Question

How is \(\left(\mathrm{C}_{6} \mathrm{H}_{5}\right)_{2} \mathrm{C}=\mathrm{CH}_{2}\) prepared by a Grignard reaction with \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{Br}\) and \(\mathrm{CH}_{3} \mathrm{COOC}_{2} \mathrm{H}_{5}\) as the starting materials?

Short answer

Question: Outline the steps to prepare the compound \((\mathrm{C}_{6} \mathrm{H}_{5})_{2}\mathrm{C}=\mathrm{CH}_{2}\) using a Grignard reaction, starting from \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{Br}\) and \(\mathrm{CH}_{3}\mathrm{COOC}_{2} \mathrm{H}_{5}\). Answer: The preparation of the compound involves the following steps: 1. Formation of Grignard reagent (\(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{MgBr}\)) by reacting \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{Br}\) with magnesium in ether. 2. Reaction of the Grignard reagent with the ester compound (\(\mathrm{CH}_{3}\mathrm{COOC}_{2} \mathrm{H}_{5}\)) to form a ketone (\(\mathrm{C}_{6} \mathrm{H}_{5}\-\mathrm{C}(\mathrm{H})=\mathrm{O}\)). 3. Reaction of the formed ketone with another mole of Grignard reagent, resulting in the alkoxide salt of the target compound. 4. Protonation of the alkoxide salt using an aqueous acid or water, leading to the desired compound \(\left(\mathrm{C}_{6} \mathrm{H}_{5}\right)_{2}\mathrm{C}=\mathrm{CH}_{2}\) and a magnesium salt side product.

Step-by-step solution

Formation of Grignard reagent

To begin, the Grignard reagent is formed by the reaction between \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{Br}\) and magnesium. The magnesium atom inserts itself between the carbon atom and the bromine atom, forming the Grignard reagent \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{MgBr}\). The reaction is as follows: $$\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{Br} + \mathrm{Mg} \xrightarrow{\text{ether}} \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{MgBr}$$

Reaction of Grignard reagent with the ester compound

Now, the formed Grignard reagent reacts with the ester compound \(\mathrm{CH}_{3}\mathrm{COOC}_{2} \mathrm{H}_{5}\). In the presence of a Grignard reagent, an ester is first converted to a ketone by the nucleophilic attack of the Grignard reagent on the carbonyl carbon and subsequent breakage of the carbon-oxygen double bond: $$\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{MgBr} + \mathrm{CH}_{3} \mathrm{COOC}_{2} \mathrm{H}_{5} \xrightarrow{} \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{MgOOC}_{2} \mathrm{H}_{5} + \mathrm{C}_{6} \mathrm{H}_{5}\-\mathrm{C}(\mathrm{H})=\mathrm{O}$$

Reaction of ketone with Grignard reagent

Next, the ketone formed in the previous step reacts with another mole of Grignard reagent. Upon nucleophilic attack by the Grignard reagent, the alkoxide salt of the target compound is formed: $$\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{MgBr} + \mathrm{C}_{6} \mathrm{H}_{5}\-\mathrm{C}(\mathrm{H})=\mathrm{O} \xrightarrow{} \left(\mathrm{C}_{6} \mathrm{H}_{5}\right)_{2}\mathrm{C}(O^{\ominus}\mathrm{Mg}^{\oplus}\mathrm{Br})(\mathrm{H})$$

Protonation of the alkoxide salt

Finally, the alkoxide salt is protonated using an aqueous acid like dilute \(\mathrm{HCl}\) or water. The magnesium salt formed is a side product and the desired alkene is obtained: $$\left(\mathrm{C}_{6} \mathrm{H}_{5}\right)_{2}\mathrm{C}(O^{\ominus}\mathrm{Mg}^{\oplus}\mathrm{Br})(\mathrm{H}) + \mathrm{H}_{2}\mathrm{O} \xrightarrow{} \left(\mathrm{C}_{6} \mathrm{H}_{5}\right)_{2}\mathrm{C} = \mathrm{CH}_{2} + \mathrm{Mg}(\mathrm{OH})\mathrm{Br}$$ In conclusion, the compound \(\left(\mathrm{C}_{6} \mathrm{H}_{5}\right)_{2}\mathrm{C}=\mathrm{CH}_{2}\) can be prepared through a series of reactions involving the formation of a Grignard reagent from \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{Br}\), reaction with the ester \(\mathrm{CH}_{3}\mathrm{COOC}_{2} \mathrm{H}_{5}\), and subsequent steps leading to the final product.

Key concepts

Grignard Reagent Preparation

Grignard reagents are a cornerstone in the field of organic synthesis. They are typically formed by the reaction of an organohalide, such as bromobenzene (\(\mathrm{C}_6 \mathrm{H}_5 \mathrm{Br}\)), with magnesium metal. The process requires a solvent like diethyl ether, which stabilizes the reactive Grignard reagent formed. Here, the magnesium inserts itself into the carbon-bromine bond, resulting in the Grignard reagent, phenylmagnesium bromide (\(\mathrm{C}_6 \mathrm{H}_5 \mathrm{MgBr}\)).

The stability and reactivity of Grignard reagents are influenced by several factors:

• The use of dry, inert conditions to prevent reaction with moisture or oxygen.
• The solvent's ability to solubilize the magnesium and stabilize the reactive intermediate.

This preparation is a critical step because the Grignard reagent acts as a nucleophile, attacking electrophilic centers in subsequent steps.

Nucleophilic Attack

Once the Grignard reagent is prepared, it is poised for a nucleophilic attack on an ester compound, such as methyl acetate (\(\mathrm{CH}_3\mathrm{COOC}_2 \mathrm{H}_5\)). The oxygen in the carbonyl group (\(\mathrm{C}=\mathrm{O}\)) is electron-rich, making the carbon atom electron-deficient and susceptible to attack.

The Grignard reagent donates electrons to this carbonyl carbon, leading to the breakage of the double bond and the formation of an intermediate alkoxide. This turns the ester into a ketone and an alkoxide ion.

Understanding nucleophilic attack is crucial in organic chemistry as it allows the transformation of carbon skeletons and the integration of new groups into molecules. It highlights how nucleophiles and electrophiles interact, a concept that is foundational in organic reaction mechanisms.

Protonation of Alkoxide

After the alkoxide ion has been formed, it needs to be protonated to produce the desired hydrocarbon. Protonation involves the addition of a proton (\(\mathrm{H}^+\)) from a source such as dilute \(\mathrm{HCl}\) or water. During this step, the reactive alkoxide ion reacts with a proton to form the alcohol intermediate. The by-product of this reaction is \(\mathrm{Mg(OH)Br}\).

Protonation is an essential step because it stabilizes the reactive species and allows the synthesis to proceed to completion. Without protonation, the reaction would halt, leaving behind unstable intermediates. This step also transforms the temporary negative charge on the oxygen atom into a neutral group, facilitating the removal of unwanted side-products as stable entities.

This demonstrates how simple acid-base chemistry can dramatically change the outcome of an organic synthesis process.

Organic Synthesis Steps

Bringing all these steps together showcases the beautiful complexity of organic synthesis. The sequence in this reaction starts with the preparation of the Grignard reagent, which sets the stage for subsequent transformations. The next move is to perform the nucleophilic attack, transforming the ester to an intermediate ketone, and followed by a final attack to afford an alkoxide. Finally, protonation of this alkoxide gives rise to the target alkene.

Each step in this process must proceed under carefully controlled conditions to ensure purity and yield of the product.

• Carefully control stoichiometry to prevent side reactions.
• Use proper solvents and conditions to maintain reagent stability.

Mastery of these elements allows chemists to design and execute elegant, efficient syntheses of complex organic molecules. By understanding each step in detail, students can appreciate not just the steps themselves, but the logic and creativity involved in devising such reaction sequences.