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Chemical Reaction of Alkenes - Addition of Hydrogen Halides (HX), Acid-Catalyzed Hydration, Hydroboration-Oxidation Reaction and Oxymercuration-Demercuration Reaction, Hydrogenation Reaction of Alkenes and Their Mechanisms, Practice Problems and FAQs

Chemical Reaction of Alkenes - Addition of Hydrogen Halides (HX), Acid-Catalyzed Hydration, Hydroboration-Oxidation Reaction and Oxymercuration-Demercuration Reaction, Hydrogenation Reaction of Alkenes and Their Mechanisms, Practice Problems and FAQs

Have you ever seen beautiful, multi-coloured clear marble-style glass gems at craft stores or people wearing plain or customized rings? They are alcohol ink rings. Alcohol ink contains alcohol dispersed with coloured dyes

Also, take a look at how nicely alcohol inks merge and mix to create beautiful swirling effects. But how do we get the alcohol?

Alkene is the answer. Three alternative chemical methods can be used to convert alkenes to alcohol. An alkene is employed as a starting component in a variety of reactions, some of which are discussed here.

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Table of Contents

  • Addition of Hydrogen Halides (HX)
  • Acid-catalyzed hydration
  • Hydration by Oxymercuration-Demercuration
  • Hydroboration-Oxidation Reaction
  • Hydrogenation reaction of alkenes
  • Polymerization
  • Practice Problems
  • Frequently Asked Questions

Alkenes are rich sources of loosely held π electrons. Hence, alkenes give addition reactions in which the electrophiles add on to the carbon-carbon double bond to form the addition products. An illustration of the weak π bond in ethene is given. These loosely held π electrons are responsible for the addition reactions in alkenes.

Addition of Hydrogen Halides (HX)

Hydrogen halides (HCl, HBr, HI) when added to alkenes give alkyl halides. This is known as the hydrohalogenation process. It is an electrophilic addition reaction.

Alkene + HX Alkyl halide

Addition of HBr to symmetrical alkenes

The electrophilic addition mechanism in which the hydrogen ion attaches to one carbon and bromide attaches to the second carbon of the double bond occurs in the addition reaction of HBr to symmetrical alkenes.

Addition of HBr to unsymmetrical alkenes

When HBr is introduced to unsymmetrical alkenes, there are two options for double bonds to break in such a way that one carbon receives the negative charge and the other receives the positive charge. The bond is broken in such a way that the bond-forming carbocation is the most stable of the three.

To know the major product formed upon the addition of HBr to an unsymmetrical alkene, Markovnikov’s rule (The rule states that the negative part of the addendum (adding molecule) gets attached to that carbon atom which possesses a lesser number of hydrogen atoms) is followed.

Mechanism (Markovnikov Addition)

Step 1: The H+ ion attacks the double bond (electrophilic addition of H+) and gets attached to the carbon having more hydrogen atoms.

Step 2: Since, the 2o carbocation is more stable than 1o carbocation, so the stable carbocation will be available more for the attack of Br- and the product formed from 2o carbocation is the major product.

Rearrangement of Carbocation

The H+ ion attacks the double bond and it gets attached to the carbon having more hydrogen atoms. The 2o carbocation undergoes rearrangement (1,2-hydride shift) to form a more stable 3o carbocation. Finally, Br- ion attacks on the 3o carbocation to form the major product.

Order of reactivities of hydrogen halides

As we move down the group, the bond length of HX increases and the bond strength decreases (i.e., the H-X bond becomes weak and can be easily dissociated). Thus, the reactivity of hydrogen halides towards addition reactions increases down the group.

HF < HCl < HBr < HI

Mechanism (Anti-Markovnikov Addition)

Anti-addition markovnikov's happens when alkenes are treated with HBr in the presence of peroxides, where the HBr H-atom is connected to the C-atom with the fewest H-atoms. It's also known as the Kharasch effect or the peroxide effect.

In the presence of peroxide, the addition of HBr to an alkene occurs via a free radical process.

Where (C6H5CO)2O2 is benzoyl peroxide

The mechanism is given as follows:

  1. Chain initiation

Step 1: The peroxide bond of RCO–O–O–CO–R breaks into radicals (i.e., RCOO) in the presence of sunlight, which is then followed by the removal of carbon dioxide and the formation of alkyl radical (i.e., R).

Step 2: Now, the alkyl free radical (R) (i.e., phenyl radical) attacks on HBr and results in the formation of R−H (i.e., benzene in benzoyl peroxide) and Br radicals.

  1. Chain propagation

The order of stability of the free radicals is 3oradical>2oradical>1oradical. The two intermediates formed are given and we know that a 2o free radical is more stable than a 1o free radical.

  1. Chain Termination

The hydrogen radical formed from HBr will attack the 2o free radical to give the major product. Hence, in the major product, Br will be attached to the carbon with more hydrogen atoms, which is in contradiction to Markovnikov’s rule. Hence, it is an anti-Markovnikov addition.

Addition of Water

There are three methods by which water can be added to an alkene

  1. Acid-catalyzed Hydration
  2. Oxymercuration-Demercuration Reaction
  3. Hyboration-Oxidation Reaction

Acid-catalyzed hydration:

This is also known as the hydration of alkenes. The addition of H2O/H+(acid-catalyzed hydration) to alkenes is a Markovnikov addition reaction.

Example: Addition of H2O to isobutene

Mechanism of Acid-catalyzed hydration:

Step 1: Protonation and formation of carbocation

When the double bond breaks, it forms a stable carbocation. Here, a tertiary carbocation is formed

instead of a primary carbocation. This is because the tertiary carbocation is more stable than the

primary carbocation.

Step 2: Nucleophilic attack of H2O

Step 3: Deprotonation to form alcohol

Hydration by Oxymercuration-Demercuration:

Alkenes can be converted to alcohol using the oxymercuration-demarcation reaction. An alkene is treated with mercuric acetate in a Tetrahydrofuran–water solution to produce a product, which can then be reduced with NaBH4 to produce alcohol. Markovnikov's rule operates in this reaction.

Mechanism of Oxymercuration-Demercuration

The hydroxyl group joins the most substituted carbon atom, while the hydrogen atom joins the least substituted carbon atom, as described by Markonikov's regioselectivity rule.

Characteristics of Oxymercuration-Demercuration

  1. Stereospecific -anti addition
  2. Markovnikov mechanism

The reaction mechanism is shown in the diagram below.

Step 1: The nucleophile C=C bond attacks the electrophile Hg in this step, leaving the acetate ion as the leaving group and forming the cyclic mercurinium ion.

Step 2: In this step, the nucleophile attacks one of the carbons linked to Hg, causing the C-Hg bond to cleave.

Step 3: In this step, oxonium ion is deprotonated in the presence of a base acetate ion, resulting in alcohol.

Step 4: The addition of sodium borohydride in the final stage substitutes a hydrogen atom for the acetyl mercury, resulting in the synthesis of alcohol via a new C-H bond.

Hyboration-Oxidation Reaction:The hydroboration oxidation reaction converts alkenes into primary alcohols and alkynes into ketones or aldehydes. A two-stage technique, which involves a hydroboration step and an oxidation step, is used to accomplish this. Using an anti-Markovnikov Rule, a net addition of water (across the entire double bond) is achieved.

Consider the Hyboration oxidation of the alkene hex-1-ene as an example.

Mechanism of Alkene Hydroboration-Oxidation

The mechanism of hydroboration oxidation can be thought of as an anti-Markovnikov reaction in which a hydroxyl group attaches itself to the less substituted carbon.

Characteristics of Hydroboration

  1. Stereospecific -syn addition
  2. Anti Markovnikov mechanism
  3. Primary alcohol formation

The conversion of alkenes into neutral alcohols takes place here. The entire reaction can be broken down into two steps, as explained below.

Step-1: The Hydroboration Process

The first step is to add borane in the form of BH3 to the given double bond. This results in the transfer of a hydrogen atom to the carbon atom next to the carbon atom bonded with the boron atom. The hydroboration step is now repeated twice, yielding three alkenes attached to the boron atom from the starting BH3.

Step-2: The Oxidation Process

The second step in the hydroboration process can now begin after the trialkyl borane has formed. The hydroperoxide ion, which is nucleophilic in nature, attacks the boron atom in this stage. The R group has been reorganized, along with its electron bond pair to the nearby oxygen atom.

The hydroxide ion has now been eliminated. The result is trialkyl borate, which is made by repeating this process three times. The needed primary alcohol is now produced by treating the trialkyl borate with water. The mechanism's next stage is depicted below.

Hydrogenation reaction of alkenes

Alkenes adds up one molecule of dihydrogen gas in the presence of finely divided nickel, palladium, or platinum to form alkanes.

When Ni is used as a catalyst, this reaction is known as the Sabatier-Sanderson reaction.

Ethene is adsorbed across the catalyst surface in this demonstration of the addition of dihydrogen to ethene in the presence of nickel. Syn addition (adding H2 from the same side) of dihydrogen occurs, resulting in the formation of a four-membered high energy transition state. Ethene is finally converted to ethane.

Polymerization

Polythene bags and sheets are certainly familiar to you. Polythene is created by merging a large number of ethene molecules in the presence of a catalyst at high pressure and temperature. Polymers are the large molecules that are produced as a result of this process. This reaction is known as polymerisation.

Polymer Applications

Polymers are used to make plastic bags, squeeze bottles, refrigerator dishes, toys, pipelines, radio and television cabinets, and other goods. Milk cartons, plastic buckets, and other moulded items are made of polypropene. Despite the fact that polythene and polypropylene are now widely used, their excessive use is a matter of concern for all of us.

Practice Problems

1. 

Predict the correct statement about isomerism exhibited by products A, B and C

  1. B and C are Homomers
  2. A and B are Positional Isomers
  3. A and C are Functional Isomers
  4. B and C are Positional Isomers

Solution: The oxymercuration-demarcation reaction can convert alkenes to alcohol. In a THF–water solution, an alkene is treated with mercuric acetate to produce a product, which can then be reduced with NaBH4 to produce alcohol. According to Markownikov's rule, addition occurs in this reaction.

The hydroboration oxidation reaction is an organic chemical reaction that converts alkenes to neutral alcohols or alkynes to ketones or aldehydes. This is accomplished via a two-step procedure that includes a hydroboration step and an oxidation step that employs an anti-Markovnikov Rule.

A Markovnikov addition reaction occurs when H2O/H+ (acid catalyzed hydration) is added to alkenes.

Product formed form hydroboration oxidation reaction and oxymercuration-demarcation reaction that are B and C exhibit positional isomerism as Skelton of hydrocarbons is same but the difference in position of the hydroxyl group.

Hence, the correct option is (D).

2. What will be the major product when unsymmetrical non-terminal alkene C5H10 is hydrolysed in acidic conditions?

  1. Pentan-3-ol
  2. Pentanal
  3. Pentan-2-one
  4. Pentan-2-ol

Solution: Two alkenes can be possible with this chemical formula

Pent-2-ene being unsymmetrical non-terminal alkene on reaction with H2O in the presence of dilute H2SO4 yields Pentan-2-ol and Pentan-3-ol as the products. Secondary carbocations are generated when hydrogen ions are added to a carbocation. Because it has 5 hyperconjugation structures with regard to 5 hydrogens, the latter secondary carbocation is more stable than the former, which has 4 hyperconjugation structures with respect to 4 hydrogens. As a result, Pentan-2-ol is the main product.

Hence, the correct answer is an option (D).

3. What would be the expected product when 4,4-dimethyl pent-2-ene is treated with mercury acetate followed by sodium borohydride?

  1. 4,4-dimethyl pentan-2-ol
  2. 4,4-dimethyl pentan-3-ol
  3. 2,3-dimethyl pentan-2-ol
  4. None of these

Solution: Alkenes can be converted into alcohol using the oxymercuration-demarcation method in which first step we use mercury acetate and in second step we use sodium borohydride. In a THF-water solution, an alkene is treated with mercuric acetate to produce a product that may then be reduced with NaBH4 to produce alcohol. According to Markownikov's rule, addition occurs in this reaction. Consequently, the suggested response in option (A) is accurate.

4. In the presence of a platinum catalyst, Hydrocarbon P adds hydrogen to form n-pentane. When hydrogen bromide is supplied to P instead of hydrogen, only one Bromo compound is formed?

  1. CH2-CH2-CH=CH-CH3
  2. CH2=CH-CH2-CH2-CH3
  3. Both (a) and (b)
  4. None of these

Solution: As in option (A) and (B) CH3-CH2-CH=CH-CH3 (pent-2-ene) is a unsymmetrical alkene, only one product will be obtained on the addition of HBr, i.e., 3-Bromopentane.

As CH2=CH-CH2-CH2-CH3 (pent-1-ene) is an unsymmetrical alkene, The again only product will be obtained as 2-Bromopentane.

As a result, choice (C) is the proper response.

Frequently Asked Questions

1. What applications does alkene have?

A. They are used in the production of plastics such as polythene for buckets, bowls, and bags and polystyrene for car battery cases and refrigerator parts. They are used in the production of ethane-1,2-diol, which is used as an anti-freeze agent in automobile radiators.

2. What are the applications of the Hydroboration and Oxymercuration-Demercuration reactions?
A.
It is used in the production of alcohol from alkenes. It produces more stereospecific and regioselective alcohols than other oxidation reactions used in alcohol formation.

3. Why are the lead salts and quinoline in Lindlar catalysts 'Poisoned'?
A.
Palladium catalysts often have significant catalytic activity and can even reduce double bonds. Using such catalysts, alkanes can be produced during the hydrogenation of alkynes (the alkene products undergo further hydrogenation under the influence of the catalyst). The Lindlar catalyst has been poisoned and is unable to reduce double bonds. As a result, when this catalyst is used in the hydrogenation of alkynes, no alkanes are formed.

4. What exactly is the distinction between dehydration and dehydrogenation?
A.
These terms have perplexed us all. The major distinction between hydration and hydrogenation is that hydration includes the addition of water molecules to an organic compound, whereas hydrogenation involves the addition of a hydrogen molecule.

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