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Chemistry > Hydrocarbons > Nomenclature and Preparation of Alkenes
Hydrocarbons

Nomenclature and Preparation of Alkenes

Alkenes are necessary chemical compounds that exist in our homes and daily lives. However, we have never really given much thought to it. Let’s discuss some very common example of how alkenes have found their way to our home. We use different forms of plastic bottles and containers of plastics for storage purpose. Several times we have come across alkenes like ethane and propene. However, are you aware of the fact that these simple alkenes are important components of polystyrene and polyvinyl chlorides (PVC)? These chemical compounds are necessary for manufacturing many types of plastics. Alkene such as 1, 3- butadiene is used for making rubber products. In this topic, we will study the different methods of preparation of alkenes, a simple but necessary group of hydrocarbons.

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Alkenes

By now, we know that alkenes are important components of rubber and plastic products but what are alkenes. They are organic compounds containing double bonds in their chemical structure. They are interchangeably known as olefins. Alkenes are class of unsaturated hydrocarbons containing carbon and hydrogen atoms having one or more carbon-carbon double bonds in its chemical structure.

The unsaturation is because of the presence of one or more double bonds in its structure. The general chemical structure of alkene is RC=CR’. Acyclic alkenes are the subdivision of alkene containing only 1 double bond. However, it lacks any other functional groups, also refers to as mono-enes.

The general formula of the homologous series of hydrocarbon is CnH2n. Hence, alkene contains 2 hydrogen atoms less than its corresponding alkane. Ethylene having the formula C2H4 is the simplest alkene. The IUPAC name of ethylene is ethane and it has the largest industrial production among all the alkenes.

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Nomenclature of Alkenes

We have to select the longest chain of carbon atoms having the double bond in order to name an alkene according to the IUPAC system. We start numbering from the carbon chain near to the double bond. “Ene” replaces “ane” suffix present in the alkanes. Even though the first compound of ethane is methene but it has a short lifecycle.

Therefore, the first stable compound is ethylene or ethene (C2H4). Nomenclature or naming alkenes is very similar to alkanes according to the IUPAC system. Let us once go through the summary of the rules for nomenclature of alkenes with an example.

Rule 1

Identification of the longest continuous chain containing the carbon-carbon double bond is necessary. The parent name of the alkene compound will be similar to its corresponding alkane. Just the suffix “ene” will replace the suffix “ane”, in case of the alkene. Suffix “ene” represents the double of the alkene. For instance, if an organic compound continuous chain having a double-bond containing five carbon atoms, the compound will be pentene.

Rule 2

The numbering of the carbon atom starts on the side nearest to the double bond of the continuous chain. Refer to the example below:

alkenes

Hence, the numbering starts from right to left because only in this arrangement the double bonds remain in between the second and third carbon atom of the chain. On the contrary, if numbering begins from left to right, the placement of double bond will be in between the third and fourth carbon of the chain.

Hence, the rule of numbering from the nearest side of the double bond will not be possible. Therefore, the numbering starts from right to left in the above example. Moreover, it is important to indicate the position of the double bond. Thus, it is necessary to place the lower of the pair of numbers to the double containing carbon atoms before the name of the alkene. Therefore, in the above example, the name of the compound can be written as 2-pentene.

Rule 3

It is important to specify the location as well as the name of any substituent molecule present in the compound. For example, the below example can be written as 5-chloro-2-hexene.

alkenes

Rule 4

Another important rule to note in the nomenclature of an alkene is to correctly indicate the three-dimensional relationship of the organic compound, in this case of the given alkene. This means that whether the compound containing double bonds is present in cis or trans conformation. Therefore, to complete the nomenclature the compound, assigning the conformation is important. For example, the below example is cis-5-chloro-2-hexene.

alkenes

Few alkenes and their IUPAC names are:

alkenes

Preparation of Alkenes

Preparation of Alkenes from Alkynes

Alkynes undergo partial reduction with a particular quantity of dihydrogen to produce alkenes. The reaction takes place in the presence of palladised charcoal that has been deactivated with poisonous compounds such as quinolone or sulfur compounds, also known as Lindlar’s catalyst.

The reaction leads to the formation of compounds having cis-geometric conformation. However, if the same alkynes reduction takes placed with sodium present in liquid ammonia, the resultant product will be trans alkenes.

alkenes

alkenes

Preparation of Alkenes from Alkyl Halides

R-X or Alkyl Halides undergo heating with alcoholic potash to form alkenes. Dissolving potassium hydroxide in an alcohol such as ethanol leads to the formation of alcoholic potash. In this reaction, the heating of alkyl halides with alcoholic potash will remove one molecule from halogen acid leading to the formation of alkenes. Thus, it is a β-elimination reaction example because removal of hydrogen atom takes place from the β carbon atom.

alkenes

The rate of reaction is dependent on the alkyl group and nature of halogen atom. The order, in this case, is iodine > bromine > chlorine. The rate of reaction according to the alkyl group is tertiary > secondary > primary.

Preparation of Alkenes from Vicinal Dihalides

Vicinal halides are compounds containing two halogen atoms present on two adjacent carbon atoms. Vicinal dihalides undergo treatment zinc metal to form an alkene. The treatment of vicinal dihalides with zinc metal will help in removal of a molecule from ZnX2 thereby resulting in the formation of the alkene. The reaction is an example dehalogenation.

alkenes

Preparation of Alkenes from Acidic Dehydration

Hydroxy derivatives of alkanes in simple terms refer to as alcohols. R–OH represents an alcohol, where the formula of R is CnH2n+1. Alcohol reacts with the concentrated sulphuric acid in heating conditions to form alkenes. The reaction will remove one water molecule to form an alkene.

The reaction is an example of acidic dehydration of alcohols because the reaction results in the elimination of a water molecule in the presence of an acid, in this case, sulphuric acid. Furthermore, the reaction is also an example of β-elimination reaction because the functional group of alcohol (-OH) eliminates extracts one hydrogen atom from β-carbon atom.

alkenes

Solved Questions for You

Question 1: Write IUPAC names of the following compound:alkenes

Solution: 2, 8-Dimethyl-3, 6- decadiene

Question 2: Write IUPAC names of the following compound:

alkenes

Solution: 4-Ethyl-2, 6-dimethyl-dec-4-ene

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