Question

In which reaction, toluene is obtained as a product? (A) \(\mathrm{PhMgBr}+\mathrm{CH}_{3}-\mathrm{Br} \longrightarrow\) (B) \(\mathrm{PhMgBr}+\left(\mathrm{CH}_{3}\right)_{3} \mathrm{C}-\mathrm{Br} \longrightarrow\) (C) \(\mathrm{Ph} \mathrm{Mg} \mathrm{Br}+\mathrm{CO}_{2} \longrightarrow\) (D) \(\mathrm{PhMgBr}+\mathrm{Cl}_{2} \longrightarrow\)

Short answer

The reaction that forms toluene is (A) \(\mathrm{PhMgBr}+\mathrm{CH}_{3}-\mathrm{Br} \longrightarrow \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{3} + \mathrm{MgBr}_2\).

Step-by-step solution

Identify the reactants

In each of the given reactions, we have one reactant as \(\mathrm{PhMgBr}\), which is Phenylmagnesium bromide, a Grignard reagent. We need to look at the additional reactants to check for the possibility of forming toluene. (A) \(\mathrm{PhMgBr}+\mathrm{CH}_{3}-\mathrm{Br}\) (B) \(\mathrm{PhMgBr}+\left(\mathrm{CH}_{3}\right)_{3} \mathrm{C}-\mathrm{Br}\) (C) \(\mathrm{Ph} \mathrm{Mg} \mathrm{Br}+\mathrm{CO}_{2}\) (D) \(\mathrm{PhMgBr}+\mathrm{Cl}_{2}\)

Analyze each reaction

(A) When the Phenylmagnesium bromide reacts with Bromomethane (\(\mathrm{CH}_{3}-\mathrm{Br}\)), an exchange occurs between the \(\mathrm{MgBr}\) and the \(\mathrm{Br}\), which forms toluene: \(\mathrm{PhMgBr}+\mathrm{CH}_{3}-\mathrm{Br} \longrightarrow \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{3} + \mathrm{MgBr}_2\) (B) If Phenylmagnesium bromide reacts with Tert-Butyl bromide (\(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{C}-\mathrm{Br}\)), it will not be able to form toluene since there are too many carbon atoms connected to the exchange carbon. (C) The reaction between Phenylmagnesium bromide and Carbon dioxide (\(\mathrm{CO}_{2}\)) yields benzoic acid (\(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COOH}\)) and not toluene. (D) The reaction between Phenylmagnesium bromide and Chlorine gas (\(\mathrm{Cl}_{2}\)) doesn't form toluene because chlorine will not attack the carbon framework of aromatic compounds like toluene.

Identify the correct reaction

From our analysis, the reaction that forms toluene is (A) as it shows the necessary exchange reaction to generate the required toluene molecule: \(\mathrm{PhMgBr}+\mathrm{CH}_{3}-\mathrm{Br} \longrightarrow \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{3} + \mathrm{MgBr}_2\)

Key concepts

Phenylmagnesium Bromide

Phenylmagnesium Bromide (\( \text{PhMgBr} \)) is a powerful organometallic reagent known as a Grignard reagent. Grignard reagents are synthesized through the reaction of magnesium metal with an organic halide, in an ether solvent.- **Key Characteristics:** - They are highly reactive and tend to act as nucleophiles. - Grignard reagents are exceptionally useful in forming carbon-carbon bonds, making them indispensable in organic synthesis.

- **Reactivity:** - These reagents react by transferring a phenyl group (\( \text{C}_6\text{H}_5^- \)) to an electrophile, usually a carbon atom bearing a positive charge or partial positive charge, such as one in an alkyl halide.When you consider the structure of phenylmagnesium bromide, it consists of a phenyl group connected to magnesium, which is then connected to a bromide ion. The carbon that is part of the phenyl ring is typically considered highly reactive in this context, ready to partake in various organic reactions, such as those leading to toluene formation.

Toluene Formation

Toluene is a simple aromatic hydrocarbon, and its formation from phenylmagnesium bromide involves a nucleophilic substitution reaction. In this scenario, the nucleophile, \( \text{PhMgBr} \), reacts with bromomethane (\( \text{CH}_3\text{-Br} \)), an alkyl halide, to produce toluene.- **Process:** - The phenyl group from \( \text{PhMgBr} \) attacks the methyl group in bromomethane. - A bond forms between the phenyl group and the methyl group. - During this process, magnesium bromide is released as a by-product.Thus, the chemical reaction taking place is as follows:\[\text{PhMgBr} + \text{CH}_3\text{-Br} \longrightarrow \text{C}_6\text{H}_5\text{CH}_3 + \text{MgBr}_2\]This reaction is particularly important due to its ability to form a new carbon-carbon bond, showcasing the significance of Grignard reagents in synthetic organic chemistry. Toluene itself is a significant solvent and chemical precursor utilized in various industries like pharmaceuticals and polymers.

Organic Reaction Mechanisms

Understanding organic reaction mechanisms is crucial to mastering the complexities of organic chemistry. Reaction mechanisms describe step-by-step sequences of elementary reactions by which overall chemical change occurs. - **Grignard Reagents in Mechanisms:** - Grignard reagents such as phenylmagnesium bromide typically participate in nucleophilic addition or substitution reactions. - They add versatility and functionality to organic reaction chemistry by enabling the synthesis of alcohols, acids, and hydrocarbons such as toluene. When analyzing the reaction forming toluene, the mechanism follows a general pathway: 1. **Formation of the Grignard reagent:** Phenylmagnesium bromide is prepared in situ under anhydrous conditions. 2. **Nucleophilic Attack:** The phenyl carbanion acts as a nucleophile, attacking the electrophilic carbon in halogenated methane. 3. **By-product Formation:** After nucleophilic substitution, magnesium bromide is formed as a by-product. These steps underscore the importance of mechanism understanding, as it dictates the feasibility and efficiency of synthesis. In the example given, the precise interactions facilitated by the Grignard reagent lead to successful toluene formation, critical for industrial and laboratory preparations.