Question

(i) Suggest a method to prepare \(2-\) methyl \(-2\) - propanol using Grignard reagent. (ii) What happens when this alcohol is passed over copper heated to \(573 \mathrm{~K} ?\)

Short answer

#Answer# 2-methyl-2-propanol can be prepared using a Grignard reagent by reacting methyl magnesium bromide with acetone. When this alcohol is passed over copper heated to 573 K, it undergoes dehydrogenation to form 2-methylpropene and water.

Step-by-step solution

Identify the Grignard reagent and the substrate

To prepare 2-methyl-2-propanol (C4H10O) using a Grignard reagent, we need to identify a suitable Grignard reagent and a substrate that will react to form the desired product. We can choose the Grignard reagent as methyl magnesium bromide (CH3MgBr) and the substrate as acetone (CH3COCH3).

Write the reaction

The reaction between methyl magnesium bromide (CH3MgBr) and acetone (CH3COCH3) can be written as: CH3MgBr + CH3COCH3 → (CH3)3COH + MgBrOH So, the method to prepare 2-methyl-2-propanol using Grignard reagent is to react methyl magnesium bromide with acetone. (ii) Reaction of 2-methyl-2-propanol when passed over heated copper:

Write the reaction

When 2-methyl-2-propanol is passed over copper heated to 573 K, it undergoes dehydrogenation (removal of hydrogen) to form an alkene. The reaction can be written as: C4H10O (2-methyl-2-propanol) → C4H8 (2-methylpropene) + H2O So, when 2-methyl-2-propanol is passed over heated copper at 573 K, it forms 2-methylpropene and water.

Key concepts

2-methyl-2-propanol preparation

To prepare 2-methyl-2-propanol using a Grignard reagent, we can utilize a straightforward method. Grignard reagents are vital in organic synthesis due to their ability to create carbon-carbon bonds. In this process, the Grignard reagent used is methyl magnesium bromide \(CH_3MgBr\), which reacts with a substrate, acetone \(CH_3COCH_3\).

Acetone is a simple ketone that contains a carbonyl group \(C=O\). The carbon atom in this group is electrophilic, making it reactive towards a nucleophile like the Grignard reagent. Methyl magnesium bromide is the nucleophilic agent here that attacks the carbonyl carbon. This addition reaction results in the formation of 2-methyl-2-propanol along with magnesium bromide by-product.

The overall reaction can be summarized as:

• CH₃MgBr (Grignard reagent) + CH₃COCH₃ (acetone) → (CH₃)₃COH (2-methyl-2-propanol) + MgBrOH (by-product)

Through this method, the desired alcohol, 2-methyl-2-propanol, is synthesized efficiently.

dehydrogenation

Dehydrogenation refers to the removal of hydrogen atoms from a molecule, often converting single bonds into double bonds or eliminating hydrogen to form new structures. In this context, 2-methyl-2-propanol undergoes dehydrogenation when passed over copper that has been heated to 573 K.

Here, under the influence of the copper catalyst, the alcohol loses hydrogen atoms, resulting in the formation of an alkene. Specifically, this process transforms 2-methyl-2-propanol into 2-methylpropene. The reaction is as follows:

• C₄H₁₀O (2-methyl-2-propanol) → C₄H₈ (2-methylpropene) + H₂O

The dehydrogenation process not only produces an unsaturated compound, in this case, an alkene, but also water as a by-product. This reaction is important in various industrial applications where alkenes are produced from alcohols.

acetone reaction with methyl magnesium bromide

The interaction between acetone and methyl magnesium bromide, an essential step in the preparation of 2-methyl-2-propanol, is a classic example of a nucleophilic addition reaction. Methyl magnesium bromide, a Grignard reagent, serves as a nucleophile and attacks the electrophilic carbon in the carbonyl group of acetone.

This attack transforms the planar carbonyl structure, resulting in a tetrahedral alkoxide intermediate, which further protonates to give 2-methyl-2-propanol. In essence, the presence of the magnesium atom in the Grignard reagent plays a crucial role here. It momentarily interacts with the oxygen of the carbonyl group, facilitating the nucleophilic attack.

The reaction is typical of Grignard reagents interacting with ketones:

• CH₃MgBr + CH₃COCH₃ → (CH₃)₃COH + MgBrOH

This example demonstrates how Grignard reagents effectively create alcohols through reactions with ketones, showcasing their utility in organic synthesis.