Problem 159 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following statements regarding chemical properties of acetophenone are wrong? I. it is reduced to methylphenylcarbinol by sodium and ethanol II. it is oxidized to benzoic acid with acidified \(\mathrm{KMnO}_{4}\) III. it does not undergo electrophillic substitution like nitration at meta position. IV. it does not undergo iodoform reaction with iodine and alkali. (a) I and II (b) II and IV (c) I and III (d) III and IV

Step-by-Step Solution

Answer

Option (d) III and IV are wrong.

Step 1: Analyze Statement I

Statement I says acetophenone is reduced to methylphenylcarbinol by sodium and ethanol. In presence of sodium and ethanol, acetophenone undergoes reduction to form the alcohol methylphenylcarbinol. Therefore, statement I is correct.

Step 2: Analyze Statement II

Statement II claims acetophenone is oxidized to benzoic acid with acidified \(\mathrm{KMnO}_{4}\). Acetophenone typically oxidizes to form benzoic acid upon reaction with acidified \(\mathrm{KMnO}_{4}\), making statement II correct.

Step 3: Analyze Statement III

Statement III suggests acetophenone does not undergo electrophilic substitution like nitration at the meta position. Due to the electron-withdrawing nature of the carbonyl group in acetophenone, it directs electrophilic substitution reactions to the meta position. Hence, statement III is incorrect.

Step 4: Analyze Statement IV

Statement IV mentions acetophenone does not undergo the iodoform reaction with iodine and alkali. The iodoform reaction is a test for methyl ketones (like acetophenone) or alcohols with the \(\text{-OH}\) group adjacent to a methyl group, therefore, acetophenone does undergo this reaction. Statement IV is incorrect.

Step 5: Conclude Wrong Statements

Based on the analysis above, the wrong statements are III and IV.

Key Concepts

Reduction Reactions

Reduction reactions in organic chemistry often involve the addition of hydrogen to a compound or the removal of oxygen. In the case of acetophenone, reduction can be achieved using common reducing agents like sodium and ethanol. This process converts the carbonyl group (C=O) in acetophenone into an alcohol group (C-OH), resulting in methylphenylcarbinol.
A reduction reaction like this is characterized by:

• The use of a reducing agent, such as sodium or ethanol.
• A change in the oxidation state of carbon within the molecule.

Overall, the reduction of ketones like acetophenone results in the formation of secondary alcohols. It's a fundamental reaction in organic synthesis, allowing for the transformation of functional groups.

Oxidation Reactions

Oxidation reactions involve the addition of oxygen or the removal of hydrogen from a compound, often resulting in an increase in the oxidation state of the molecule. For acetophenone, an oxidative agent like acidified potassium permanganate (\(\mathrm{KMnO}_{4}\)) can be used.
In this specific reaction with acetophenone:

• The reagent \\(\mathrm{KMnO}_{4}\) acts as an oxidizing agent.
• The transformation involves converting acetophenone into benzoic acid.
• The carbonyl carbon of acetophenone gains an oxygen to form -COOH group, characteristic of carboxylic acids.

This reaction exemplifies the oxidative process where the conversion of a ketone to a carboxylic acid occurs through the breaking of a C-H bond and the formation of a C-O bond.

Electrophilic Substitution

Electrophilic substitution reactions are a key feature of aromatic chemistry, involving the replacement of a hydrogen atom in an aromatic cycle by an electrophile. Acetophenone, with its carbonyl group, behaves uniquely in these reactions.
Key points about electrophilic substitution in acetophenone:

• The presence of the electron-withdrawing carbonyl group directs substitutions to the meta position. This is contrary to certain other substituents that might direct ortho or para.
• The meta-directing nature of acetophenone makes nitration and similar reactions occur at the meta position.

Understanding how functional groups affect the orientation of electrophilic substitution is critical in predicting the behavior of aromatics in various chemical reactions.

Iodoform Reaction

The iodoform reaction is a traditional chemical test to identify methyl ketones and secondary alcohols with a methyl group in the alpha position. For acetophenone, this is applicable because it possesses a methyl ketone structure.
The iodoform test can be summarized by:

• The reaction involves iodine and an alkali, typically sodium hydroxide.
• The presence of a methyl carbonyl group such as in acetophenone will undergo halogenation followed by hydrolysis to form iodoform (CHI3), a yellow precipitate.

This reaction is a classic example of compound identification, providing clear evidence of methyl groups next to ketones, and showcases practical organic chemistry applications.