Problem 50 - Advanced Problems in Organic Chemistry for Competitive Examinations

Question

\(\frac{\mathrm{BrMg}-\left(\mathrm{CH}_{2}\right)_{4}-\mathrm{MgBr}}{\text { then } \mathrm{H}_{3} \mathrm{O}^{+}}\)

Step-by-Step Solution

Answer

Hexane is formed.

Step 1: Analyze the Reagent

The reagent given is a Grignard reagent. Here, \( \text{BrMg}-(\text{CH}_2)_4-\text{MgBr} \) implies that we have a di-Grignard reagent with the formula \( \text{BrMg(CH}_2\text{)}_4\text{MgBr} \). This reagent consists of two Grignard units with a -(CH2)4- chain.

Step 2: Reaction with Acid

Grignard reagents react with water or acids like \( \text{H}_3\text{O}^+ \) to form hydrocarbons. In this case, the di-Grignard reagent will react with water to produce hexane, as the \( -(\text{CH}_2)_4- \) chain becomes protonated on each end.

Step 3: Predict the Product

The protonation of the Grignard leads to the formation of hexane. All magnesium and bromine atoms are removed in the form of magnesium bromide salts.

Key Concepts

Organic Synthesis

Organic synthesis involves building complex molecules from simpler ones. In the world of chemistry, it's like constructing a skyscraper with the right combination of building blocks, beams, and structural supports.
A crucial tool in organic synthesis is the Grignard reagent. Grignard reagents are highly reactive substances formed by reacting magnesium metal with an alkyl or aryl halide. These reagents are used to create carbon-carbon bonds, which is essential in forming the backbone of many organic structures.

• Grignard reagents open the door to endless possibilities in creating complex organic molecules.
• They enable the conversion of smaller, simpler molecules into larger, more complex ones.

In the given exercise, the Grignard reagent takes the form of a di-Grignard compound, meaning it has two reactive Grignard sites. These sites can participate in multiple reactions, significantly enhancing the complexity of the molecules that can be synthesized.

Protonation Reaction

Protonation is the process of adding a proton (H⁺) to an atom, molecule, or ion. This process often involves acids, which easily donate protons in solution.
For Grignard reagents, protonation usually involves water or acids. When these reagents come into contact with a proton source, a reaction occurs where the Grignard reagent is transformed into a hydrocarbon.

• Water or an acid like \(\text{H}_3\text{O}^+\) supplies the necessary protons.
• The reaction results in the conversion of the Grignard reagent into a saturated hydrocarbon, while the magnesium bromide and other by-products are removed.

In the exercise, the di-Grignard reagent undergoes protonation at each end of the \(-(\text{CH}_2)_4-\) chain, converting it into hexane. This process is straightforward but essential in organic synthesis, as it enables the creation of alkanes from more reactive starting materials.

Reaction Mechanism

Understanding the mechanism of reactions is crucial in chemistry, as it allows us to predict the behavior of molecules under different conditions. A reaction mechanism is a step-by-step description of how a chemical reaction occurs.
For Grignard reactions, the mechanism often starts with the nucleophilic nature of the Grignard reagent. It acts as both a nucleophile and a base.

• The reagent's carbon atoms are extremely reactive towards electrophiles like protons due to the highly polar bond between carbon and magnesium.
• Upon encountering a proton source, the carbon atoms of the reagent rapidly grab onto the protons, leading to the formation of new bonds.

In our exercise, the di-Grignard compound reacts in a two-step mechanism: first, each end of the \(-(\text{CH}_2)_4-\) chain grabs a proton, leading to the loss of the Grignard functionality and the formation of magnesium bromide as a by-product. Finally, hexane is formed as the main product, showcasing the versatility and usefulness of Grignard reagents in synthetic chemistry.