Problem 206 - 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) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{OH}\) (c) CC(C)CO (d) \(\mathrm{PhCHOHCH}_{3}\)

Step-by-Step Solution

Answer

Option (c) gives a positive iodoform test because it contains isopropanol, which has the required -CH(OH)CH₃ group.

Step 1: Understanding the Iodoform Test

The iodoform test is used to identify methyl ketones or alcohols with the structure \(-CH(OH)CH_3\). The test is positive if the compound contains the \(C-CH_3\) group, which can be oxidized to a ketone or aldehyde, that then reacts with \(I_2\) and \(NaOH\) to form a yellow precipitate of iodoform \((CHI_3)\).

Step 2: Analyzing Option (a)

Option (a), \(\mathrm{CH}_3 \mathrm{CH}_2 \mathrm{CH}(\mathrm{OH}) \mathrm{CH}_2 \mathrm{CH}_3\), does not have a \(-CH(OH)CH_3\) group or a \(C-CH_3\) group adjacent to a carbonyl, so it will not give a positive iodoform test.

Step 3: Analyzing Option (b)

Option (b), \(\mathrm{C}_6 \mathrm{H}_5 \mathrm{CH}_2 \mathrm{CH}_2 \mathrm{OH}\), follows a primary alcohol structure. This molecule lacks a methyl group adjacent to a carbonyl group or the specific alcohol structure required for a positive iodoform test.

Step 4: Analyzing Option (c)

Option (c), represented by CC(C)CO, translates to \(\mathrm{CH}_3 \mathrm{C(OH)CH}_3\) or isopropanol. This secondary alcohol has a \(-CH(OH)CH_3\) group, which can give a positive iodoform test as it can be oxidized to acetone \((\mathrm{CH}_3 \mathrm{COCH}_3)\), a methyl ketone.

Step 5: Analyzing Option (d)

Option (d), \(\mathrm{PhCHOHCH}_3\), contains a \(-CH(OH)CH_3\) group adjacent to a phenyl group. This can potentially produce a positive iodoform test.

Step 6: Decision

Between options (c) and (d), option (c) clearly fits the criteria for a positive iodoform test due to isopropanol’s structure \(CH_3CH(OH)CH_3\), whereas option (d) primarily forms non-methyl ketones upon oxidation. Therefore, the correct answer is option (c).

Key Concepts

Methyl Ketones

Methyl ketones are organic compounds that feature the structural designation –COCH eolithic mousse_3. These structures are notable for their reactivity in the iodoform test, where they give a positive result. The characteristic methyl group ( CH _3) is located next to the carbonyl group ( C=O), which is critical for this chemical behavior.

• The iodoform test hinges on the presence of this methyl carbonyl formation.
• When subjected to iodine ( I _2) and a base, usually sodium hydroxide ( NaOH), methyl ketones generate a yellow iodoform precipitate ( CHI _3).
• This reaction provides an effective diagnostic method to identify such compounds in organic chemistry.

Thus, methyl ketones are easily recognized by their ability to form a specific yellow compound, making them unique among organic groups.

Secondary Alcohols

Secondary alcohols contain an OH group that is bonded to a carbon atom, which is itself connected to two other carbon atoms. An example of a secondary alcohol is isopropanol, also known as CH _3 CH(OH) CH _3. These substances have distinctive properties related to their oxidation:

• They can be oxidized to produce ketones, which then enable them to undergo further reactions.
• For the iodoform test, it is crucial that the resulting ketone possesses a methyl group next to the carbonyl. This configuration permits the formation of iodoform.

A classic case is isopropanol. Upon oxidation, it forms acetone. Acetone reacts in the iodoform test, providing a clear indication of secondary alcohol presence. Thus, recognizing secondary alcohols and understanding their transformation into ketones is key in identifying them via specific reactions.

Organic Chemistry Reactions

Organic chemistry encompasses reactions involving the rearrangement, creation, or breaking of bonds between carbon-containing compounds. Reactions range from simple substitution and addition to complex rearrangements and oxidation-reduction processes. Each reaction follows a set of principles developed to elucidate mechanisms, predict outcomes, and comprehend the transformation of molecules:

• The iodoform reaction targets specific structural features in organic molecules, such as methyl ketones.
• Understanding the structure and reactivity of compounds allows chemists to infer the potential pathways during reactions.
• This knowledge is powerful in practical applications such as drug design, synthesis of materials, and analytical chemistry.

By mastering the nuances of organic chemistry reactions, one gains insights into how molecular transformations occur, paving the way for practical advancements and innovation.