Question

Eventhough a carboxylic acid contains \(-\mathrm{C}\) - group, it does not form oxime with hydroxyl amine because (a) hydroxyl amine is less reactive to carboxylic acids. (b) in this case hydroxyl amine acts as a reducing agent, oxidized to \(\mathrm{HNO}_{2}\). (c) the \(\sum \mathrm{C}=\mathrm{O}\) group enters into resonance with lone pairs of \(-\mathrm{OH}\) group. (d) the -OH group of carboxylic acid reacts with \(\mathrm{NH}_{2} \mathrm{OH}\).

Short answer

Based on the analysis of the given options, the reason why carboxylic acids do not form oximes with hydroxylamine is because the carbonyl group (\(\sum \mathrm{C}=\mathrm{O}\)) enters into resonance with the lone pairs of the hydroxyl group (\(-\mathrm{OH}\)). This resonance causes the carbonyl carbon to be less electrophilic, limiting the nucleophilic attack by the nitrogen atom of hydroxylamine (\(\mathrm{NH}_2\mathrm{OH}\)) and preventing the formation of an oxime. Option (c) is the correct answer.

Step-by-step solution

Understand the reaction to form oximes

When aldehydes or ketones react with hydroxylamine (\(\mathrm{NH}_2\mathrm{OH}\)), an oxime is formed. Typically, the reaction is driven by the nucleophilic attack on the carbonyl \(\sum \mathrm{C}=\mathrm{O}\) group by the nitrogen atom of hydroxylamine. The oxygen atom of the \(\sum \mathrm{C}=\mathrm{O}\) is then protonated and replaced with the nitrogen atom.

Examine the structure of carboxylic acid

A carboxylic acid has a structure \(\mathrm{R-COOH}\), which is an \(\mathrm{R-C}\) bond with a carbonyl group (\(\sum \mathrm{C}=\mathrm{O}\)) and a hydroxyl group (\(-\mathrm{OH}\)).

Analyze the given answers

(a) Although hydroxylamine can be less reactive to carboxylic acids, it is not the main reason why it does not form oximes with carboxylic acids. (b) In this case, hydroxylamine does not act as a reducing agent and is not oxidized to \(\mathrm{HNO}_2\) during the reaction. (c) Option (c) suggests that the carbonyl group \(\sum \mathrm{C}=\mathrm{O}\) enters into resonance with the lone pair of electrons of the hydroxyl group (\(-\mathrm{OH}\)). Due to resonance, the delocalization of electrons causes the carbonyl carbon to be less electrophilic, so the nucleophilic attack by the nitrogen atom of hydroxylamine (\(\mathrm{NH}_2\mathrm{OH}\)) is less likely to occur, thus not forming an oxime. (d) The -OH group of the carboxylic acid does not react with \(\mathrm{NH}_2\mathrm{OH}\) in this case.

Identify the correct answer

Based on the analysis, the correct answer is (c): the \(\sum \mathrm{C}=\mathrm{O}\) group enters into resonance with the lone pairs of the \(-\mathrm{OH}\) group. This resonance makes the carbonyl group less electrophilic and less reactive for nucleophilic attack by hydroxylamine, resulting in no oxime formation.

Key concepts

Oximes

Oximes are chemical compounds formed when aldehydes or ketones react with hydroxylamine ( NH_2OH ). This reaction converts the carbonyl group ( C=O ) of an aldehyde or ketone into an oxime group ( C=N-OH ).
The process is facilitated by the nucleophilic attack of the nitrogen atom on the carbonyl carbon. Once the nitrogen binds to the carbon, the oxygen of the carbonyl is replaced by the nitrogen of hydroxylamine, creating the oxime structure.
Oximes are useful in various chemical synthesis processes and can often be used as intermediates in the preparation of other chemical compounds. However, it is important to note that not all carbonyl-bearing compounds can form oximes. Carboxylic acids, for example, do not form oximes with hydroxylamine due to their specific structural properties which we will explore further.

Resonance

Resonance is a concept in chemistry where certain molecules can be represented by more than one Lewis structure. This occurs due to the delocalization of electrons, often in pi bonds or lone pairs, across different atoms within a molecule. In the case of carboxylic acids, the resonance involves the carbonyl group and the adjacent hydroxyl group.
For carboxylic acids, resonance plays a crucial role as it stabilizes the molecule by allowing electrons to be distributed over different atoms. The resonance in carboxylic acids makes the carbonyl carbon less electrophilic than in aldehydes and ketones, because the resonance leads to electron delocalization which reduces the positive charge on the carbonyl carbon.
This delocalization affects reactions with nucleophiles like hydroxylamine. As the carbonyl carbon becomes less electrophilic, it is less susceptible to nucleophilic attack. Consequently, this is a key reason why carboxylic acids do not form oximes with hydroxylamine, unlike aldehydes and ketones.

Nucleophilic Attack

The process of nucleophilic attack is pivotal in many organic reactions. A nucleophile is an electron-rich species that targets electron-deficient areas in other molecules to form a chemical bond. In the reaction to form oximes, the nucleophilic attack occurs when the nitrogen atom of hydroxylamine targets the carbonyl carbon of an aldehyde or ketone.
This attack is driven by the attraction between the electron-rich nitrogen and the electron-deficient carbonyl carbon. However, not all carbonyl compounds allow such an interaction. Carboxylic acids, due to their resonance-stabilized structure, have a less electrophilic carbonyl carbon that makes it difficult for the nitrogen in hydroxylamine to initiate a nucleophilic attack.
As such, the chemistry of nucleophilic attack explains why oximes form with aldehydes and ketones but not with carboxylic acids, emphasizing the importance of molecular structure in chemical reactivity.

Hydroxylamine Reaction

The reaction between hydroxylamine ( NH_2OH ) and carbonyl-containing compounds demonstrates the specificity of chemical reactivity. Hydroxylamine is generally used to convert aldehydes and ketones into oximes through nucleophilic attack. However, its interaction with carboxylic acids reveals much about chemical structure dependencies.
Hydroxylamine is a versatile reagent and its chemistry with aldehydes and ketones generally follows a predictable pattern leading to oxime formation. This reaction involves the replacement of the carbonyl oxygen with the nitrogen from hydroxylamine.
In the context of carboxylic acids, however, the hydroxylamine reaction does not proceed to form oximes. The resonance in carboxylic acids alters the reactivity of the carbonyl group, preventing the nucleophilic attack necessary for oxime formation. This highlights that, while hydroxylamine can act with different carbonyl groups, the presence of a stable resonance structure in carboxylic acids inhibits the usual reaction pathway seen with aldehydes and ketones.