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Chapter 22: Problem 36

Propose a synthesis of triphenylmethane from benzene, as the only source of aromatic rings, and any other necessary reagents.

Short Answer

Expert verified
Question: Outline the synthesis of triphenylmethane from benzene, using appropriate reagents and reaction conditions for each step. Answer: To synthesize triphenylmethane from benzene: 1. Brominate benzene using Br2 and FeBr3 catalyst to form bromobenzene. 2. Convert bromobenzene to a Grignard reagent (phenylmagnesium bromide, Ph-MgBr) using Mg and ether solvent. 3. React the Grignard reagent with benzoyl chloride (Ph-CO-Cl) to form triphenylmethanol, using acidic water (H3O+) for quenching. 4. Dehydrate triphenylmethanol using a strong acid, such as H2SO4, to form the desired product, triphenylmethane.

Step by step solution

01

Bromination of benzene

First, we need to introduce a functional handle on the benzene ring to allow further reactions. The bromination of benzene in the presence of a catalyst, such as FeBr3, will accomplish this. The reaction is as follows: C6H6 + Br2 <-> Ar-Br + HBr, where Ar-Br represents bromobenzene.
02

Conversion of bromobenzene to Grignard reagent

The next step is to create a Grignard reagent from the bromobenzene. This is done by reacting the bromobenzene with magnesium metal in an ether solvent, such as diethyl ether or tetrahydrofuran (THF). This reaction produces a phenylmagnesium bromide Grignard reagent (Ph-MgBr).
03

Formation of the central carbon atom of triphenylmethane

Now we need to prepare a molecule that will react with the Grignard reagent to make the central carbon-phenyl bond. To do this, we can react benzoyl chloride (also called phenylcarbonyl chloride, Ph-CO-Cl) with the phenylmagnesium bromide (Ph-MgBr) Grignard reagent from Step 2. The Grignard reagent will attack the carbonyl carbon to form a ketone intermediate. Then, a second equivalent of the Grignard reagent will attack the ketone, forming a new carbon-carbon bond. Finally, the reaction mixture is quenched with acidic water (H3O+), and the magnesium salts are removed to give the desired product, triphenylmethanol (Ph3COH).
04

Dehydration of triphenylmethanol to triphenylmethane

The final step is to convert the triphenylmethanol (Ph3COH) into triphenylmethane (Ph3C-H). This can be done through dehydration, which can be carried out by treating the triphenylmethanol with a strong acid, such as sulfuric acid (H2SO4). The acid will protonate the alcohol group, and a molecule of water is lost to form the corresponding carbocation. A base present in the reaction mixture will then remove the remaining proton from the central carbon, generating the desired product, triphenylmethane.

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