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Hydrogenation of Alkenes

Alkenes are unsaturated hydrocarbons with double covalent bond between carbon atoms of parent chain. General formula for alkenes is $C_nH_{2n}$ and first member of this series contains two carbon atoms unlike alkanes in which first member is $CH_4$ (methane). In alkenes, the double bonded carbon atoms are $sp^2$ hybridized. Each double bonded carbon atom of alkene contains three $sp^2$ hybridized orbitals which are arranged in trigonal planer manner. All hybridized orbitals involve in formation of sigma bonds with other carbon and hydrogen atoms. Along with hybridized orbital, each double bonded carbon atom also has one un-hybridized $p_z$ orbital with one electron. This orbital is placed perpendicular to the axis of sigma bond and involve in pi-bond formation. The pi-electron cloud remains perpendicular to sigma bond and arranged half above the plane and half below the plane as it is formed by side way overlapping of orbitals. Due to this orientation, pi-bonds are weaker compare to sigma bonds and presence of pi-bond also restricts the rotation of sigma bond. The restricted rotation towards $C - C$ bond is responsible for cis and trans-isomers. 

Structure of Alkene

Since pi-bonds are weaker than sigma bonds therefore they can easily break to form saturated compounds which were stable compare to unsaturated compounds. In other words alkenes can readily show addition reactions with appropriate reagent. Because of presence of high electron density of pi-electrons, they usually exhibit electrophilic addition reactions. 


Hydrogenation of Alkenes Reaction

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Hydrogenation is a type of addition reaction in which hydrogen atoms are bonded with double bonded carbon atoms with the cleavage of $C - C$ pi bond. Usually metal catalysts like $Pt$, $Pd$, and $Ni$ etc. are used for hydrogenation of alkenes. Platinum and palladium are most common metal catalyst for hydrogenation of alkene. Platinum catalyst is used in the form of $PtO_2$ and is known as Adams catalyst. Since metal catalyst remains in solid state so it is an example of heterogeneous catalysis. In this catalysis, hydrogen molecule changes to $H$ atoms with cleavage of $H - H$ bond and absorb over metal surface. Later, alkene molecule also absorbs over metal surface and form bond with hydrogen atoms with cleavage of pi bond. In the last step of mechanism, alkane molecule desorbs from the metal surface. Hydrogenation reaction of alkene is an example of exothermic reaction as it happens with the release of some amount of energy. The presence of catalyst reduces the activation energy for the reaction so reaction occurs with fast speed at low temperature. The energy level of product is lower than reactant as some energy is released. This energy that is released during hydrogenation reaction is called as heat of hydrogenation. 
Hydrogenation of Alkene

The hydrogenation of 2-butene forms butane in the presence of $Pd-C$ as catalyst. Similarly cyclohexene converts to cyclohexane and 2-methylpropene converts to 2-methylpropane after reaction with Hydrogen and metal catalyst. 

Hydrogenation of Alkene with Catalyst
Metal catalysts are very expensive therefore they are usually separate out from the reaction mixture by filtration and can then be recycled.

Hydrogenation of Alkenes Mechanism

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The hydrogenation of alkene is an example of electrophilic addition reaction in which hydrogen atoms added to double bonded carbon atoms over metal surface. 
It is a heterogeneous catalysis which initiated with absorption of hydrogen atoms over metal surface. It forms a complex with alkene molecule which involves the transfer of H-atoms over double bonded carbon atoms and forms alkane. Later alkane diffuses away from the metal surface and metal’s active site will be free for absorption of more hydrogen and alkene. 

Mechanism of Hydrogenation of Alkene

Hydrogenation of Alkenes by Wilkinson's Catalyst

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Metal catalyst for hydrogenation of alkanes is an example of heterogeneous catalysis because gaseous reactants absorb over solid metal surface. Hence both reactants and catalyst are not in same phase. Hydrogenation of alkene can be done by homogenous catalysis with the use of appropriate catalyst. Usually organo-metallic complexes are used for homogeneous catalysis. For example chloridotris(triphenylphosphane)rhodium(I) or Wilkinson’s catalyst can be used for hydrogenation of alkene. Reaction occurs either in gaseous or liquid medium for this catalyst.

It is a coordination compound with structural formula $RhCl(PPh_3)_3$. Here $Rh$ is Rhodium that is center metal atom. It is bonded with four ligands; 3 – triphenylphosphine (-$PPh_3$) and one –Cl. It is a square planer 16-electron system. In the first step of hydrogenation, hydrogen atoms are bonded with center metal atom to form an 18 –electron octahedral complex which further reacts with alkene to form a pi-complex with the replacement of one –$PPh_3$ ligand. This pi-complex shows insertion of ligand again with shifting of H atom over alkene group. Last step is reductive elimination which forms a 14-electrons complex with alkane. 

Hydrogenation with Wilkinsons Catalyst

Preparation of Wilkinson's Catalyst

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The structural formula for Wilkinson’s catalyst is [RhCl(PPh3)3]. It was discovered by Fred Jardine, a student of Geoffrey Wilkinson who was trying to prepare [RhCl3(PPh3)3]. He used hydrated rhodium trichloride and excess triphenylphosphine with boiling ethanol but the final product he formed was [RhCl(PPh3)3]. This happened because of the excess of phosphine which acts as a reducing agent for the reaction. The chemical reaction for the formation of Wilkinson’s catalyst can be written as given below.

$RhCl_3. 3 H_2O + 4 PPh_3  \rightarrow [RhCl(PPh_3)_3] + Ph_3PO + 2 HCl + 2 H_2O$.

The structure of Wilkinson’s catalyst is as given. 
Wilkinson Catalyst

Asymmetric Hydrogenation of Alkenes

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Addition reactions over asymmetric alkenes Markovnikov's rule and such addition reactions are called as Markovnikov’s addition. According to this rule, the addition of reagent over an asymmetric alkene is determined by the positive and negative part of reagent. In a reagent HX, the positive part is H and negative part is X than the positive part of reagent will form bond with that double bonded carbon atom which has greatest number of hydrogen atoms. At the same time, the negative end of reagent will form bond with that carbon atom which has less number of hydrogen atoms. Let’s take an example. The addition reaction of HBr with propene leads to formation of two products.

$CH_3-CH = CH_2  +  HBr \rightarrow CH_3-CH_2CH_2-Br + CH_3CH(Br)CH_3$
Propene                1-bromopropane                2-bromopropane

Here 2-bromopropane forms as major product because it follows Markovnikov’s rule and H of HBr is added to that carbon atom which carries greatest number of hydrogen atoms.

Does the Markovnikov’s rule also apply over hydrogenation of asymmetric alkene? No! It is not.  This is because in hydrogenation, two hydrogen atoms are bonded over double bonded carbon atoms. Both hydrogen atoms are same and form by cleavage of sigma bond of H-H molecule. In other words, hydrogen molecule is a non-polar covalent molecule so there will be no positive or negative end in the molecule. Hence the product will always be same for asymmetric or symmetric acid. For example; hydrogenation of 2-butene and 1- butane will form same product that is butane. No doubt the heat of hydrogenation would be different in both cases but the product will be same.
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