Question

Addition of Grignard reagents, \(\mathrm{RMgX}\), to diethyl carbonate, \(\mathrm{O}=\mathrm{C}\left(\mathrm{OC}_{2} \mathrm{H}_{5}\right)_{2}\), gives tertiary alcohols, \(\mathrm{R}_{3} \mathrm{COH}\), on hydrolysis. Write the steps involved in this reaction.

Short answer

Grignard reagents react with diethyl carbonate to form tertiary alcohols via nucleophilic attacks and hydrolysis.

Step-by-step solution

Formation of the Grignard Reagent

The Grignard reagent is prepared by reacting an alkyl halide, \( \mathrm{R}\mathrm{X} \), with magnesium metal in an ether solvent. This results in the formation of the Grignard reagent \( \mathrm{R}\mathrm{MgX} \). In this case, \( \mathrm{X} \) is typically Br or Cl.

Nucleophilic Attack on Diethyl Carbonate

The Grignard reagent \( \mathrm{R}\mathrm{MgX} \) acts as a nucleophile and attacks the carbonyl carbon of diethyl carbonate, \( \mathrm{O} = \mathrm{C}(\mathrm{OC}_{2}\mathrm{H}_{5})_2 \). The nucleophilic attack opens the double bond, creating a tetrahedral intermediate.

Formation of a Ketone Intermediate

The tetrahedral intermediate formed in the previous step collapses back to a carbonyl compound by expelling one of the ethoxy groups (\( \mathrm{OC}_{2}\mathrm{H}_{5} \)). This step yields a ketone intermediate, \( \mathrm{RCOR} \), and an ethoxide ion \( \mathrm{C_2H_5O^-} \).

Second Nucleophilic Attack

A second equivalent of the Grignard reagent \( \mathrm{R}\mathrm{MgX} \) performs another nucleophilic attack on the newly formed ketone intermediate. This results in the formation of a tetrahedral intermediate once more.

Hydrolysis to Form Tertiary Alcohol

The tetrahedral alkoxide intermediate generated in the second nucleophilic attack is then treated with water or an acid, such as \( \mathrm{HCl} \). This step hydrolyzes the compound, resulting in the formation of the tertiary alcohol \( \mathrm{R}_3 \mathrm{COH} \) and releases \( \mathrm{Mg(OH)X} \) as a byproduct.

Key concepts

Nucleophilic Attack

In the world of organic chemistry, a nucleophilic attack is a common and crucial reaction mechanism. It describes a scenario where a nucleophile, which is an electron-rich species, donates an electron pair to an electron-deficient atom, known as an electrophile. In the case of Grignard reagent reactions, the Grignard reagent \( \mathrm{R}\mathrm{MgX} \) functions as the nucleophile. This reagent will target the carbonyl carbon in diethyl carbonate \( \mathrm{O} = \mathrm{C}(\mathrm{OC}_{2}\mathrm{H}_{5})_2 \). The Grignard reagent has an important job in this step: breaking the highly polarized carbon-oxygen double bond, resulting in the formation of a new carbon-carbon single bond, which ultimately leads to the creation of a tetrahedral intermediate.
Understanding this step is crucial as it explains how new bonds form in organic synthesis, using the electron-rich nature of the Grignard reagent to create more complex molecules.

Tertiary Alcohol Formation

The ultimate goal in this reaction involving the Grignard reagent and diethyl carbonate is the formation of a tertiary alcohol \( \mathrm{R}_3 \mathrm{COH} \). This alcohol type is recognized by its central carbon atom being bonded to three alkyl groups and one hydroxyl group (OH). The significance in organic chemistry comes from the fact that tertiary alcohols are often used as intermediates in organic synthesis processes. They are highly stable due to steric hindrance and are less reactive than primary or secondary alcohols in oxidation-reduction reactions.
The journey to forming this structure involves multiple reactions and intermediate formations, highlighting the complexity and intricacy that is typical in multi-step organic synthesis sequences.

Ketone Intermediate

A key step in the reaction forming a tertiary alcohol through Grignard reagent is the production of a ketone intermediate. After the initial nucleophilic attack on diethyl carbonate, a ketone is formed when the intermediate expels one of the ethoxy groups, specifically \( \mathrm{OC}_{2}\mathrm{H}_{5} \). This leads to a simplified molecule, \( \mathrm{RCOR} \), known as a ketone, where the carbon of the carbonyl group is bonded to two other carbon atoms.
Ketones play a pivotal role in organic reactions as intermediate molecules. Their structure allows them to further react in subsequent steps, making them versatile tools in synthetic routes. Their formation and transformation in these processes underscore their importance, acting as both linkages and building blocks towards more complex products.

Hydrolysis Reaction

The final step in the transformation of diethyl carbonate to a tertiary alcohol using a Grignard reagent involves a hydrolysis reaction. In chemistry, hydrolysis refers to a reaction with water that results in the breaking down of a compound. Here, the tetrahedral alkoxide intermediate reacts with water, or sometimes an acid like \( \mathrm{HCl} \), to complete the transformation. This process substitutes a hydroxyl group (OH) in place of the alkoxy group bonded to the central carbon atom.
Hydrolysis is brilliantly efficient because it finalizes the structure of organic molecules, like converting the intermediate to the desired alcohol while ensuring stability for the final product. It demonstrates the importance of water or dilute acid in finalizing organic synthesis, emphasizing their roles in concluding and polishing reaction sequences.

Organometallic Chemistry

Organometallic chemistry is the study of chemical compounds containing bonds between carbon and a metal. This field is key to understanding reactions involving Grignard reagents because these reagents are quintessential organometallic compounds. They integrate a magnesium atom bonded to a carbon-based group, usually an alkyl or aryl group from the original halide \( \mathrm{RX} \).
These organometallic compounds can act as nucleophiles due to the polarity of the \( \mathrm{C-Mg} \) bond. This bond is highly polar with a partial negative charge on the carbon, endowing the compound with its nucleophilic characteristics. Grignard reagents are thus pivotal in organic synthesis for their ability to form new ⁠— often complex ⁠— organic compounds. Understanding organometallic chemistry provides crucial insights into not only the mechanics but also the applications of these powerful synthetic tools.