Problem 175 - A Complete Resource Book in Chemistry for JEE Main

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- \((\mathrm{A})\) 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.

Step-by-Step Solution

Answer

Statements (b), (c), and (d) are true for compound (A).

Step 1: Reaction Mechanism Identification

The given reaction involves the reaction of benzaldehyde (\(\mathrm{Ph}-\mathrm{CHO}\)) with hydroxylamine (\(\mathrm{NH}_2\mathrm{OH}\)) to form an oxime, which is compound (A). This reaction is a typical condensation reaction that leads to the formation of an oxime, specifically benzaldoxime (\(\mathrm{Ph{-}CH{=}NOH}\)).

Step 2: Understand Isomer Forms of Compound (A)

Oximes can exist in two isomeric forms due to syn and anti configurations around the C=N bond. Therefore, the statement that compound (A) exists in two isomeric forms is true.

Step 3: Assess Reactivity with PCl5

Treating an oxime such as benzaldoxime with \(\mathrm{PCl}_5\) in ether leads to Beckmann rearrangement, which would typically produce N-substituted amides. In the case of the syn isomer, N-phenyl formamide is produced.

Step 4: Understand Suitable pH for Syn/Anti Oxime Formation

The optimal pH for oxime formation is generally in the range of 4 to 6, where the reaction proceeds most efficiently. This means that the condition given is also true.

Step 5: Analyze Anti Isomer Reaction

Upon treatment of the anti isomer with \(\mathrm{PCl}_5\) and subsequent hydrolysis, the product yields may include decomposition to aniline and formic acid due to the breakdown of the cyclic intermediates formed during the reaction.

Key Concepts

Condensation reaction

A condensation reaction is a chemical process where two molecules combine to form a larger molecule, accompanied by the loss of a small molecule such as water. It's a key mechanism in organic chemistry for forming new bonds.
For the specific reaction involving benzaldehyde (Ph-CHO) and hydroxylamine (NH_2 ext{OH}), this reaction leads to the formation of benzaldoxime (Ph-CH=NOH). During this process, a water molecule is eliminated and a double bond forms between carbon and nitrogen.
This type of reaction is significant because it is used to transform aldehydes and ketones into oximes, which are intermediates in various chemical syntheses.

Oxime isomerism

Oximes can exhibit isomerism due to the configurations around their C=N (carbon-nitrogen double bond). The two geometric isomers are known as syn and anti isomers.
The syn isomer has the hydroxyl group adjacent to the neighboring group on the opposite side of the C=N bond, while the anti isomer positions the groups on opposite sides. This is similar to the familiar concept of cis-trans isomerism seen in such bonds.
These isomers are important in chemical reactions as they can display different reactivities and form different products in further transformations.

Beckmann rearrangement

The Beckmann rearrangement is a chemical reaction commonly applied to oximes, converting them into amides when treated with a reagent like phosphorus pentachloride (PCl_5).
For benzaldoxime, when the syn-isomer undergoes this rearrangement, N-phenyl formamide is a typical product. This transformation is instrumental in making a variety of amides from less complex structures.
The rearrangement proceeds via an initial activation of the oxime by PCl_5, forming a cyclic intermediate, which then rearranges to yield the amide.

Reaction pH range

The pH of the reaction medium can significantly affect the formation of oximes. For the reaction between benzaldehyde and hydroxylamine forming benzaldoxime, the optimal pH is typically between 4 and 6.
This slightly acidic environment is crucial as it facilitates the removal of water, promoting the condensation process. The reaction pH influences the equilibrium positions and ensures the maximum yield of the desired product.
It's a practical consideration buried in the subtle pH-dependent manipulation of reaction conditions in synthetic organic chemistry.

Hydrolysis of anti isomer

When the anti isomer of benzaldoxime is treated with PCl_5 and subsequently hydrolyzed, it can lead to the formation of aniline and formic acid. This follows the breakdown of cyclic intermediates formed during the rearrangement.
Unlike the syn isomer, which generally forms amides, the anti isomer tends to undergo further decomposition leading to these simpler compounds.
Understanding the pathway and ultimate decomposition products of the anti isomer is useful in planning synthetic routes and predicting possible side reactions in chemical processing.