Question

Ethyl acetate is treated with excess of methyl magnesium iodide in dry ether. The reaction mixture is then treated with water. The organic products obtained are

Short answer

The organic products obtained from treating ethyl acetate with an excess of methyl magnesium iodide in dry ether and then treating the reaction mixture with water are 2-methoxy-2-methylpropanol (CH3CH2OC(OH)(CH3)2).

Step-by-step solution

Identify the reactants in the Grignard reaction

Ethyl acetate (CH3COOCH2CH3) is treated with excess of methyl magnesium iodide (CH3MgI) in dry ether as the solvent. In this reaction, CH3MgI is the Grignard reagent, and it will act as a nucleophile attacking the carbonyl group of ethyl acetate.

Perform the Grignard reaction

The Grignard reagent reacts with the carbonyl group (C=O) of ethyl acetate, and the methyl anion in the Grignard reagent acts as a nucleophile attacking the electrophilic carbonyl carbon. This results in the addition of the methyl group to the carbonyl carbon and the formation of a new C-C bond. The intermediate formed after this reaction is a magnesium alkoxide, with the following structure: CH3CH2OC(O-)(CH3)MgI.

Add water to the reaction mixture

After the Grignard reaction, the reaction mixture is treated with water. This step is essentially a hydrolysis, and it results in the protonation of the alkoxide group, leading to the formation of an alcohol. The magnesium salt will form as a byproduct. The products formed after this step are a tertiary alcohol and magnesium salts: CH3CH2OC(OH)(CH3)2 and MgI(OH).

Identify the organic products

The organic product obtained from this reaction is the tertiary alcohol, which has the structure CH3CH2OC(OH)(CH3)2. This compound is called 2-methoxy-2-methylpropanol.

Key concepts

Nucleophile Attack

A key step in many organic reactions, including the Grignard reaction, involves a nucleophile attack. In the context of this reaction, the Grignard reagent, methyl magnesium iodide (\( \text{CH}_3\text{MgI} \)), acts as the nucleophile. A nucleophile is a species that is rich in electrons and is attracted to positive charges. Here, the methyl anion (\( \text{CH}_3^- \)) from the Grignard reagent is electron-rich and seeks out an electron-deficient center to bond with.

In this reaction, ethyl acetate (\( \text{CH}_3\text{COOCH}_2\text{CH}_3 \)) serves as the substrate. The carbonyl carbon in ethyl acetate, located at the \( \text{C=O} \) group, is electrophilic due to the partial positive charge it carries. This happens because oxygen, being highly electronegative, pulls electrons towards itself, creating a polar bond.

The methyl anion attacks this electrophilic carbon, forming a new \( \text{C-C} \) bond. This nucleophilic attack is the driving force behind the formation of the magnesium alkoxide intermediate and is crucial for advancing the reaction towards the desired alcohol formation.

Magnesium Alkoxide

After the nucleophilic attack in the Grignard reaction, an important intermediate called a magnesium alkoxide is formed. This intermediate is a salt-like compound that features both organic and inorganic characteristics due to the presence of the magnesium ion.

In the reaction between ethyl acetate and methyl magnesium iodide, the organic intermediate formed can be denoted as \( \text{CH}_3\text{CH}_2\text{OC(O-)(CH}_3)\text{MgI} \). This structure shows the alkoxide ion (the \( \text{O}^- \) group) attached to the magnesium cation.

Magnesium alkoxides are generally not the final desired product in a Grignard reaction; instead, they act as transient species on the pathway to forming alcohols. The strong metal-oxygen bond provides stability for the intermediate, which is why magnesium is so valuable in Grignard reactions.

• Provides stability to the intermediate form.
• Acts as a native part of the reaction mechanism leading to alcohol formation.

Next, the reaction will typically involve a step to convert this magnesium alkoxide into the desired alcohol by addition of water or another proton source.

Tertiary Alcohol Formation

The ultimate goal of the Grignard reaction in this exercise is to produce an alcohol. Following the formation of the magnesium alkoxide, the next critical step is the addition of water, which serves to hydrolyze this intermediate.

This process, generally known as hydrolysis, involves the magnesium alkoxide reacting with water to replace the metal group with a hydrogen atom. The water molecule donates a proton (\( \text{H}^+ \)) to the alkoxide ion, which results in the formation of a tertiary alcohol and magnesium salts as byproducts.

The specific alcohol formed in this example is 2-methoxy-2-methylpropanol, whose structure is \( \text{CH}_3\text{CH}_2\text{OC(OH)(CH}_3)_2 \). This tertiary alcohol contains a central carbon atom bonded to three other carbon-containing groups, making it structurally different and typically more stable than primary or secondary alcohols.

• Tertiary alcohols have a carbon atom connected to three carbon groups.
• This central carbon bears the hydroxyl group (\( \text{OH} \)).
• These alcohols are often less reactive than their primary and secondary counterparts.

The completion of this step highlights the Grignard reaction's ability to synthesize complex alcohols from simpler components.