Question

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

Short answer

Three different oximes are formed; none are optically active.

Step-by-step solution

Understand the smallest ketone

The smallest ketone is acetone, with the molecular formula \( ext{CH}_3 ext{COCH}_3\). It consists of two methyl groups bonded to a carbonyl group.

Identify the next homologue

The next homologue of acetone is butan-2-one (or methylethylketone) with the molecular formula \( ext{CH}_3 ext{COCH}_2 ext{CH}_3\). It consists of an ethyl group and a methyl group bonded to a carbonyl group.

Reaction with hydroxylamine

When acetone and butan-2-one are reacted with hydroxylamine \( ext{NH}_2 ext{OH}\), they each form oximes. The general reaction with a ketone \( ext{R}_2 ext{C=O}\) and \( ext{NH}_2 ext{OH}\) gives an oxime \( ext{R}_2 ext{C=NOH}\).

Oxime formation from acetone

Acetone reacts with \( ext{NH}_2 ext{OH}\) to form a single oxime, \( ext{CH}_3 ext{C=NOH} ext{CH}_3\), because there is no structural difference in the methyl groups.

Oxime formation from butan-2-one

Butan-2-one reacts with \( ext{NH}_2 ext{OH}\) to form two geometrical isomers of the oxime: \( ext{CH}_3 ext{C=NOH} ext{C}_2 ext{H}_5\) can exist as two different forms due to the different environments of the methyl and ethyl groups around the C=N bond. These are generally referred to as E and Z isomers.

Identify optically active oximes

For an oxime to be optically active, it must have a stereocenter or lack an internal plane of symmetry in a way that generates enantiomers. This is not the case for simple oximes like those formed from these ketones, as the stereoisomers are geometric (cis/trans or E/Z), not optical isomers.

Conclusion

In total, three different oximes are formed: one from acetone and two (E and Z isomers) from butan-2-one. All are geometric isomers, and none are optically active.

Key concepts

Geometric Isomerism

In organic chemistry, geometric isomerism is an essential concept that arises due to the restricted rotation around a double bond. This phenomenon is particularly observed in molecules containing a carbon-carbon double bond or a carbon-nitrogen double bond, as seen in oximes. Geometric isomers, often referred to as cis-trans or E-Z isomers, differ in their spatial arrangement.

For example, when butan-2-one reacts with hydroxylamine, it forms two geometric isomers due to the presence of different groups attached to the carbon-nitrogen double bond. These isomers are the E (trans) and Z (cis) configurations. Even though these isomers have the same connectivity of atoms, their different spatial arrangements result in varied physical and chemical properties.

Understanding geometric isomerism allows chemists to predict the behavior of molecules during reactions and the types of products that are likely to form. It's crucial in pharmaceutical industries where different isomers of a compound can have drastically different biological activities.

Acetone Reactivity

Acetone, the smallest ketone, is a highly reactive molecule due to the presence of the carbonyl group (C=O). This group is polar, with the oxygen atom being more electronegative than carbon, creating a dipole moment. This polarity makes acetone susceptible to nucleophilic attacks.

In the formation of oximes, acetone reacts with hydroxylamine (NH₂OH). The reaction involves the replacement of the oxygen atom in the carbonyl group with the oxime group (-C=NOH). Since acetone has two identical methyl groups attached to the carbonyl carbon, the resulting oxime forms without introducing any new structural diversity. This is why only a single oxime product is formed from acetone.

Acetone's reactivity isn't limited to forming oximes; it participates in various organic reactions, including aldol condensations and nucleophilic addition reactions. Its versatility makes acetone a valuable solvent and reactant in organic synthesis.

Butan-2-one Chemistry

Butan-2-one, also known as methyl ethyl ketone, displays interesting chemical behavior due to its structure. With an ethyl and a methyl group bonded to the carbonyl carbon, it introduces subtle asymmetry that plays a role in its reactivity and the potential for isomerism.

When butan-2-one reacts with hydroxylamine, the formation of oxime leads to two distinct geometric isomers: E and Z forms. This occurs because the methyl and ethyl groups provide different environments around the C=N bond, allowing for different spatial orientations. Understanding the chemistry of butan-2-one gives insight into how subtle structural differences can affect the number and type of products formed in reactions.

Butan-2-one is also widely used in industrial applications, serving as a solvent and an intermediate in chemical reactions due to its suitable volatility and reactivity profile.

Optical Activity in Organic Compounds

Optical activity is a property of compounds to rotate plane-polarized light, which occurs in chiral molecules that lack a plane of symmetry. For a compound to exhibit optical activity, it typically must have one or more chiral centers.

In the case of oxime formation from acetone and butan-2-one, none of the resulting compounds are optically active. This is because both oximes lack the necessary symmetry-breaking features required for optical activity. They exhibit geometric isomerism (cis/trans or E/Z) rather than optical isomerism (R/S configurations). Geometric isomers can exhibit different physical properties, but they do not rotate plane-polarized light.

Understanding optical activity is crucial in organic chemistry, especially in pharmaceutical applications where the optical isomer of a drug can significantly affect its efficacy and safety. However, in the exercise given, it is important to note that the oximes formed do not exhibit this property.