Question

\(\mathrm{Ph}-\mathrm{CHO}+\mathrm{NH}_{2} \mathrm{OH} \rightarrow(\mathrm{A})\) Which of the following is/are true about compound (A)? (a) The syn- (A) on treatment with \(\mathrm{PCl}_{5}\) in ether gives N- phenyl formamide. (b) The product (A) exists in 2 isomeric forms. (c) The suitable \(\mathrm{pH}\) for the reaction is between 4 to 6 (d) The anti- (A) on treatment with \(\mathrm{PCl}_{5}\) in ether followed by hydrolysis gives aniline and formic acid.

Short answer

Options (a), (b), (c), and (d) are all correct.

Step-by-step solution

Identify the Reaction

The reaction presented is the formation of an oxime. The reagent \(NH_2OH\) (hydroxylamine) will react with \(\text{Ph-CHO}\) (benzaldehyde) to form \(\text{Ph-CH=NOH}\), which is an oxime.

Understand Syn and Anti Forms

Oximes exist as two stereoisomers: syn and anti forms. This occurs due to the double bond between carbon and nitrogen, which creates a plane of symmetry.

Analysis of Option (a)

Consider the syn-oxime derivative of benzaldehyde. Upon treatment with \(\text{PCl}_5\), chlorination occurs, resulting in the formation of \(\text{N-phenylformamide}\) due to syn geometry aiding rearrangement where the \(\text{OH}\) group is positioned for easy substitution.

Deciding on Option (b)

Identify that oximes, due to their syn and anti configurations, exist in two isomeric forms. Thus, (A) indeed has two isomers.

Evaluate Option (c)

The condensation reaction forming an oxime is best at a slightly acidic pH of 4 to 6. This protonates the carbonyl group, making it more susceptible to a nucleophilic attack by hydroxylamine.

Examine Option (d)

For anti-oxime upon treatment with \(\text{PCl}_5\) followed by hydrolysis, it results in the formation of aniline and formic acid due to the rearrangement and subsequent hydrolysis reactions.

Key concepts

Oxime Formation

Oxime formation is a fundamental reaction in organic chemistry where a carbonyl group (like those in aldehydes or ketones) reacts with hydroxylamine ( NH_2OH) to form an oxime. This results in a compound with the structural feature C=N-OH. The reaction typically proceeds with aldehydes or ketones, where the carbonyl carbon is nucleophilically attacked by the NH_2 group of hydroxylamine. Then, the loss of a water molecule ( -H_2O) completes the process.
The significance of oxime formation lies in its role in structural determination and as intermediates in the synthesis of other compounds. Oximes are key in the Beckmann rearrangement, which can convert them into amides. Oxime formation is a reversible reaction under specific conditions, allowing chemists to work backwards from oximes to their precursor aldehydes or ketones for analytical purposes.

Stereoisomerism

Stereoisomerism is crucial in understanding oximes, where it leads to the presence of isomers: specifically, syn- and anti-forms. These isomers arise due to the restricted rotation around the C=N bond. The syn form is when the larger groups attached to the carbon and nitrogen are on the same side, and the anti form is when they're on opposite sides.
This stereochemistry is important because it affects the chemical reactivity and physical properties of the oximes. For example, syn- and anti- isomers might interact differently with other chemicals or catalysts, leading to divergent reaction pathways. Additionally, these forms can display different melting and boiling points, which is a crucial factor in purification processes.

Reactivity of Hydroxylamine

The reactivity of hydroxylamine is pivotal in its role in forming oximes. Hydroxylamine is a potent nucleophile due to its nitrogen atom with a lone pair of electrons. It actively seeks out electrophilic centers, like the carbonyl carbon in benzaldehyde (  ext{Ph-CHO}), to form oximes.
Hydroxylamine also displays versatility in its reactions. Beyond forming oximes, it can be used to synthesize nitrones, or as a reactant in the reduction of nitro compounds to amines. Its reactivity, however, demands caution. It can form explosive mixtures when heated or under certain conditions, necessitating careful handling and storage.

pH Conditions for Reactions

The pH conditions for oxime formation are instrumental in driving the reaction forward. The optimal pH range is between 4 to 6, slightly acidic, to facilitate the reaction.
At this pH, the carbonyl oxygen is protonated, enhancing its electrophilic character. This heightened electrophilicity makes the carbonyl carbon more susceptible to nucleophilic attack by hydroxylamine. If the pH is too low (highly acidic), the reaction might be less efficient due to excessive protonation interfering with hydroxylamine's nucleophilicity. Conversely, if the pH is too high (basic), hydroxylamine might not attack efficiently, as the electrophilic character of the carbonyl would not be appropriately enhanced. Thus, maintaining the pH within this range is critical to achieving a good yield in oxime syntheses.