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

Question

Among the following the one that gives positive iodoform test upon reaction with \(\mathrm{I}_{2}\) and \(\mathrm{NaOH}\) is [2006] (a) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}(\mathrm{OH}) \mathrm{CH}_{2} \mathrm{CH}_{3}\) (b) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{OH}\) (d) \(\mathrm{PhCHOHCH}_{3}\)

Step-by-Step Solution

Answer

Compound (d) \(\mathrm{PhCHOHCH}_3\) gives a positive iodoform test.

Step 1: Understand the Iodoform Test

The iodoform test is a chemical reaction useful for detecting methyl ketones or alcohols where the hydroxyl group is adjacent to a methyl group. Compounds containing the \(-\text{COCH}_3\) or \(-\text{CHOHCH}_3\) group will give a positive iodoform test, resulting in a yellow precipitate of iodoform.

Step 2: Analyze Compound (a)

Compound (a) is \(\mathrm{CH}_3\mathrm{CH}_2\mathrm{CH}( ext{OH})\mathrm{CH}_2\mathrm{CH}_3\). This is a secondary alcohol, but it doesn't contain a methyl group adjacent to the hydroxyl group. Therefore, it will not give a positive iodoform test.

Step 3: Analyze Compound (b)

Compound (b) is \(\mathrm{C}_6\mathrm{H}_5\mathrm{CH}_2\mathrm{CH}_2\mathrm{OH}\). In this structure, the hydroxyl group is not adjacent to a methyl group, thus it doesn't meet the criteria for a positive iodoform test.

Step 4: Analyze Compound (d)

Compound (d) is \(\mathrm{PhCHOHCH}_3\), which is a secondary alcohol with the structure \(\mathrm{C}_6\mathrm{H}_5\mathrm{CHOHCH}_3\). This structure has a \(- ext{CHOHCH}_3\) group, which is known to give a positive iodoform test.

Step 5: Conclusion

Based on the analysis of each compound, only compound (d), \(\mathrm{PhCHOHCH}_3\), has the functional group arrangement required for a positive iodoform test. Thus, it will produce a yellow precipitate of iodoform when reacted with \(\mathrm{I}_2\) and \(\mathrm{NaOH}\).

Key Concepts

Methyl Ketones

Methyl ketones are a specific type of organic compound characterized by the structure R-CO-CH3, where R can be any hydrocarbon group. These compounds are a key focus in organic chemistry because of their unique reactivity, particularly in the iodoform test.
The methyl group ( (-CH3) ) connected to a carbonyl group ( (-CO-) ) is what makes these molecules react under certain conditions, such as with iodine ( (I_2) ) and sodium hydroxide ( (NaOH) ).

• Methyl ketones give a positive iodoform test, resulting in the formation of a yellow compound known as iodoform ( (CHI_3) ).
• The presence of the (-COCH_3) group allows a visible chemical reaction, primarily because the iodination and subsequent hydrolysis occur here.
• This test is utilized not just in educational settings but also in organic synthesis labs to verify the presence of specific methyl ketones.

Understanding the behavior of methyl ketones is crucial for both students and professionals who interpret organic reactions. They are not only common in laboratory studies but also important in various industrial applications.

Chemical Reaction Analysis

Chemical reaction analysis involves understanding and interpreting chemical reactions by identifying reactants, conditions, and products. The iodoform test is a great example for this kind of analysis.
In the iodoform reaction:

• Iodine ( (I_2) ) reacts with a methyl ketone in the presence of a strong base such as sodium hydroxide ( (NaOH) ).
• The process involves halogenation, where iodine replaces the hydrogen atoms of the methyl group, followed by hydrolysis, resulting in the formation of iodoform ( (CHI_3) ) and a carboxylate ion.
• The reaction is observable due to the formation of the yellow iodoform precipitate, which is the basis for the test's usage in detecting methyl ketones and certain types of alcohols.

This type of analysis allows chemists to predict the outcomes of reactions and determine the presence of specific functional groups. Learning how this test works offers a solid grounding in interpreting complex organic reactions and understanding how molecule structures impact reactivity.

Functional Groups Detection

Functional groups are specific groups of atoms within molecules responsible for the chemical reactions characteristic of those molecules. Detecting them is crucial in organic chemistry to understand and manipulate the reactivity of compounds.
For example, the iodoform test specifically detects:

• Methyl ketones, containing the (-COCH_3) functional group.
• Certain alcohols with the structure (-CHOHCH_3) , where the hydroxyl group is adjacent to a methyl group.
• These structural requirements are essential for testing because the reaction will only yield a positive result if these groups are present and accessible for the halogenation process.

Recognizing these functional groups helps identify substances capable of forming iodoform, contributing to both academic understanding and practical applications in organic synthesis and identification solutions. This detection process is part of the broader framework of learning to identify the behavior and characteristics of organic compounds based on their structures and functional groups.