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Reactions of Alkenes

The organic compounds which are composed of carbon and hydrogen are called as hydrocarbons. On the basis of single and multiple covalent bonds in the molecules, they can be classified as alkanes, alkenes and alkynes. Alkanes are saturated hydrocarbons with single covalent bonds in the between the carbon atoms. Alkenes and alkynes have multiple covalent bonds either double or triple covalent bonds in the molecule.
Alkenes
The presence of multiple covalent bonds makes the molecule unsaturated in nature. The unsaturated hydrocarbons show different chemical and physical properties compare to alkanes. In a double bond, there is one pi bond and one sigma bond. Since the pi bond is weaker compare to sigma bond therefore alkenes can readily show addition reactions and involves the cleavage of the pi bond in the molecule. For example, the addition of hydrogen to alkene results the formation of alkanes. Let’s have a look on some preparation methods and properties of alkenes.

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Synthesis of Alkene

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Alkene is an unsaturated hydrocarbon which can be synthesized either by using elimination reactions of saturated hydrocarbons like alkane or haloalkanes or by controlled addition reaction of alkyne or substituted alkynes.

Some methods to synthesis alkene are as follows.

1. Dehydrohalogenation of haloalkanes

  1. The reaction of a haloalkane with an alcoholic solution of potassium hydroxide at high temperature forms an alkene with the elimination of a molecule of hydrogen.
  2. The hydrogen of an alkyl halide which is eliminated is come from a beta-carbon and the halogen comes from alpha-carbon.
  3. Since in this reaction the hydrogen is lost from beta-carbon, therefore the reaction is classified as beta-elimination.

For example, dehydrohalogenation of 2-Bromopropane forms propene in the presence of alcoholic solution of base like sodium hydroxide. 
Example of Dehydrohalogenation


Reaction proceeds through the attack of base like hydroxy ion on double bonded carbon atoms and remove hydrogen in the form of water followed by the removal of halide ion which further reacts with sodium ion to form sodium bromide.

However, if the structure of haloalkane is such type that it can undergo elimination in two different ways, due to the availability of different types of ß-hydrogen atoms , then the more highly substituted alkene which have less number of hydrogen atoms on double bonded carbon atoms is the major product of dehydrohalogenation. This generalization is called as Satyzeff’s rule.

Example of Dehydrohalogenation of Haloalkanes

For example, dehydrohalogenation of 2-Bromo-3-methylbutane forms more substituted alkene 2-Methylbut-2-ene as a major product with less substituted 3-Methylbut-1-ene as minor one. Thus the rate of dehydrohalogenation of different haloalkanes increases with increasing the number of beta-hydrogen atoms on double bonded carbon atoms.The rate of dehydrohalogenation for different haloalkanes.
(CH3)3C-Cl> (CH3)2CHCl>CH3CH2Cl
hence,

Tertiary haloalkane > Secondary haloalkane > primary haloalkane

For same alkyl group with different halo groups the rate of reaction increases from.

R-I > R-Br > R-Cl > R-F
As the rate determining step involve the cleavage of carbon-halogen bond.


2. Dehydration of alcohols


Dehydration that is removal of water molecules, of alcohols can lead to the formation of alkene. This dehydration can be carried out either in the presence of protonic acid like concentrated sulfuric acid or phosphoric acid or catalyst like anhydrous zinc chloride or alumina.

When the primary alcohols are heated with conc. H2SO4 at 433 - 443 K temperature, they undergo intramolecular dehydration to form alkenes. For example; ethanol forms ethene in the presence of conc. H2SO4.
Conc.H2SO4
CH3CH2OH $\to$CH2=CH2 + H2O
433-443K
However the dehydration of secondary and tertiary alcohol takes place under milder conditions like 85-20% of conc. H2SO4 at comparatively low temperature. For example, dehydration of propan-2-ol takes place at 440 k temperature with 85% H2SO4 to gives propene.
85%.H2SO4
CH3CH(OH)CH3 $\to$ CH3CH=CH2 +H2O
440K
Similarly dehydration of 2-methyl-2-propanol gives 2-methylpropene at 358K temperature with 20% H2SO4.
20% H2SO4
CH3C(CH3)(OH)CH3 $\to$CH3C(CH3)=CH2 +H2O
358 K
The reactivity of alcohols for dehydration follows the following order.

Tertiary alcohol> Secondary alcohol> Primary alcohol

The mechanism of dehydration involves certain steps.
  • Attack of proton (H+) from acid on alcoholic oxygen atom leads to the formation of protonated alcohol.
  • Loss of water to form carbocation.
  • Loss of proton to give an alkene.
  • Generally primary alcohols follow E2 path in elimination through the formation of carbocation as an intermediate.
Dehydration of Alcohols
While secondary and tertiary alcohols follow E1 mechanism through transition state as an intermediate in dehydration process. 

Intermediate in Dehydration

If there is possibility to get stable carbocation by alkyl or hydride shifting, than stable carbocation will form. For example; dehydration of 2,2-dimethylpropan-1-ol show the conversion of primary carbocation to more stable secondary carbocation by alkyl shifting and form 2-methylbut-2-ene as major product with 20methylbut-1-ene as minor product.

Example of Dehydration of 2-2 Dimethylpropanol

3. Reduction of alkynes


The selective reduction of alkynes leads to the formation of either E- or Z-alkenes by choosing appropriate reaction conditions. For example, with an inactivated transition metal catalyst like the Lindlar catalyst (Pd/CaCO3), alkyne reduces to Z-alkenes. 


Reduction of Alkynes

Reaction follows the formation of alkenyl anion radical as an intermediate which further reacts with ammonia to form E/trans-alkene.

Formation of Alkenylanion Radical


However in the presence of alkali metal like sodium dissolved in ammonia gives E-alkenes. Some other catalyst like palladium in charcoal or platinum in the form PtO2, and Raney-Nickel can be used for same process. Hydrogenation of alkene is an example of heterogeneous catalysis which is a multistep process based on absorption.

First the hydrogen molecules get absorbed on the metal surface and form metal-hydrogen bond. In the same way, alkyne also absorbed on the surface of metal and a hydrogen atom is then transferred to the alkene and form carbon-hydrogen bond. Similarly another hydrogen atom bonded to alkene to form alkene. Since this addition takes place on flat metal surface, therefore hydrogen must be added on same side and show syn-addition to form cis-alkene.

Catalytic Hydrogenation Mechanism 

4. Electrolysis of salts of dicarboxylic acids


This process leads to the formation of alkene from sodium of potassium salts of dicarboxylic acids. Reaction is also called as Kolbe’s Electrolysis. For example; electrolysis of sodium salts of dibasic acid like pentanedioic acid (Glutaric acid) forms propene. 

When an aqueous solution of a sodium salt of pentanedioic acid is electrolyzed, an alkene is produced with carbon dioxide at anode. 
-OOC-CH2- CH2-CH2-COO- $\to$ CH3CH=CH2 + 2CO2(g) +2e-
At cathode hydrogen gas releases with hydroxyl ion which further react with sodium to form sodium hydroxide.

2H2O +2e- $\to$ 2OH- +H2(g)
2K+ +2OH- $\to$ 2KOH

Electrolysis of Salts of Dicarboxylic Acids

5. Dehalogenation of vicinal dihalides


The dehalogeantion of vicinal dihalide gives alkene. The reaction takes place in the presence of alcoholic solution of zinc at high temperature. 

For example, Dehalogenation of 2, 3-Dibromobutane leads to the formation of 2-Butene in the presence of catalyst.

Dehalogenation of Vicinal Dihalides

6. Cracking


The cleavage of large hydrocarbon molecules to smaller more useful molecules in the presence of high pressure and temperature is called as cracking. Cracking of higher saturated hydrocarbons form a mixture of lower alkanes and alkenes.

For example, cracking of kerosene or candle wax is used in formation of ethene gas. The kerosene which soaked in sand first vaporizes and then cracks into smaller molecules like ethene which passed over water and collected in gas jar by displacement of water. No doubt this ethene gas is not pure as the cracking of kerosene formed many other hydrocarbons also.

Cracking of Kerosene

Generally cracking process follows free radical mechanism. Like cracking of propane form many lower hydrocarbons including alkene. These products are ethene, methane, ethane, propene and butane.
Cracking Process

7. From vinyl halide

  • Some common synthetic reagents like Grignard reagent of dialkyl copper also used in synthesis of alkenes.
  • The reaction of vinyl chloride with Grignard reagent gives higher.
  • The alkene will have more number of carbon atoms compare to Grignard reagent and vinyl halide.
For example, Ethyl magnesium bromide with vinyl bromide forms 1-Butene by substitution reaction.
CH3CH2MgBr + BrCH=CH2 $\to$ CH3CH2CH=CH2 + MgBr2
Similarly Vinyl halides can also react with dialkyl copper to form higher alkenes. 
2CH2=CHBr + R2Cu $\to$ 2CH2=CHR + CuCl2

Cyclic Alkenes

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Some Points about Cyclic Alkenes:
  1. Cyclic alkenes are the type unsaturated hydrocarbon which contains of contains a closed ring of carbon atoms with double bond.
  2. But in cyclic alkene, double bonds are not in conjugation; hence they are not aromatic in nature.
  3. In IUPAC nomenclature cyclic alkenes named as cycloalkenes.
  4. The general formula of cyclic alkene is CnH2n-2.
  5. Some examples of cyclic alkenes are Cyclobutene and Cyclopentene. Cycloalkenes have wide applications in synthesis of different polymers.
  6. Just like aliphatic alkenes, cycloalkenes can show geometrical isomerism. Generally cis-form of cyclic alkene is more common compare to trans-form.
Cyclic Alkene

Just like aliphatic alkenes, cycloalkenes can be synthesized following methods.


1. By dehydration of cyclic alcohols


Cyclohexanol undergoes dehydration in the presence of concentrated acids like phosphoric acid forms Cyclohexene.
Cyclohexanol Undergoes Dehydration

Dehydration of cyclic alcohol involves the formation phenyl cation which further lose proton to form cyclic alkene.
Dehydration of Cyclic Alcohol


2. By Diels-Alder cycloaddition


Diels-Alder cycloaddition is a [4+2]-cycloaddition of a conjugated diene and an alkene or alkyne also termed as dienophile to form cyclic alkenes. It is a type of pericyclic or an electrocyclic reaction which involves the 4 p-electrons of the diene and 2 p-electrons of the dienophile to form cycloalkenes. Reaction involves the cleavage of less stable pi bond to more stable sigma bonds. 

Diels Alder Cycloaddition
The most common example of Diels-Alder cycloaddition is additional reaction of 1,3-butadien and ethene to form Cyclohexene.

Example of Diels Alder Cycloaddition

If there is any electron withdrawing group (EWG) bonded to dienophile, rate of reaction increases. Due to presence of electron withdrawing group, they pull the electrons away from the dienophile and make it susceptible for the attack of diene and form substituted cyclic alkenes. Some common examples of electron withdrawing groups are carbonyl , nitrile, trifluoro and nitro groups.

Example of Electron Withdrawing Group

Alkene Stability

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Alkenes are stable hydrocarbons but less stable compare to stable hydrocarbons; alkane. The presence of pi bond in molecules makes the molecules more reactive and less stable. The most common chemical reaction of alkene is the addition to a carbon-carbon double bond. 

Generally addition reactions of alkenes are exothermic in nature because of the Carbon-Carbon pi-bond is relatively weak has bond energy 63 kcal/mole compare to the sigma-bonds formed to the atoms or groups of the reagent. The bond dissociation energy of carbon-carbon double bond is 146 kcal/mol and for single bond it is 83 kcal/mol.Bond dissociation energies BDE

C=C BDE = 146 kcal/mol
C-C BDE = - 83 kcal/mol
----------------------------------------------
Stability of Pi bond = 63 kcal/mol 

In any chemical reactions some bond energies required to break all the covalent bonds in the molecule. If in any chemical reaction the bond energies of the product molecules are more than the bond energies of the reactants, hence reaction will be exothermic in nature.>The reactions of alkenes are exothermic in nature, like hydro halogenation of ethene releases 16.5 kcal/mol as the energy release in the formation of new bond is more than the amount of energy requires to cleave bonds in reactant molecules. The heat of energy involve in a chemical reaction is depends on the stability of alkenes. The stability of alkenes depends on three factors.

1. Degree of substitution


Since there is one or more double bond present in alkenes, hence the number of hydrogen is less in alkene compare to an alkane having same number of carbon atom and that is the reason it is referred to as unsaturated. For example, ethene and ethane both have two carbon atoms.
Degree of Substitution
The general formula of alkane is CnH2n+2. A hydrocarbon can be unsaturated because of the presence of double bond or triple bond or by the formation of a ring or by the combination of both. For example all these given molecules are unsaturated with C6H10 molecular formula.
Example of Unsaturated Molecular Formula
All these unsaturated molecules have molecular formula C6H10. The formula for saturated hydrocarbon with six carbon atoms will be; CnH2n where n =6

Hence it will be C6 H(2x6+2) = C6H14

It stands that all these unsaturated molecules have 14-10 = 4 less hydrogen atoms. Hence the degree of unsaturation will be 4/2 = 2 in these molecules.

Two degree of unsaturation stands for two double bonds or one double bond with one ring or one triple bond. The general formula of degree of unsaturation is

Formula of Degree of Unsaturation

Where DU stands for degree of unsaturation with in is the number of atoms with Vi valence electrons.
As the degree of unsaturation increases; stability of alkene decreases. It can proves by using heat of hydrogenation. For example, the heat of hydrogenation of butene and 1,3-butadiene is 30.3 kcal/mol and 56.5 kcal/mol respectively.

Example of the Heat of Hydrogenation of Butene

As the substitution increases in alkene, stability increases. Symmetrical alkenes are more stable compare to unsymmetrical alkenes. Hence the order of stability of different
substituted alkene would be 

Tetra substituted alkene (R2C=CR2) > trisubstituted alkene (R2C=CHR) > Symmetrical disubstituted alkene (RCH=CHR)> Unsymmetrical disubstituted alkene (R2C=CH2)> monosubstituted alkene (RCH=CH2) > unsubstituted alkene (CH2=CH2)

Stability of substituted alkene can be proves by using heat of hydrogenation. Alkenes with high heat of hydrogenation are less stable.

Alkene

The stabilization due to substitution can also be explained by using the concept of hyper conjugation. As the substitution of alkene is increases the delocalization of double bond with adjacent carbon-hydrogen bond increases.

Substitution of Alkene


2. Stereochemistry


Because of the presence of pi bond in an alkene, the free rotation of carbon-carbon bond gets restricted and molecule show two different configurations as the pi electron density distributed above and below the plane of sigma bond. If the double bonded carbon atoms in an alkene has WXC = CWX type of arrangements, they can show cis-trans isomerisation also known as geometrical isomerism. 

Stereochemistry

In trans-isomers same groups bonded to double bonded carbon atom located at opposite side while in cis-isomer same groups located at same side. The interconversion of cis and trans-isomer require breaking the pi bond. The cleavage of pi bond needed 63 kcal/mol , which is not possible to provide under normal conditions.

In case of trans-isomer same groups located at opposite position leads to less steric hindrance, hence trans-isomer is more stable than cis-isomer which is less stable due to crowding on same side.

Cis-2-Butane and Trans-2-Butane


3. Conjugation in alkenes


If there is a possibility of conjugation of double bond of alkene with lone pair of electron or another double bond, it would be more stable than an isolated alkene.
More topics in Reactions of Alkenes
Cis and Trans Isomers Degrees of Unsaturation
Alkene Reactions Addition Reaction
Carbocation Stereochemistry
Hammond Postulate Alkene Nomenclature
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