbon-carbon atom then alkenes (C=C) are formed. In case if it shares a triple bond between carbon-carbon atom then it is alkynes (C≡C). A large number of pie bonds are present in alkanes that are closely held due to which they show a variety of chemical properties.

Chemical Property of Alkenes

  • Alkenes belong to the family of hydrocarbons containing a double bond between carbon-carbon atoms.

  • Alkenes are less stable than alkanes and more stable than alkynes.

Single bond > Double bond >Triple bond

  • Alkenes exits in all three solid-liquid and gaseous states.

  • Alkenes are less soluble in water due to weak Van-Der-Waal forces

  • The boiling point of alkenes depends on the molecular structure, the longer will be the molecular chain, higher will be its boiling point.

  • Functional groups are responsible for the polarity of alkenes.

Reactions of Alkanes

Unsaturated alkenes compounds are highly reactive. They form a double bond between carbon-carbon atoms which makes them less stable due to loosely held pi bonds.

Addition of Halogens 

Alkene group reacts with halogens such as bromine, chlorine except for iodine as it’s an exception and it does not react in normal conditions. When the alkene group reacts with bromine, then bromine reduces its reddish-orange color because of the addition of an unsaturated carbon long chain.

For the testing of the presence of unsaturation in a solution, it is reacted with bromine if bromine loses its reddish-brown color in carbon tetrachloride solution then there is the presence of unsaturation site in the solution.

These types of reactions are called electrophilic reactions. One of the electrophilic reaction is as follows: 

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Addition of Water

These reactions follow the Markovnikov Rule according to which on the reaction of water with an alkene group there is the formation of alcohols i.e (OH) functional group and the reaction occurs in the presence of sulphuric acid.


The reaction of an alkene with water is as shown: 

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Addition of Hydrogen Halides to Alkenes

Whenever hydrogen halides are reacted with an alkyl group they form alkyl halides. Alkyl halides are formed with the help of two rule one Markovnikov rule and another Anti-Markovnikov rule.

Reactivity order of halogens is:

                            HI>HBr>HCL

According to Markovnikov Rule:

When hydrogen halides are added with an unsymmetrical alkyl group then the molecule or the negative portion which needs to be added to the carbon chain gets attached with the carbon atom which is attached to the least number of hydrogen atoms.

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According to Anti-Raskolnikov Rule:

When hydrogen halides are reacted with an unsymmetrical alkyl group in the presence of peroxide then the product obtained is opposite of markovnikov rule. Molecules that need to be added to the carbon chain attach themselves to the carbon atom which is attached to more number of hydrogen atoms.

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Addition of Dihydrogen

When alkenes groups are reacted with one molecule of dihydrogen i.e one diatomic molecule of hydrogen then this reaction results in the formation of alkanes in the presence of different types of catalysts such as palladium, nickel, cobalt or platinum.

Catalyst, when added to the reaction results in the increase of the rate of reaction or we can say they help to speed up the reaction without affecting the thermodynamics of the reaction.

Oxidation Reaction

  • Combustion Reaction:- The combustion reaction formed by alkenes is very exothermic because as a result a huge amount of thermal energy is produced. Welding of metals is a practical example of a combustion reaction of alkene which is also known as oxyethylene welding.

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  • Oxidation by Potassium Permanganate:- Another reaction used for testing the presence of unsaturation in solution. In this reaction, alkenes are reacted with Baeyer's reagent i.e with cold dilute KMnO4 and as a result, vicinal glycols are formed. Moreover, the pink color of the KMnO4 also gets faded. Hence, this reaction can also be preferred for testing the presence of an unsaturation site.

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Ozonolysis

Another significant chemical property of alkene compounds which on the addition of ozone or three molecules led to the formation of ozone at which upon reduction with zinc dust and water produces aldehydes and ketones respectively. This reaction is also considered as a method of preparation for aldehydes and ketones.

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Oxidation reaction and Ozonolysis are prominently exhibited in the chemical properties of alkanes 

FAQ (Frequently Asked Questions)

1.Why are Alkenes Important Chemicals?

Alkenes are important compounds as they are used in diverse applications by the industries. They are used for the synthesis of alcohol, plastics, fuels, and detergents. There are many alkenes which are important among which ethene, propene, and 1,3-butadiene are the most important ones.

  • Ethene is used for the production of crude oil and natural gases. It is also used as feedstock for chemical products as vinyl chloride and ethanol.

  • Propane is used for the production of oxidation products such as acrylic acids, glycerol, ester, etc.

  • 1,3-butadiene is used for the synthesis of various types of synthetic rubbers. 

2. Why are Alkenes More Reactive than Alkanes?

Alkenes are the compounds made up of double bonds between carbon atoms with loosely held pi bonds. Alkenes are relatively stable compounds but more reactive than alkanes as pi bonds are stronger than sigma bonds. Alkenes belong to the family of hydrocarbons containing a double bond. Most alkenes reactions involve the addition of a pi bond to obtain a new single bond compound. Whereas alkanes have single bonds and sp3-sp3 hybridization with sigma bonds. Sigma bond requires more energy for the action that’s why it is less reactive than alkenes compounds.