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Electrophilic Addition Reaction of Osmium Tetroxide on Alkenes - Syn Dihydroxylation, Practice Problems and FAQs

Electrophilic Addition Reaction of Osmium Tetroxide on Alkenes - Syn Dihydroxylation, Practice Problems and FAQs

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Let us now look at the formation of various diols using osmium tetroxide!

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Table of Contents:

  • Electrophilic addition reaction of osmium tetraoxide -Hydroxylation of Alkene
  • Syn-Dihydroxylation using osmium tetroxide
  • Mechanism:
  • Practice Problem
  • Frequently Asked Questions

Electrophilic addition reaction of osmium tetroxide - Dihydroxylation of Alkene:

The carbon-hydrogen bond in the alkene oxidises to a carbon-hydroxyl bond during hydroxylation. Alkene hydroxylation is an oxidation reaction. An oxidizing agent is a reagent that increases the oxidation number.

Alkenes can be hydroxylated via two distinct stereochemical routes:

  1. Anti-dihydroxylation
  2. Syn-dihydroxylation

The anti-dihydroxylation mechanism is produced by employing m-CPBA (meta-chloro peroxy benzoic acid) followed by hydrolysis, whereas potassium permanganate or osmium tetroxide produces syn-dihydroxylated products.

The electrophilic addition reaction with osmium tetroxide can occur in two steps:

Step-1: Addition of OsO4 in pyridine

Step-2: Reduction of cyclic intermediate using H2S or NaHSO3.

Syn-Dihydroxylation using osmium tetroxide:

Dihydroxylated products are created by reacting alkene with aqueous osmium tetroxide in pyridine and then using H2S or NaHSO3 in the second step to produce the desired product.

Mechanism:

We would expect syn-stereoselectivity in the bonding to oxygen based on the mechanism shown here.

During strong oxidation with aqueous osmium tetroxide in pyridine, there is a formation of an intermediate. In the second step, the cyclic intermediate gets reduced with H2S or NaHSO3 and yield diol as a major product.

Practice Problem:

Q1. What will be the expected product in the given reaction?

a. Cyclohexane-1,2-dione
b. Cyclohexane-1,2-diol
c. Cyclohexane-1,2-diamine
d. Cyclohexane-1,2-dione

Answer: (B)

Solution: Dihydroxylated products are formed in this reaction when alkene reacts with osmium tetroxide in a solvent pyridine. The reaction should have syn-stereoselectivity. Hence, when cyclohexene is treated with osmium tetroxide, it will yield the dihydroxylated product Cyclohexane-1,2-diol.

Hence the correct answer is an option (B).

Q2. What will be the expected product in the given reaction?

Answer: (B)

Solution: Dihydroxylated products are formed in this reaction when alkene reacts with osmium tetroxide in pyridine. Hence, when 1,2-dimethylcyclohex-1-ene is treated with osmium tetroxide, it will yield the dihydroxylated product. The reaction should have syn-stereoselectivity. Hence the correct answer is option (B).

Q3. What is the expected outcome of the given reaction?

(A) (B)

(C) (D)

Answer: (D)

Solution: This reaction produces dihydroxylated products when alkene reacts with osmium tetroxide in pyridine. Syn-stereoselectivity should be present in the reaction. As a result, when the reactant is treated with osmium tetraoxide, the cis-dihydroxylated product is formed. But the position of hydroxyls is anti with respect to each other in options (B) and (C ), hence these can not be the answer. In the remaining options, let us analyze them one by one.

In option (A), a bulky tert-butyl group is present above the plane and hydroxyl products are also present above the plane. Hence, steric hindrance makes this product less stable.

In option (D), a bulky tert-butyl group is present above the plane and hydroxyl products are present below the plane. Hence, steric hindrance makes this product stable.

As a result, the option is the correct answer (D).

Q4. When 1,2-dichlorohexane is treated with 1 mole of (A), it will result in the formation of 1-chlorohexene which is further treated with again 1 mole of (A) and result in the formation of (B) which give a positive result with baeyer’s reagent. The compound (B) is treated with Lindlar catalyst and the product is hexene which is treated further with osmium tetroxide and H2S or NaHSO3 to yield the product (C). Predict the A, B and C.

a. Alcoholic KOH, hexane and hexane-1,2-diol
b. Alcoholic KOH, hexyne and hexane-1,2-diol
c. Aqueous KOH, hexene and hexane-1,2-diol
d. Aqueous KOH, hexyne and hexane-1,2-diol

Answer: (B)

Solution: The scheme for the given question can be designed as:

In the reaction scheme, when 1,2-dichloro hexane is treated with 1 mole of (A), it will result in the formation of 1-chlorohexene which is further treated with again 1 mole of (A) and result in the formation of (B) which give a positive result with baeyer’s reagent.

In general, alcoholic KOH is used to create Alkene from Alkyl Halides, whereas aqueous KOH is used to create alcohol from Alkyl Halides. Dehydrohalogenation is accomplished using alcoholic KOH. Hence, The reagent (A) should be alcoholic KOH.

The compound (B) gives a positive Bayer's reagent test which is used for the detection of unsaturation. Hence, there should be again Dehydrohalogenation which is accomplished using alcoholic KOH (reagent(A)). Hence, product (B) should be hexyne.

The compound (B) is treated with Lindlar catalyst and the product is hexene which is treated further with osmium tetroxide and H2S to yield the product (C). This reaction produces dihydroxylated products and, the product here should be hexane-1,2 -diol.

Hence, the correct answer is option (B).

 

Frequently Asked Questions-FAQs:

1. How do we make osmium tetroxide?
Answer:
It is made by oxidizing osmium metal or its compounds with high heat in the presence of air. It is typically prepared by heating osmium metal to red heat in the air.

2. What is the nature of Osmium tetroxide?
Answer:
Osmium tetroxide is a highly toxic and volatile substance. When present in vapor form, it can cause eye irritation.

3. What are the uses of osmium tetroxide?
Answer:
Because it is a costly and toxic substance, it is used in the synthesis of fine chemicals in pharmaceuticals. It is also used for degradative purposes in structural characterization.

4. Is hydroxylation a reduction reaction?
Answer:
No, hydroxylation of alkynes is an oxidation reaction. Osmium tetroxide acts as an oxidizing agent or oxidant which reduces itself and oxidizes the given reactant.

5. How can osmium tetraoxide's toxicity be reduced?
Answer:
Because osmium tetroxide is both expensive and toxic, it can be avoided by combining catalytic amounts of OsO4 with stoichiometric amounts of an oxidizing agent like hydrogen peroxide.

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