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Chemical Reaction of Alkenes - Addition of Water- Acid-Catalysed Hydration, Hydroboration-Oxidation Reaction and Oxymercuration-Demercuration Reaction and their Mechanisms, Practice Problems, FAQs

Chemical Reaction of Alkenes - Addition of Water- Acid-Catalysed Hydration, Hydroboration-Oxidation Reaction and Oxymercuration-Demercuration Reaction and their Mechanisms, Practice Problems, FAQs

Getting a vaccine or other routine injection is a painful experience for many of us. The fact that the injection site is sanitised with a disinfectant swab is reassuring. Alcohol is the disinfectant in this case.

Before an injection, alcohol swabbing is a common infection control procedure. It gets rid of microorganisms that can get pushed into the puncture and cause an infection. But how can we get ready for them?

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

  • Chemical reaction of alkenes - Addition of Water
  • Acid-catalyzed hydration
  • Mechanism of Acid catalysed hydrations
  • Hydroboration-Oxidation Reaction
  • Mechanism of Alkene Hydroboration-Oxidation
  • Hydration by Oxymercuration-Demercuration:
  • Mechanism of Oxymercuration-Demercuration
  • Practice Problems
  • Frequently Asked Questions

Chemical reaction of alkenes - Addition of Water

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

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

Acid catalysed hydration:

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

Example: Addition of H2O to isobutene

Mechanism of Acid catalysed 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

In the second example, the rearrangement by methyl shift will take place to form a stable tertiary

Carbocation. Acid catalysed hydration yields secondary or tertiary alcohols only. The product yield will be low because of the equilibrium phenomenon.

Hyboration-Oxidation Reaction:The hydroboration oxidation reaction is an organic chemical reaction that is used to convert alkenes into primary alcohols or alkynes into ketones or aldehyde. This is accomplished through a two-step procedure that includes a hydroboration step and an oxidation step. This is accomplished through a net addition of water (across the entire double bond)using an anti-Markovnikov Rule.

Let us consider the example of Hyboration oxidation of alkene hex-1-ene.

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. Herbert Charles Brown, an English-born American chemist, reported the hydroboration oxidation reaction for the first time in the second half of the 1950s.

For this work, he was awarded the Nobel Prize in Chemistry in 1979.

Characteristics of Hydroboration

  1. Stereospecific -syn addition
  2. Anti Markovnikoff 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.

  1. The Hydroboration Process

The hydroboration process occurs in three steps.

The first step involves the addition of electron deficient borane across the electron rich double bond of the alkene. Boron in borane always get attached to the steric free carbon ie. terminal carbon and the syn addition of Hydrogen to te next carbon atom of the double bond.

His addition is gets repeated twice with two more alkene giving a trialkyl substituted borane.

  1. The Oxidation Process

Now that the trialkyl borane has been produced, the second step in the hydroboration process can begin. The boron atom is attacked in this step by the hydroperoxide ion, which is nucleophilic in nature. The R group, along with its electron bond pair to the adjacent oxygen atom, is now rearranged.

The ion of hydroxide has now been removed. This process is repeated three times to produce trialkyl borate as the product. This trialkyl borate is now treated with water to produce the required neutral alcohol. This step of the mechanism is illustrated below.

The trialkylorane is still electron deficient. This molecule is treated with hydrogen peroxide in the second step.

Hydrogen peroxide ion is nucleophilic and attacks the alkyl borane A rearrangement of one alkyl group from Boron atom to oxygen with elimination of hydroxyl group takes place, producing an alkyl borate. This step is repeated twice so as to produce a trialkyl borate

3 Hydrolysis.

In the third and final step, the trialkyl borate is hydrolysed with water to give three molecules of a primary alcohol and boric acid.

Hydration by Oxymercuration-Demercuration:

In a two step process. mercuric acetate adds on to an alkene, and the reduction of the intermediate with sodium borohydride yields a secondary alcohol. Addition reaction is known as oxymercuration while the reduction step is called demercuration.

Mechanism of Oxymercuration-Demercuration

The mechanism of this reaction follows Markonikov's regioselectivity rule, with the hydroxyl group joining the most substituted carbon atom and the hydrogen atom joining the least substituted carbon atom.

Characteristics of Oxymercuration-Demercuration

  1. Stereospecific -syn addition
  2. Markovnikoff mechanism

The reaction mechanism is depicted below.

Step 1: In this step, the nucleophile C=C bond attacks the electrophile Hg, resulting in the acetate ion leaving as the leaving group and the formation of 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: In the final step, the addition of sodium borohydride replaces the acetyl mercury with a hydrogen atom, resulting in the formation of alcohol via a new C-H bond.

Reactions

Stereochemistry

Product

Rule

Acid catalyzed Hydration

Stereospecific -anti addition

Secondary, tertiary

Markovnikov mechanism with rearrangement

Hyboration-Oxidation Reaction

Stereospecific -syn addition

Primary

Anti-Markovnikov mechanism

Oxymercuration-Demercuration Reaction

Stereospecific -syn addition

Secondary, tertiary

Markovnikov mechanism

Practice Problems

1. In the given reaction, what will be the major product?

a. Pentan-2-ol
b. Pentan-3-ol
c. Pentanal
d. Pentan-2-one

Solution: Pent-2-ene on reaction with H2O in the presence of dilute H2SO4 yields Pentan-2-ol and Pentan-3-ol as the products. The carbocations formed from the addition of hydrogen ions are

secondary carbocations. The latter secondary carbocation is more stable because it has 5 hyperconjugation structures with respect to 5 hydrogens whereas the former carbocation is stabilised by 4 hyper conjugating structures with respect to 4 hydrogens. Hence, the major product is Pentan-2-ol.

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

2. In the given reaction, what will be the major product?

  1. 2-(1-methyl cyclopentyl) Ethan-1-ol.
  2. 1,1-Dimethylcyclohexan-2-ol.
  3. 2,2-Dimethylcyclohexan-1-ol.
  4. 1,2-Dimethylcyclohexan-1-ol.

Solution: Step 1: The first step is the protonation of the alkene, which results in the formation of a secondary carbocation. However, in this case, the carbocation formed results in the ring expansion and forms a six-membered stable ring.

Step 2: The addition of OH- occurs and the product formed is shown as follows:

After rearrangement and addition of H2O, the final product is 1,2-dimethyl cyclohexan--ol.

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

3. In which case will hydroboration oxidation and acid hydration produce different products?

(A)

(B)

(C) CH2=CH2

(D) CH3-CH=CH-CH3

Solution: The hydroboration oxidation reaction converts alkenes into alcohol by an organic chemical reaction. A two-stage technique, which involves a hydroboration step and an oxidation step, is used to produce this. Using an anti-Markovnikov Rule, a net addition of water is achieved.

Acid catalyzed hydration is a chemical reaction in which water is added to an unsaturated substrate while an acid catalyst is present. The hydration of ethene is one example.

Cyclohexene (option B), CH2=CH(option C) and CH3-CH=CH-CH3 (option D) all are symmetrical alkene. Hence, the Markovnikov Rule is not applicable in this case. As products formed from Markovnikov or anti-Markovnikov are the same compound.

In option (A), the unsymmetrical alkene will yield a different product concerning the reagent and can be shown as

Hence, the correct answer is option (A).

4.

Predict the product A, B and C are

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 catalysed hydration) is added to alkenes.

Hence, the correct option is (A).

Frequently Asked Questions

1. What are the uses of alcohol?
Answer:
Alcohols are very useful as solvents, raw meterial in synthesis, beverages and as sterilisers.
Ethanol particularly is used in beverage manufacture, fuel additive and in sterilization of hand, hospital equipments etc..

2. What applications does alkene have?
Answer: They are used in the production of plastics such as polythene for buckets, bowls, and bags, as well as 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.

3. Who discovered the oxidation reaction of hydroboration?
Answer: Herbert Charles Brown, an English-born American chemist, reported the hydroboration oxidation reaction for the first time in the second half of the 1950s. For this work, he was awarded the Nobel Prize in Chemistry in 1979.

4. What are the applications of the Hydroboration and Oxymercuration-Demercuration reactions?
Answer: 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.

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