Phenols: Preparation & Properties

Hint: Phenol undergoes oxidation and form benzoquinone

1. When phenol reacts with CO₂ in the presence of a base then an electrophilic substitution reaction takes place. In this reaction, CO₂ acts as an electrophile. Phenol with CO₂ in the presence of base forms an intermediate, in the next step intermediate is acidified with acid and a salicylic acid or a Ortho hydroxybenzoic acid is obtained as a product. 

The reaction is known as Kolbe’s reaction. The reaction is as follows:

2. When phenol is treated with chloroform in the presence of sodium hydroxide, a –CHO group is introduced at ortho position of a benzene ring. This reaction is known as Reimer – Tiemann reaction. The intermediate substituted benzal chloride is hydrolyzed in the presence of alkali to produce salicylaldehyde. 

The reaction is as follows:

The mechanism is as follows:

3. When phenol is heated with zinc dust then benzene is obtained as a product. The reaction is as follows: 

4. Oxidation of phenol with chromic acid produces a conjugated diketone known as benzoquinone. The reaction is as follows:

The structure of phenol is as follows:

1. When aniline reacts with NaNO₂/HCl then diazonium salt is formed. When diazonium salt reacts with water then phenol is obtained as a product. 

2. Cumene (isopropylbenzene) is oxidized in the presence of air to cumene hydroperoxide. It is converted to phenol and acetone by treating it with dilute acid. Acetone, a by-product of this reaction, is also obtained in large quantities by this method. The reaction is as follows:

3. Chlorobenzene does not react with NaOH at STP condition. It reacts with NaOH at high pressure and temperature. Thus, in the third reaction phenol is not obtained as a product. 

4. Benzene is sulphonated with oleum and benzene sulphonic acid so formed is converted to sodium phenoxide on heating with molten sodium hydroxide. Acidification of the sodium salt gives phenol. The reaction is as follows:

Hence, option third is the correct answer. 
 

In substituted phenols, the presence of electron-withdrawing groups such as nitro group enhances the acidic strength of phenol. This effect is more pronounced when such a group is present at ortho and para positions.

It is due to the effective delocalization of negative charge in phenoxide ion when the substituent is at ortho or para position. 

The acidic strength of monosubstituted nitrophenol is higher than phenol because of the electron-withdrawing nitro group. Hence, the statement first is correct. 

The pK_a value of given acids are as follows:

o-nitrophenol = 7.2
m-nitrophenol = 8.3
p-nitrophenol = 7.1
phenol = 10.0

Hence, statement second is incorrect. 

The phthalein dye test is commonly used to identify the presence of certain functional groups in organic compounds. This test uses phthalein dyes, which are specific for certain groups, especially phenolic compounds.

Phthalein Dye Test:

• Phenolphthalein and other phthalein dyes are often used in acid-base titrations. They undergo color changes depending on the pH of the solution.
• The phthalein dye test is specifically used to detect phenolic compounds. In the test:
  • If a phenolic compound is present, the phthalein dye will turn a pink color in an alkaline medium. This is because the hydroxyl group (-OH) in phenolic compounds reacts with the dye, causing the color change.

Explanation of the Options:

1. Phenolic: Correct. Phthalein dye (especially phenolphthalein) is commonly used to identify phenolic compounds. The color change in the test indicates the presence of a phenolic hydroxyl group (-OH).
2. Alcohol: Incorrect. While alcohols also contain hydroxyl groups, phthalein dye is more specific for phenols, not alcohols. Alcohols typically do not cause the same color change with phthalein dyes.
3. Aldehyde: Incorrect. The phthalein dye test is not used to identify aldehydes. Aldehydes are typically detected using other reagents, like Tollens’ reagent or Fehling’s reagent.
4. Carboxylic acid: Incorrect. Carboxylic acids can be detected using other tests (e.g., reacting with sodium bicarbonate to release CO₂), but phthalein dyes are not specific for carboxylic acids.