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Chapter 3: Problem 172

Statement 1 Pure phenol is a colourless solid but turns reddish brown on keeping. and Statement 2 Phenol is slowly oxidized by air to quinone.

Short Answer

Expert verified
Answer: Phenol turns reddish-brown over time when exposed to air due to the slow oxidation process that converts phenol to quinone. The formation of quinone causes the reddish-brown color as it absorbs light in the visible spectrum due to its increased conjugation in comparison to phenol.

Step by step solution

01

Understanding the structure of phenol and quinone

Phenol is an aromatic compound with the formula C6H5OH, which consists of a benzene ring with a hydroxyl (-OH) group attached to one of its carbon atoms. Quinone, on the other hand, is an organic compound with the formula C6H4O2. In the case of phenol oxidation, the quinone formed is 1,2-benzoquinone, also called simply 'quinone'. It has a benzene ring with two carbonyl groups (=O) in the 1,2-positions (adjacent carbon atoms).
02

Discussing the oxidation process from phenol to quinone

Over time, when phenol is exposed to air, it undergoes a slow oxidation process. Oxidation is a chemical reaction that involves the loss of electrons. In the case of phenol, the hydroxyl group (-OH) loses its hydrogen atom as a proton, and the remaining oxygen forms a double bond with the benzene ring. This results in the formation of the two carbonyl groups (=O) on the quinone molecule.
03

Explaining how the formation of quinone causes the reddish-brown color in phenol

As phenol slowly oxidizes to quinone in the presence of air, the resulting quinone molecules have a reddish-brown color. This color change is mostly due to the increased conjugation in the quinone molecule as compared to phenol, which causes quinone to absorb light in the visible spectrum. The more quinone that forms in the sample, the more pronounced the reddish-brown color becomes. Therefore, when pure phenol is kept for an extended period, its color turns from colorless to a reddish-brown due to the formation of quinone by slow oxidation in the presence of air.

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Most popular questions from this chapter

When phenol is exposed to air for a long period or treated with chromic acid, it is oxidized to (a) p-benzoquinone (b) glyoxal (c) \(\mathrm{CO}_{2}\) (d) oxalic acid

Which of the following reagents can effect hydroxylation at an aromatic carbon? (a) aqueous KOH (b) alcoholic KOH (c) alkaline \(\mathrm{KMnO}_{4}\) (d) Fenton's reagent

Phenol on warming with conc. \(\mathrm{H}_{2} \mathrm{SO}_{4}\) and \(\mathrm{NaNO}_{2}\) gives brown colour. On adding \(\mathrm{NaOH}\) solution, the colour changes to green or blue. This test is called (a) Victor Meyer's test (b) Libermann's test (c) Lucas test (d) Baeyer's test

Phenol + phthalic anhydride \(\frac{\text { conc. } \mathrm{H}_{2} \mathrm{SO}_{4}}{\text { heat }}{\longrightarrow}(\mathrm{X})\). The product \((\mathrm{X})\) is used as (a) an antipyretic (b) a catalyst (c) an indicator (d) a laboratory reagent

Why is pyrrole a much weaker base than pyridine?
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