Problem 5 - Advanced Problems in Organic Chemistry for Competitive Examinations

Question

Which of the following pairs will form the same product with phenyl hydrazine/H (a) \(\mathrm{CH}_{2} \mathrm{OH}-\mathrm{CO}-\mathrm{CH}_{3}\) and \(\mathrm{CHO}-\mathrm{CHOH}-\mathrm{CH}_{3}\) (b) Glucose and mannose (c) Glucose and fructose (d) Glucose and sucrose

Step-by-Step Solution

Answer

Option (a) will form the same product with phenylhydrazine.

Step 1: Understand the Reaction

Phenylhydrazine reacts with carbonyl-containing compounds, such as aldehydes and ketones, to form phenylhydrazones. The reaction specifically targets the carbonyl group (C=O). Different carbonyl compounds can form the same phenylhydrazone if they are interconvertible enol-keto structures or have similar mechanisms.

Step 2: Analyze Option (a)

For option (a), - \(\mathrm{CH}_2 \mathrm{OH}-\mathrm{CO}-\mathrm{CH}_3\) is a type of ketone. - \(\mathrm{CHO}-\mathrm{CHOH}-\mathrm{CH}_3\) is an aldehyde. Both compounds can tautomerize to form the same enol structure, meaning they will form the same phenylhydrazone.

Step 3: Analyze Option (b)

Glucose and mannose are both aldohexoses differing only in the configuration at the C-2 position. They are epimers, which means they have different configurations around one specific carbon atom, but this slight structural change will lead to different phenylhydrazones.

Step 4: Analyze Option (c)

Glucose is an aldohexose, while fructose is a ketohexose. Although they have the same chemical formula, their differing structures mean they will produce different phenylhydrazones products when reacted with phenylhydrazine.

Step 5: Analyze Option (d)

Glucose is a simple sugar, while sucrose is a disaccharide made from glucose and fructose. Being entirely different in structure, glucose and sucrose will form different phenylhydrazones.

Step 6: Conclusion

From the analysis above, option (a) (\(\mathrm{CH}_2 \mathrm{OH}-\mathrm{CO}-\mathrm{CH}_3\) and \(\mathrm{CHO}-\mathrm{CHOH}-\mathrm{CH}_3\)) are interconvertible in their enol-keto tautomeric forms and thus will form the same product with phenylhydrazine.

Key Concepts

Phenylhydrazine Reaction

The phenylhydrazine reaction is a significant process in organic chemistry, particularly when dealing with carbonyl-containing compounds such as aldehydes and ketones. This reaction involves phenylhydrazine reacting with the carbonyl group (C=O) to form a compound known as phenylhydrazone. This is instrumental in identifying or characterizing carbonyl compounds.
This reaction is particularly useful because it identifies the presence of a carbonyl group, forming a solid hydrazone that can be easily isolated. The properties of the resulting phenylhydrazone can be utilized to determine the structure or identity of unknown carbonyl compounds, as different carbonyl compounds may produce similar phenylhydrazones under certain conditions, like enol-keto tautomerism.

Enol-Keto Tautomerism

Enol-keto tautomerism is a fascinating concept where compounds exist in two forms simultaneously – an enol form (containing an -OH group bonded to a C=C) and a keto form (containing a C=O group). These two forms are easily interconvertible.

In the context of phenylhydrazine reactions, enol-keto tautomerism is significant because it can lead to the same phenylhydrazone product. For instance, some carbonyl compounds may tautomerize to the same enol state, allowing them to react in an identical fashion with phenylhydrazine. This underscores the flexibility and reactivity of carbonyl compounds, as their tautomeric forms can often predict similar reaction pathways.

Carbonyl Compounds

Carbonyl compounds are a broad class of organic compounds featuring a carbon atom double-bonded to an oxygen atom (C=O). This functional group is key in reactions such as the phenylhydrazine addition reaction.

Carbonyl compounds are divided into two main types: aldehydes and ketones, based on the location of the carbonyl group. Aldehydes have the carbonyl group at the end of the carbon skeleton, while ketones have it within the chain.

• Aldehydes: An example is \( \mathrm{CHO}-\mathrm{CHOH}-\mathrm{CH}_{3} \).
• Ketones: An example is \( \mathrm{CH}_{2} \mathrm{OH}-\mathrm{CO}-\mathrm{CH}_{3} \).

These compounds are versatile in organic synthesis due to their ability to participate in a variety of chemical reactions, facilitating the formation of enols or reacting to form hydrazones as seen in the phenylhydrazine reaction.

Aldoses and Ketoses

Aldoses and ketoses are terms used to categorize simple sugars based on the carbonyl group they contain. These sugars are vital in biology and chemistry, especially when examining reactions with phenylhydrazine.

Aldoses contain an aldehyde group, and common examples include glucose and mannose. Ketoses, on the other hand, have a ketone group; fructose is a familiar ketose.

• Glucose and mannose: Both are aldohexoses and differ only at the C-2 position, making them epimers. Despite this similarity, they form different phenylhydrazone products due to slight structural differences.
• Glucose and fructose: These sugars share the same chemical formula, yet their structural variations (aldose vs. ketose) result in different products when reacted with phenylhydrazine.

Understanding the difference between aldoses and ketoses is crucial in predicting and analyzing their reactions, particularly when comparing the phenylhydrazine products they form.