Addition of Hydrogen — Process of Hydrogenation
Generally, alkenes and alkynes undergo an addition reaction with hydrogen (H2) molecules such a type of addition of hydrogen in a substance is called hydrogenation or reduction reaction. Hence the addition of hydrogen to an alkene or alkyne in the presence of a catalyst (nickel, platinum, palladium, or Raney) to form a saturated hydrocarbon is called hydrogenation. The catalyst used in hydrogenation reactions is usually metal. In a hydrogenation reaction, two hydrogen atoms are added across the double bond of an alkene, resulting in a saturated alkane. Hydrogenation is an exothermic reaction and the amount of heat released when one mole of an alkene is hydrogenated is called heat of hydrogenation. Let us take the example of the addition of hydrogen in the ethene molecule in the presence of finely divided metals such as nickel and platinum.
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Addition of Hydrogen Halide to Alkenes
The addition reaction to the C=C double bond is the most prevalent form of reaction for alkenes. As a result of the addition process, a tiny molecule is added to multiple bonds, and one pi bond is transformed into two sigma bonds. When a hydrogen halide is added to an alkene, it creates an alkyl halide as a result. This type of reaction is called hydrohalogenation. This is an electrophilic addition reaction of hydrogen halides to unsaturated hydrocarbons such as alkene, alkyne, and arene to yield the corresponding haloalkane. For example:
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In this reaction, the halogen halides are reacted in the following order: HI > HBr > HCl. This ranking of reactivity is based on the bond dissociation energies of halogen halides; the lower the bond dissociation energy, the more reactive the halogen halide. The actual product formed, however, depends upon whether the alkene is symmetrical or unsymmetrical.
Addition of Hydrogen Halides to Symmetrical Alkenes
A symmetrical alkene is one in which the substituents at both ends of the double bond are the same. The addition of halogen halides to symmetrical alkenes gives only one product which is theoretically possible. For examples:
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Mechanism: The addition of hydrogen halides to alkenes occurs through an electrophilic addition reaction. This reaction is initiated by the ionisation of HCl to produce hydrogen cation and chloride anion.
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The reaction takes place in the following two-step:
Step 1: The double bond is utilised to build a bond with an electrophile that has arrived. This reaction is slow and the rate determines the step of the reaction.
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Step 2: Attack the nucleophile with ethyl carbocation to form chloroethane.
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Addition of Hydrogen Halide to Unsymmetrical Alkene
An unsymmetrical alkene has a pair of substituents on one double bond carbon that is distinct from the pair on the other. The Markovnikov rule governs addition in the situation of unsymmetrical alkynes. When an unsymmetrical reagent (HBr, HCl) reacts with an unsymmetrical alkene, the reagent's negative portion attaches to the unsaturated carbon atom with the fewest hydrogen atoms. Let us take the example of the addition of hydrogen bromide to propene.
According to Markovnikov's rule, adding hydrogen bromide to propene produces isopropyl bromide, which is the main product.
What is Markovnikov’s Rule?
The acidic hydrogen of a protic acid (HX) binds to the carbon atom with the most hydrogen substituents, while the halide group bonds to the carbon atom with the most alkyl substituents. To get a better understanding of this mechanism, use the same example as before, namely the addition reaction of hydrobromic acid with propene. The rule mechanism of Markovnikov can be broken down into two phases, as shown below.
Step 1: Protonation of the alkene yields a more stable carbocation. Carbocations are the most stable whenever the charge is on a tertiary carbon, and the least stable whenever the charge is on a primary carbon. Positive carbocation charges will gravitate towards the most stable conformation.
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Step 2: The halide ion nucleophile now approaches the carbocation. An alkyl halide is formed as a result of this reaction. Since the synthesis of the secondary carbocation is favoured, 2-bromopropane would be the main result of this reaction. As a result, Markovnikov's addition uses the more stable intermediate carbocation.
Conclusion
In this article, we learned about hydrogenation, hydrohalogenation, and the Markovnikov rule. The synthesis of halogenated compounds begins with the hydrohalogenation of numerous bonds. Although carbon-carbon double bonds in the alkenes can be electrophilically added to hydrogen halides to yield alkyl halides, carbon-carbon triple bonds in the alkynes cannot. This is due to the lower stability of the cationic intermediate derived from the electronegative sp carbon compared to that derived from sp2 carbon. The outcome of the addition reaction of protic acid HX to an asymmetric alkene, also known as hydrohalogenation, is predicted by Markovnikov's rule (under normal situations).
FAQs on Addition of Hydrogen with Examples for JEE
1. What is the difference between the Markovnikov and Anti Markovnikov rules?
The main difference between the Markovnikov and Anti-Markovnikov rules is that in an addition reaction, hydrogen atoms are attached to the carbon atom with the most hydrogen substituents, whereas in a subtractive reaction, hydrogen atoms are attached to the carbon atom with the least hydrogen substituents.
In the absence of peroxides, the addition of hydrogen halide happens according to the Markonikov rule. The electrophilic addition mechanism is used for this addition. The addition of HBr to an unsymmetrical alkene occurs in the presence of peroxides such as benzoyl peroxide, which is contradictory to the Markonikov rule. The peroxide effect is also called the Kharasch effect. This addition is made possible by the free-radical process.
2. What products are formed with the addition of hydrogen iodide to ether?
The most notable reaction that ethers undergo is an acid-catalysed cleavage that happens when hydroiodic acid (HI) reacts with them. A nucleophilic substitution mechanism is used to carry out this reaction. The Sn2 mechanism is used by primary and secondary alkyl ethers, while the Sn1 mechanism is used by tertiary, benzylic, and alkylic ethers. When the ether is bonded to just primary, secondary, or methyl alkyl groups, an Sn2 mechanism is used to selectively cleave the ether. The ether oxygen is first protonated by a strong acid. The protonated ether is then attacked by the halide conjugate base at the less sterically hindered alkyl substituent, yielding a halogen product. As a leaving group, the ether's more sterically hindered alkyl substituent is expelled, forming an alcohol product.
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3. What is the peroxide effect, also known as the anti-Markovnikov rule?
When HBr adds the "wrong way around" in the presence of organic peroxides, the peroxide effect, also known as anti-Markovnikov addition, occurs. In the absence of peroxides, hydrogen bromide adds to propene via an electrophilic addition process. As a result, the expected product by Markovnikov's Rule is obtained. It is simply a matter of reaction rates that hydrogen bromide adds in an anti-Markovnikov fashion in the presence of organic peroxides. The free radical mechanism is significantly faster than the electrophilic addition mechanism.