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Alkenes

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Hydrocarbons are commonly defined as the compounds which are mainly composed of carbon and hydrogen atoms. On the basis of saturation, hydrocarbons can be classified as alkanes, alkenes and alkynes. Alkanes are saturated hydrocarbons with single covalent bonds between carbon atoms.

Alkenes and alkynes are unsaturated hydrocarbons with double or triple covalent bonds between carbon atoms. In this article we will discuss about chemical and physical properties of alkenes with their uses. Before starting this discussion let’s have a look on the difference in the bonding of alkenes from alkane.     

Definition

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Alkenes are a family of hydrocarbons with at least one double bond between carbon atoms of the parent chain of the hydrocarbon. Some common examples of alkenes are ethene, propene, butene etc.
The un-hybridized  orbital over each carbon atom involves in side-way overlapping to form pi-bond. The electron density of pi-bond gets distributed in two equal halves, above and below the plane of carbon atoms.

It makes the pi-bond weaker compare to sigma bond. Since pi-bond is placed perpendicular to the sigma bond, it hinders the free rotation of carbon –carbon bond in alkene which is mainly found in alkane as they do not have any restriction on the free rotation of carbon-carbon bond. That is the major difference between alkane and alkenes.
 Alkanes show different conformations due to free rotation of carbon-carbon bond which is absent in alkene and leads the formation of geometrical isomers of alkenes. Since the pi electrons are not as fully under the control of the carbon atoms like sigma electrons therefore they can easily attack by other reagents to form additional compounds. 

Formula

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The general formula of alkene is CnH2n which stands that the number of hydrogen atoms is just double of carbon atoms in alkene molecules. In a double covalent bond between two carbon atoms, one covalent bond is sigma bond and other is pi-bond.

Both the double bonded carbon atoms are sp2 hybridized and have three sp2 hybridized orbitals with one un-hybridized 2pz orbital. These three hybridized orbitals are arraigned in trigonal planer manner with 120o of bond angle.One of the sp2 hybridized orbital gets overlap with another sp2 hybridized orbital to form a sigma bond between both carbon atoms whereas remaining two hybridized orbitals form sigma bonds with hydrogen atoms.  

Naming Alkenes

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The unsaturation of alkenes also follows the IUPAC nomenclature like alkane. The name of alkenes is named by replacing the -ane of the parent molecule with –ene suffix. For writing the name of an alkene, first find the longest chain or parent chain of the molecule. Remember the parent chain should contain the double bond of the molecule and also most substituted chain. 
Now give the lowest possible number to the double bond of alkene. In case of cycloalkane, the double bond will get always one number. Alkenes with same molecular formula but different position of double bond are called as constitutional (positional) isomers of each other.

The substituent of the alkene must be indicated with the position of them. In the presence of more than one substituent, the name of substituent must be written in alphabetical order. The cis and trans-isomers can be identified as cis and trans prefix before the root word followed by –ene suffix.
 For Example: In CH3CH=CH2,  the functional group is an alkene that indicated by suffix –ene. The longest chain or parent chain  contains three carbon atoms therefore the root word would be prop and complete name would be propene. Similarly in this alkene
Alkanes

the functional group is same that is –ene with root word But- as there are four carbon atoms in the parent chain. The position of double bond is in between C-2 and C-3 therefore the IUPAC name would be  but-2-ene or 2-butene. In case of cyclic alkenes like

Butane
the functional group is –ene with longest continuous chain of six carbon atoms. Therefore the root word would be Hex-. The cyclic structure of alkene indicates by Cyclo-prefix.  Hence the name of alkene would be Cyclohexene.  In case of branched alkenes like (CH3)2CHCH=CHCH3, the longest chain or parent chain contains five carbon atoms, hence root word would be pent-.  

The double bond is placed between C-2 and C-3, hence the locant would be 2-. There is a substituent methyl- at C-4 position, so the IUPAC name of molecule would be 4-methylpent-2-ene or 4-methyl-2-pentene.

Preparation of Alkenes

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There are many preparation methods of alkenes. Let’s have a look on some of the preparation methods of alkene:

Dehydration of alcohols: In the presence of catalyst like strong acid or aluminium chloride and high temperature alcohols dehydrate to form alkenes. As the branching of alcohols increases, the reactivity of alcohols increases and dehydration will occur with high speed. In other words, tertiary alcohols are more reactive and easily dehydrate at 25°– 80°C to form alkenes whereas primary alcohols requires 170° - 180°C for dehydration.
The simplest example is dehydration of ethanol to form ethene:

$CH_3CH_2OH \to CH_2=CH_2+H_2O$

Ethanol vapor is passed over heated Al2O3 powder to give ethene and water vapour. Similar dehydration can proceed in the presence of acid catalyst like concentrated sulphuric acid or concentrated phosphoric(V) acid.
Dehydrohalogenation of haloalkanes: Alkenes can also form with the help of alkyl halides by dehydrohalogenation reaction in the presence of alcoholic potassium hydroxide. For example dehydrohalogenation of bromoethane in the presence of alcoholic potassium hydroxide leads to the formation of ethene with potassium bromide and water as given below;