Problem 26 - A Complete Resource Book in Chemistry for JEE Main

Question

Osmium tetroxide is a reagent used for (a) hydroxylation of carbonyl compounds (b) hydroxylation of olefins to form trans-diols (c) hydroxylation of olefins to give cis-diols (d) hydroxylation of acetylenes

Step-by-Step Solution

Answer

(c) hydroxylation of olefins to give cis-diols.

Step 1: Understanding the Question

The question asks about the specific reaction that osmium tetroxide is used for as a reagent. The key term to recognize here is 'hydroxylation,' which means the addition of hydroxyl groups (OH) to a substrate.

Step 2: Identifying Known Uses of Osmium Tetroxide

Osmium tetroxide (OsO₄) is a well-known reagent in organic chemistry, particularly for transforming olefins (alkenes) into glycols, which are vicinal diols (cis-diols specifically). It predominantly adds hydroxyl groups to the same side of the double bond, making it a syn-dihydroxylation process that results in cis-diols.

Step 3: Evaluating Each Option

We need to check each option provided: (a) Osmium tetroxide does not hydroxylate carbonyl compounds. (b) Osmium tetroxide actually forms cis-diols, not trans-diols. (c) Osmium tetroxide is indeed used for the hydroxylation of olefins to give cis-diols, matching its known reactivity. (d) Osmium tetroxide is not typically used for hydroxylating acetylenes.

Step 4: Conclusion

Based on the evaluation of the options, option (c), 'hydroxylation of olefins to give cis-diols,' correctly describes the primary use of osmium tetroxide as a reagent in syn-dihydroxylation.

Key Concepts

Hydroxylation

Hydroxylation is a fundamental reaction in organic chemistry where a hydroxyl group (-OH) is added to an organic compound. When using osmium tetroxide, hydroxylation specifically targets olefins, which are compounds with carbon-carbon double bonds. Osmium tetroxide serves as an efficient and selective reagent for this transformation.

During the reaction, osmium tetroxide interacts with the double bond, facilitating the addition of hydroxyl groups. This process is key because it converts unsaturated hydrocarbons into more complex structures like diols, enhancing their functional capabilities.

This reaction is not applied to carbonyl groups or alkynes, as osmium tetroxide does not favor these substrates. Its specificity for olefins emphasizes the reagent's role in selective hydroxylation, crucial for creating reliable synthetic pathways.

Cis-diols Formation

Cis-diols are a specific type of diol where both hydroxyl groups are located on the same side of the structure. The formation of cis-diols from olefins is a prominent feature of the osmium tetroxide reaction.

During the reaction, the osmium tetroxide molecule approaches the olefin from one side of the double bond. It attaches to both carbons simultaneously in a cyclical transition state before breaking apart and leaving behind the cis-diol.

This mechanism ensures that both hydroxyl groups are added to the same side, resulting in a product that has retained its stereochemistry. Cis-diols are especially valuable in synthetic chemistry for constructing complex molecular architectures with precise spatial arrangements.

Syn-dihydroxylation

Syn-dihydroxylation refers to the addition of two hydroxyl groups across a double bond in a concerted manner, meaning both groups add to the same side or face of the molecule. Osmium tetroxide is a classic agent for syn-dihydroxylation, leading to the formation of cis-diols.

During syn-dihydroxylation, osmium tetroxide attacks the double bond to form a cyclic osmate ester intermediate. The syn addition is crucial because it ensures both hydroxyl groups end up on the same face of the molecule, preserving the stereochemistry.

This type of hydroxylation is significant for organic synthesis because it allows chemists to control the stereochemical outcome, which is vital for producing molecules with specific biological and chemical properties. Understanding syn-dihydroxylation is essential for predicting the configuration of products in synthetic reactions.