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Chapter 22: Problem 42

Propose a synthesis for 3,5 -dichloro-2-methoxybenzoic acid starting from phenol.

Short Answer

Expert verified
Answer: The final product obtained is 3,5-dichloro-2-methoxybenzoic acid.

Step by step solution

01

Formation of 2-methoxyphenol

In the first step, we will protect the hydroxyl group by ether formation. React phenol with an excess of anhydrous methanol in the presence of an acidic catalyst (e.g., H2SO4) to generate the ether 2-methoxyphenol (also known as guaiacol). This step is crucial to prevent unwanted substitution of the hydroxyl group in the subsequent reactions. The reaction can be represented as: Phenol + CH3OH (excess) → 2-methoxyphenol
02

Nitration of 2-methoxyphenol

Perform nitration of 2-methoxyphenol by reacting it with a mixture of concentrated nitric acid (HNO3) and concentrated sulfuric acid (H2SO4). Nitration of 2-methoxyphenol will result in 4-nitro-2-methoxyphenol as the major product due to the electron-donating methoxy group. The reaction can be represented as: 2-methoxyphenol + HNO3 → 4-nitro-2-methoxyphenol
03

Reduction of 4-nitro-2-methoxyphenol

Reduce the nitro group of 4-nitro-2-methoxyphenol to an amino group. This can be achieved using a reducing agent like tin (Sn) and hydrochloric acid (HCl), followed by neutralization with a strong base (e.g., NaOH). The product formed will be 4-amino-2-methoxyphenol. The reaction can be represented as: 4-nitro-2-methoxyphenol + Sn/HCl → 4-amino-2-methoxyphenol
04

Diazotization of 4-amino-2-methoxyphenol

Convert the amino group of 4-amino-2-methoxyphenol to a diazonium salt by treating it with sodium nitrite (NaNO2) in the presence of hydrochloric acid (HCl) at a low temperature (0-5°C). The product will be a diazonium salt, 4-diazonium-2-methoxyphenol chloride. The reaction can be represented as: 4-amino-2-methoxyphenol + NaNO2 + HCl → 4-diazonium-2-methoxyphenol chloride
05

Formation of 3,5-dichloro-2-methoxyphenol

Replace the diazonium group of 4-diazonium-2-methoxyphenol chloride with two chlorine atoms using cuprous chloride (Cu2Cl2) in the presence of hydrochloric acid (HCl). This reaction is known as Sandmeyer reaction and will result in the formation of 3,5-dichloro-2-methoxyphenol. The reaction can be represented as: 4-diazonium-2-methoxyphenol chloride + Cu2Cl2 → 3,5-dichloro-2-methoxyphenol
06

Formation of 3,5-dichloro-2-methoxybenzoic acid

Lastly, convert the hydroxyl group of 3,5-dichloro-2-methoxyphenol to a carboxyl group (benzoic acid). This can be done by performing a strong oxidation reaction using potassium permanganate (KMnO4) in the presence of a strong base such as potassium hydroxide (KOH). The product formed will be 3,5-dichloro-2-methoxybenzoic acid. The reaction can be represented as: 3,5-dichloro-2-methoxyphenol + KMnO4/KOH → 3,5-dichloro-2-methoxybenzoic acid

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Most popular questions from this chapter

Suggest a reason why the nitroso group, \(-\mathrm{N}=\mathrm{O}\), is ortho- para directing whereas the nitro group, \(-\mathrm{NO}_{2}\), is meta directing.

Write the stepwise mechanism for sulfonation of benzene by hot, concentrated sulfuric acid. In this reaction, the electrophile is \(\mathrm{SO}_{3}\) formed as shown in the following equation. $$ \mathrm{H}_{2} \mathrm{SO}_{4} \rightleftharpoons \mathrm{SO}_{3}+\mathrm{H}_{2} \mathrm{O} $$

When certain aromatic compounds are treated with formaldehyde, \(\mathrm{CH}_{2} \mathrm{O}\), and \(\mathrm{HCl}\), the \(\mathrm{CH}_{2} \mathrm{Cl}\) group is introduced onto the ring. This reaction is known as chloromethylation.

2,6-Di-tert-butyl-4-methylphenol, alternatively known as butylated hydroxytoluene (BHT), is used as an antioxidant in foods to "retard spoilage" (Section 8.7). BHT is synthesized industrially from 4-methylphenol by reaction with 2-methylpropene in the presence of phosphoric acid. Propose a mechanism for this reaction.

The first widely used herbicide for the control of weeds was 2,4 -dichlorophenoxyacetic acid (2,4-D). Show how this compound might be synthesized from phenol and chloroacetic acid by way of the given chlorinated phenol intermediate.
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