Question

\(\mathrm{H}_{3} \mathrm{C}-\mathrm{C} \equiv \mathrm{CH}+\mathrm{CH}_{3} \mathrm{MgI} \longrightarrow(\mathrm{A})+\mathrm{CH}_{4}\) \((\mathrm{A})+\mathrm{CH}_{3} \mathrm{I} \longrightarrow(\mathrm{B})+\mathrm{MgI}_{2}\) (A) and (B) are (a) \(\mathrm{H}-\mathrm{C} \equiv \mathrm{C}-\mathrm{MgI}\) and \(\mathrm{CH}_{4}\) (b) \(\mathrm{H}_{3} \mathrm{C}-\mathrm{C} \equiv \mathrm{C}-\mathrm{CH}_{3}\) and \(\mathrm{CH}_{4}\) (c) \(\mathrm{CH}_{3}-\mathrm{C} \equiv \mathrm{C}-\mathrm{MgI}\) and \(\mathrm{CH}_{3}-\mathrm{C} \equiv \mathrm{C}-\mathrm{CH}_{3}\) (d) \(\mathrm{H}_{3} \mathrm{C}-\mathrm{C} \equiv \mathrm{C}-\mathrm{I}\) and

Short answer

Answer: Product (A) is H-C≡C-MgI, and product (B) is CH3-C≡C-CH3.

Step-by-step solution

Reaction of alkyne with Grignard reagent

Grignard reagents react with alkynes to form alkyl magnesium halide. In our given reaction, HC≡CH (ethyne) is reacting with CH3MgI (methylmagnesium iodide) to form product (A) and CH4 (methane). To determine the structure of product (A), we must know where the bond will be formed. Grignard reagents are strong nucleophiles, and they usually attack the least substituted carbon atom in the triple bond.

Determine structure of product (A)

The least substituted carbon in HC≡CH is the terminal carbon bonded to hydrogen. Thus, the CH3 group of CH3MgI will attack the terminal carbon of HC≡CH, and the magnesium will attack the other carbon of the triple bond. This leads to the formation of H-C≡C-MgI as product (A).

Reaction of product (A) with alkyl halide

Product (A) is H-C≡C-MgI, and now it reacts with CH3I (methyl iodide) to form product (B) and MgI2 (magnesium iodide). As the Grignard reagent acts as a strong nucleophile, it will attack the carbon atom of CH3I, breaking the carbon-iodine bond.

Determine structure of product (B)

The attack of C≡C-MgI on CH3I leads to the formation of a new C-C bond, resulting in the structure CH3-C≡C-CH3 as product (B). Therefore, the correct answer is option (c) where product (A) is \(\mathrm{CH}_{3}-\mathrm{C} \equiv \mathrm{C}-\mathrm{MgI}\), and product (B) is \(\mathrm{CH}_{3}-\mathrm{C} \equiv \mathrm{C}-\mathrm{CH}_{3}\).

Key concepts

Reaction Mechanism

Understanding the reaction mechanism is essential to grasping how different chemical reactions occur. In this scenario, we focus on the way alkynes interact with Grignard reagents. Grignard reagents, such as methylmagnesium iodide (CH₃MgI), are known for their strong nucleophilic properties. This means they can donate an electron pair to form a chemical bond with an electron-deficient site, known as an electrophile.

When reacting with an alkyne, the Grignard reagent specifically targets the terminal hydrogen atom of ethyne (HC≡CH). This carbon is less hindered and thus more accessible to the nucleophilic attack. As the Grignard reagent attacks this site, the resulting product is a new organomagnesium compound (H-C≡C-MgI). This step also liberates a molecule of methane (CH₄).

Breaking down such a reaction mechanism emphasizes the importance of electron flow and bond formation in organic chemical reactions. This concept underpins the transformation from a terminal alkyne to the intermediate product (A).

Organic Synthesis

Organic synthesis is the art of constructing organic compounds through logical chemical transformations. Here, our initial synthesis involves transforming an alkyne into a more complex molecule using Grignard reagents. The immediate goal is to convert HC≡CH into H-C≡C-MgI, setting the stage for further reactions.

This initial step is pivotal as it opens the way for subsequent transformations, like the reaction of product (A) with another compound, such as methyl iodide (CH₃I). This subsequent reaction is a perfect example of how organic synthesis builds up complexity. Product (A) reacts with CH₃I, undergoing another nucleophilic substitution. The Grignard reagent acts as a nucleophile and attacks the carbon atom bound to iodine in CH₃I.

The result is the final product (B), CH₃-C≡C-CH₃, exemplifying how a simple alkyne undergoes a planned series of reactions to form a more complex organic molecule. Organic synthesis uses such strategies to design and fabricate a wide array of chemical compounds.

Grignard Reagent Chemistry

Grignard reagent chemistry is a cornerstone of organic chemistry, enabling the formation of carbon-carbon bonds. These reagents are broadly represented by the formula RMgX, where R is an organic moiety and X is a halogen. The synthesis of Grignard reagents typically occurs through the reaction of an organic halide with magnesium, forming a reagent capable of acting as a powerful nucleophile.

In the exercise, methylmagnesium iodide (CH₃MgI) is our Grignard reagent. Grignard reagents hold a unique position in chemistry due to their ability to attack a wide variety of electrophiles, facilitating diverse reactions. When used with alkynes, such as ethyne (HC≡CH), they generate new molecular entities such as organomagnesium compounds.

Grignard reagents are essential for synthesizing a broad range of organic molecules, enabling chemists to construct complex molecular architectures. They play a crucial role in industrial applications and laboratory syntheses alike, proving invaluable for making pharmaceuticals, plastics, and other significant chemical products. By manipulating these reagents under controlled conditions, chemists unlock pathways to creating new compounds from simple precursors.