Problem 102 - A Complete Resource Book in Chemistry for JEE Main

Question

The reaction of elemental sulphur with Grignard reagent followed by acidification leads to the formation of (a) sulphoxide (b) mercaptan (c) sulphonic acid (d) thioester

Step-by-Step Solution

Answer

(b) mercaptan

Step 1: Understanding the Reactants

Identify the chemical reactants: elemental sulphur (S) and a Grignard reagent (R-MgX, where R is an alkyl or aryl group and X is a halogen). The reaction will proceed between these substances.

Step 2: Reaction with Grignard Reagent

Consider the first part of the reaction where elemental sulphur reacts with the Grignard reagent, R-MgX. Grignard reagents are nucleophilic, and they will attack the sulfur, likely forming a bond to create a compound like R-S-MgX.

Step 3: Acidification

Once the intermediate R-S-MgX is formed, the compound undergoes acidification. Acidification typically means adding an acid like HCl or H2O, which provides protons (H+). This step results in the -MgX group being replaced by a hydrogen atom, forming R-SH, a thiol or mercaptan.

Step 4: Identifying the Product

After completing the previous steps, the final product is a thiol, also known as a mercaptan, with the formula R-SH.

Key Concepts

Grignard Reagent

Grignard reagents are widely used in organic chemistry as a powerful tool for making carbon-carbon bonds. These reagents are organomagnesium compounds with the general formula R-MgX. Here, "R" represents an organic group like an alkyl or aryl, and "X" denotes a halogen atom such as chlorine, bromine, or iodine.
Grignard reagents are highly reactive due to the presence of a polarized carbon-magnesium bond. The carbon from the organic group becomes highly nucleophilic, making it eager to attack electrophilic centers. This reactivity allows Grignard reagents to form new bonds, making them essential in synthesizing various organic compounds.
To create a Grignard reagent, a suitable halide is mixed with magnesium in an anhydrous (water-free) solvent like ether. This environment is crucial because Grignard reagents can easily decompose in the presence of water, yielding hydrocarbons instead.

• This quality makes them very useful in forming alcohols, acids, and other complex organic structures.
• They're broadly utilized in the pharmaceutical industry for drug synthesis.

Because Grignard reagents are so reactive, they must be handled with care. Controlling factors like solvent and temperature can optimize their reactions and prevent unwanted side reactions.

Thiols

Thiols, often called mercaptans, are a fascinating class of organic compounds characterized by the presence of a thiol group  \(\text{R-SH}\). This group consists of a sulfur atom bonded to a hydrogen atom, attached to an organic group or alkyl chain.
These compounds are analogs to alcohols, but with sulfur replacing the oxygen atom typically found in the hydroxyl group of alcohols. Thiols have a strong, often unpleasant odor, reminiscent of garlic or rotten eggs, a feature exploited in natural gas as a smell marker for leaks.

The acidity of a thiol's hydrogen atom is noteworthy; while not as acidic as carboxylic acids, it's more acidic than alcohols. This moderate acidity is essential for metal ion binding and reactivity in organic synthesis.

• Thiols can be created by the reaction of Grignard reagents with elemental sulfur.
• They are highly useful as ligands in coordination chemistry and as intermediates in synthesizing pharmaceuticals.

Overall, thiols play a crucial role in biological systems and synthetic chemistry, showcasing their versatility and importance.

Sulphur Reactions

Sulfur plays a significant role in organic chemistry, engaging in reactions that frequently involve its electrophilic nature. Among these reactions, those involving sulfur commonly result in the formation of compounds with diverse applications in industry and research.
One prominent reaction involving sulfur is its interaction with Grignard reagents. Here, the nucleophilic carbon in the Grignard reagent attacks the sulfur atom, generating an intermediate compound like \(\text{R-S-MgX}\). This step demonstrates sulfur's ability to act as a good electrophile, easily bonding with nucleophilic substances.
The subsequent step usually involves acidification, where the intermediate undergoes protonation to yield the final product, such as a thiol. This step is crucial, as the addition of an acid breaks the metal-sulfur linkage, introducing a hydrogen atom to form \(\text{R-SH}\).

• These reactions are vital in forming various sulfur-containing organic compounds used in both industrial and pharmaceutical contexts.
• They highlight sulfur's versatility as a central atom in different molecular transformations.

Sulfur-based reactions continue to advance, offering chemists new pathways to synthesize and explore innovative compounds.