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 Reagent

Osmium tetroxide (OsO₄) is a versatile reagent used in organic chemistry. It is particularly known for its ability to add hydroxyl groups across double bonds, a process known as hydroxylation.

Step 2: Evaluating the Options

Consider each option given in the exercise: - (a) Hydroxylation of carbonyl compounds: Carbonyl compounds contain a carbon-oxygen double bond. Osmium tetroxide is not commonly used for hydroxylation of carbonyls. - (b) Hydroxylation of olefins to form trans-diols: This suggests forming diols where the hydroxyl groups are on opposite sides. Osmium tetroxide does not typically form trans-diols. - (c) Hydroxylation of olefins to give cis-diols: Osmium tetroxide is known for converting olefins (alkenes) into cis-diols, where both hydroxyl groups are added to the same side of the double bond. - (d) Hydroxylation of acetylenes: Acetylenes have a triple bond, and hydroxylation of acetylenes is not a typical use of OsO₄.

Step 3: Conclusion

Based on the evaluation of options, option (c) is correct because osmium tetroxide is used for the hydroxylation of olefins to give cis-diols. It reacts with double bonds, making this its primary application.

Key Concepts

Hydroxylation

Hydroxylation is a chemical process that thoroughly changes the nature of organic compounds by introducing hydroxyl groups (-OH). This transformation is crucial in organic chemistry because it converts nonpolar molecules into polar ones, allowing for increased interactions with water and other polar substances. Osmium tetroxide is a classic reagent used for hydroxylation, particularly due to its ability to bind to the carbon atoms of double bonds. The result is the addition of hydroxyl groups across the bond, creating vicinal diols.

Organic chemists favor hydroxylation because it unfolds opportunities for further chemical reactions and synthesis. It plays a big role in making complex molecules more reactive, setting the stage for further substitutions or modifications. This makes hydroxylation not only an essential tool for synthesis but also a gateway to more advanced chemical structures.

Key points about hydroxylation include:

• Converts nonpolar molecules to polar
• Uses reagents like osmium tetroxide
• Key in synthetic organic chemistry

Cis-diols

Cis-diols are a specific type of diol, characterized by two hydroxyl groups on the same side of the carbon chain. When Osmium tetroxide reacts with olefins (simple alkenes with double bonds), it adds hydroxyl groups across the double bond, always resulting in cis configurations. This occurs because both hydroxyl groups attach to the same side during the reaction mechanism.

Understanding the structure and formation of cis-diols is important since the spatial arrangement of molecules greatly influences their chemical behaviors. Cis-diols are particularly interesting because their stereochemistry allows them to form distinct interactions and reactions compared to their trans counterparts.

Highlights about cis-diols:

• Formed by hydroxylation of alkenes using OsO₄
• Both hydroxyl groups on the same side
• Significant role in stereochemistry

Organic Chemistry Reagents

Organic chemistry relies heavily on various reagents, each playing unique roles in chemical transformations. Among them, Osmium tetroxide (OsO₄) is celebrated for its specific ability to catalyze the formation of cis-diols from olefins. This makes it an invaluable tool in synthesis where precise stereochemistry is needed.

Reagents such as OsO₄ help control the course of a reaction by enabling selective transformations. They allow chemists to build complex structures from simpler ones, orchestrating the creation of specified molecular configurations. Osmium tetroxide, despite its toxic nature, is prized for its consistency and efficiency, requiring small amounts to achieve the desired product.

Essentials about organic chemistry reagents:

• Control reaction pathways
• Enable selective chemical transformations
• OsO₄ is used sparingly due to toxicity but is highly effective