Problem 44 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following will not give an alkene when treated with \(\mathrm{Ph}_{3} \mathrm{P}=\mathrm{CH}_{2} ?\) (a) \(\mathrm{R}_{2} \mathrm{C}=\mathrm{C}=\mathrm{O}\) (b) \(\mathrm{RNCO}\) (c) RNC (d) \(\mathrm{R}_{2} \mathrm{C}=\mathrm{NR}\)

Step-by-Step Solution

Answer

Option (b) RNCO will not give an alkene when treated with Ph₃P=CH₂.

Step 1: Understanding the Reaction

The Wittig reaction involves phosphonium ylides, such as Ph₃P=CH₂, combining with carbonyl compounds to form alkenes. The general reaction is: \[ \text{R}_2\text{C}=\text{O} + \text{Ph}_3\text{P=CH}_2 \rightarrow \text{R}_2\text{C=CHR} + \text{Ph}_3\text{PO} \]

Step 2: Analyzing Each Option

We have four options to assess for their ability to partake in the Wittig reaction to form alkenes. We will consider each one separately: (a) R₂C=C=O (b) RNCO (c) RNC (d) R₂C=NR

Step 3: Evaluating Option (a)

The structure \(\text{R}_2\text{C}=\text{C}=\text{O}\) contains a ketene, which has a carbonyl group. Ketenes like carbonyl compounds can normally undergo the Wittig reaction to form alkenes.

Step 4: Evaluating Option (b)

The structure \(\text{RNCO}\) represents an isocyanate, which is not a carbonyl compound. Isocyanates cannot undergo the typical Wittig reaction, as they lack the necessary carbonyl carbon.

Step 5: Evaluating Option (c)

The structure \(\text{RNC}\) is a nitrile, which lacks a carbonyl carbon. As with isocyanates, nitriles cannot participate in a standard Wittig reaction. Thus, this compound is not capable of forming an alkene with Ph₃P=CH₂.

Step 6: Evaluating Option (d)

The structure \(\text{R}_2\text{C}=\text{NR}\) refers to an imine, where the carbon is doubly bonded to the nitrogen. Imines can potentially undergo modifications that might involve reactions similar to the Wittig, or via other mechanisms to yield alkenes.

Step 7: Conclusion

Based on the analysis, \(\text{RNCO}\) (option b) cannot undergo the Wittig reaction to form an alkene because it does not have a carbonyl group, making it incompatible with the typical requirements of a Wittig reaction.

Key Concepts

Phosphonium Ylides

Phosphonium ylides are key players in the Wittig reaction. They are special compounds consisting of a positively charged phosphorus atom, bonded to a carbon atom which holds a negative charge. This anionic carbon is typically attached to a hydrocarbon, making the compound highly reactive. The most common example used in reactions is Ph₃P=CH₂, where three phenyl groups (Ph₃) are bonded to phosphorus, and the ylidic carbon is bonded to hydrogen.
Phosphonium ylides excel in their ability to react with carbonyl groups, transforming them into alkenes. The ylide effectively donates electron density from its carbon to the carbonyl carbon,

• initiating a reaction where the carbon-oxygen double bond in the carbonyl is replaced by a higher-energy carbon-carbon double bond.
• This creates a new carbon structure, known as an alkene, accompanied by the formation of Ph₃PO as a byproduct.

Understanding the behavior of phosphonium ylides allows us to manipulate chemical structures efficiently.

Carbonyl Compounds

Carbonyl compounds are central to many essential organic reactions, including the Wittig reaction. Characterized by a carbon atom double-bonded to an oxygen atom, they provide a site of high reactivity due to the polarity of the C=O bond. This polarity means the carbon atom is electrophilic, readily reacting with nucleophiles.
In the Wittig reaction:

• Carbonyl compounds serve as the reactive partner for phosphonium ylides.
• For example, aldehydes and ketones, which contain carbonyl groups, undergo a reaction with Ph₃P=CH₂ to yield alkenes.
• The reaction involves breaking the C=O bond and forming a C=C bond, resulting in an alkene and the byproduct, Ph₃PO.

Understanding the nature of carbonyl compounds helps predict their behavior during the Wittig reaction and provides a foundational concept to many synthetic strategies in organic chemistry.

Isocyanates

Isocyanates, represented by the formula RNCO, are reactive organic compounds that contain a nitrogen atom bonded to a carbon atom, which is further double-bonded to oxygen. Unlike typical carbonyl groups, the carbon in an isocyanate is part of an N=C=O group, making it unsuitable for the Wittig reaction with phosphonium ylides. The absence of a direct carbonyl carbon-oxygen double bond means:

• Isocyanates cannot participate in forming an alkene as the reaction pairing lacks the necessary carbonyl substrate.
• Without the electrophilic carbon atom seen in typical carbonyl groups, isocyanates do not support the transfer of the ylide's carbon to form a C=C bond.

This volatility informs their practical use mainly in polymerization and other non-Wittig synthetic processes, differing from reactions involving true carbonyl compounds.

Ketenes

Ketenes are a unique class of compounds that contain a cumulated double bond structure, specifically R₂C=C=O. The presence of this cumulated structure means ketenes behave similarly to carbonyl compounds in the Wittig reaction.
This feature allows them:

• To undergo reactions with phosphonium ylides to potentially form alkenes.
• The reaction sequence follows the breaking of the C=O double bond, facilitating the formation of a new alkene product.

However, the unique structure of ketenes also means they exhibit behavior distinct from typical ketone or aldehyde reactions. Their atomic arrangement and reactivity pathways contribute to a rich chemistry that is pivotal in several industrial and research applications.