Problem 24 - Chemistry in 30 days

Question

The smallest ketone and its next homologue are reacted with \(\mathrm{NH}_{2} \mathrm{OH}\) to form oxime: (a) two different oximes are formed (b) three different oximes are formed (c) two oximes are optically active (d) all oximes are optically active

Step-by-Step Solution

Answer

(b) three different oximes are formed.

Step 1: Identify the Smallest Ketone

The smallest ketone is propanone (acetone), which has the formula \( \mathrm{CH}_3\mathrm{COCH}_3 \).

Step 2: Find the Next Homologue

The next homologue of propanone is butanone, which has the formula \( \mathrm{C}_3\mathrm{H}_7\mathrm{COCH}_3 \).

Step 3: Understand Oxime Formation

Oximes are formed when a ketone reacts with hydroxylamine (\( \mathrm{NH}_2\mathrm{OH} \)). The general reaction is: \[ \text{R}_2\mathrm{C}=\mathrm{O} + \mathrm{NH}_2\mathrm{OH} \rightarrow \text{R}_2\mathrm{C}=\mathrm{NOH}. \] This reaction replaces the ketone's oxygen with NOH.

Step 4: Analyze Propanone Oxime Formation

Propanone forms a single symmetrical oxime due to its structure \( \mathrm{(CH}_3)_2\mathrm{C}=\mathrm{NOH} \). The symmetry means only one product is possible.

Step 5: Analyze Butanone Oxime Formation

Butanone forms two different oximes because it is unsymmetrical, resulting in \( \mathrm{CH}_3\mathrm{CH}_2\mathrm{C}=\mathrm{NOH}\mathrm{CH}_3 \) and \( \mathrm{CH}_3\mathrm{C}=\mathrm{NOH}\mathrm{CH}_2\mathrm{CH}_3 \).

Step 6: Consider Optical Activity

The oxime from butanone can exist in two different geometric isomers (syn and anti forms), but neither oxime can be optically active as there is no chirality center.

Step 7: Conclusion: Determine the Number of Oximes

Combining the results, three different oximes are formed in total: one from propanone and two from butanone.

Key Concepts

Ketones

Ketones are a class of organic compounds characterized by the presence of a carbonyl group (C=O), where the carbon atom is bonded to two other carbon atoms. They are a significant functional group in organic chemistry and are usually represented in the general form as

• R₂C=O

where R is a hydrocarbon chain or group.
Propanone, also known as acetone, is the simplest ketone with the chemical formula

• CH₃C(=O)CH₃

It is noteworthy for its use as a solvent and in organic synthesis.
The next molecule in the homologous series of ketones is butanone,

• CH₃C(=O)CH₂CH₃

By adding an additional carbon atom to the chain, it differs from acetone, showcasing the nature of homologous series in organic chemistry.

Hydroxylamine Reaction

The reaction between ketones and hydroxylamine is a fascinating transformation in organic chemistry known primarily for forming oximes. When ketones react with hydroxylamine (

• NH₂OH

their carbonyl group (C=O) undergoes modification.
This reaction converts the ketone into an oxime with the general formula

• R₂C=NOH

by replacing the oxygen in the carbonyl group with the NOH group.
In this context, propanone forms a symmetrical oxime because of its identical R groups on either side of the carbonyl, allowing only one unique oxime product.
Conversely, butanone, with its asymmetrical structure, generates two distinct oxime structures, exemplifying the variability of products depending on the initial ketone.

Geometric Isomers

In the realm of chemistry, particularly with oximes, geometric isomers come into play. These isomers occur due to restricted rotation around the double bonded atoms,

• R₂C=NOH

In oxime formation, the position of the substituents relative to the double bond defines the syn and anti isomers.
For butanone oxime, the potential for two geometric isomers arises. Syn and anti denote the differing spatial arrangement of the substituents across the C=N bond:

• Syn refers to the substituents being on the same side.
• Anti indicates that they are on opposite sides.

However, in the context of oximes formed from butanone and propanone, no optical activity is displayed because they lack a chiral center.

Homologous Series

A homologous series is a group of organic compounds sharing a common functional group and a similar structure, differing mainly by a consistent unit such as

• CH₂

This concept is essential in understanding the progression of organic compounds.
Each successive member in the series differs by this repeating unit, which imparts predictability in chemical behavior and properties.
For instance, the ketone series starting with acetone (propanone) progresses to butanone, with each member contributing to the understanding of their chemical reactivity and physicochemical properties.
As seen in the oximes formation exercise, understanding the homologous nature of these compounds aids in predicting the variation in reaction products, especially as new carbon units influence the structure and symmetry of the resultant molecules.