Problem 102 - A Complete Resource Book in Chemistry for JEE Main

Question

An alcohol and a phenol can be distinguished by their reactions with (a) Ceric ammonium nitrate (b) \(\mathrm{NaOH}\) (c) Benezene diazonium chloride (d) \(\mathrm{NaHCO}_{3}\)

Step-by-Step Solution

Answer

The correct options are (a) Ceric ammonium nitrate and (b) NaOH.

Step 1: Understanding Alcohols and Phenols

Alcohols and phenols are both organic compounds that contain hydroxyl groups (-OH). However, they differ in their reactivity and acidic nature. Alcohols are generally less acidic than phenols.

Step 2: Reaction with Ceric Ammonium Nitrate

Ceric ammonium nitrate reacts with alcohols to give a red color, whereas phenols do not react with ceric ammonium nitrate under normal conditions. This can be used to distinguish between the two.

Step 3: Reaction with NaOH

Phenols react with sodium hydroxide ( ext{NaOH} ) to form phenoxide ions due to their acidic nature, resulting in a salt and water. Alcohols generally do not react with NaOH in this manner because they are not acidic enough.

Step 4: Reaction with Benzene Diazonium Chloride

Phenols can react with benzene diazonium chloride to form colored azo compounds. Alcohols generally do not participate in this reaction.

Step 5: Reaction with NaHCO3

Phenols do not react with sodium bicarbonate ( ext{NaHCO}_3) because they are not acidic enough to release  ext{CO}_2 gas, unlike stronger acids such as carboxylic acids. Alcohols also do not react with NaHCO3.

Key Concepts

Ceric Ammonium Nitrate Reaction

The Ceric Ammonium Nitrate (CAN) test is a simple, yet effective method for distinguishing alcohols from phenols. This test relies on the reaction between ceric ammonium nitrate and an alcohol, where they react to produce a distinct red colored solution. This color change occurs because the CAN complex reacts with the alcohol to form a cerium(IV) alcohol complex, which has a characteristic red hue.

This reaction serves as a quick visual confirmation that an alcohol is present. Conversely, phenols typically do not react with ceric ammonium nitrate under standard conditions, making this a reliable test to differentiate these two classes of compounds.

• Alcohols react with CAN to form a red complex.
• Phenols do not change color with CAN.
• This test is useful due to its simplicity and visual nature.

Phenol Reactivity with NaOH

Phenols exhibit a unique reactivity with sodium hydroxide (NaOH) thanks to their slightly acidic character. Phenols are more acidic than alcohols due to the stabilization of their phenoxide ion through resonance. When phenols react with NaOH, they form phenoxide ions along with water, turning into a more ionic form which can easily dissolve in a polar solvent. This is because the hydrogen ion from the hydroxyl group (OH) is replaced, forming water and a phenoxide salt.

In contrast, alcohols generally do not react with NaOH because they are not acidic enough to undergo such a transformation. This characteristic interaction with NaOH distinguishes phenols from alcohols effectively:

• Phenols react with NaOH to form water and phenoxide salts.
• Alcohols do not exhibit a similar reaction with NaOH due to their lack of acidity.
• This reactivity allows for straightforward identification of phenolic compounds.

Benzene Diazonium Chloride Test

The Benzene Diazonium Chloride test is a fascinating chemical reaction that phenols can undergo, showcased by their ability to react with benzene diazonium chloride to form azo compounds. This often results in a colorful transformation because azo compounds are very strongly colored.

Typically, when a phenol is treated with benzene diazonium chloride, it results in the formation of an azo dye, often showing bright red, orange, or yellow hues, depending on specific conditions and the substituents involved in the phenol. This reaction is characteristic of phenols and does not occur with alcohols due to their different chemical structures.

• Phenols react to form colored azo compounds.
• Azo dyes often appear as bright colors like red or orange.
• Alcohols do not form azo compounds with benzene diazonium chloride, making this a selective test for phenols.