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) RNCO (c) RNC (d) \(\mathrm{R}_{2} \mathrm{C}=\mathrm{NR}\)

Short answer

Option (c) RNC will not give an alkene.

Step-by-step solution

Understand the Role of Phosphorous Ylides

Phosphorous ylides like \( \mathrm{Ph}_{3} \mathrm{P}=\mathrm{CH}_{2} \) are used in the Wittig reaction to convert carbonyl groups, specifically aldehydes and ketones, into alkenes through the formation of a new carbon-carbon double bond.

Identify Structures Related to Carbonyl Groups

Examine each option to determine if it contains a structure similar to a carbonyl group (\( \mathrm{C}=\mathrm{O} \) ), since these are the groups that can react with phosphorous ylides to form alkenes.

Analyzing Option (a)

Option (a) is \( \mathrm{R}_{2} \mathrm{C}=\mathrm{C}=\mathrm{O} \), which contains a carbonyl group suitable for Wittig reaction, likely yielding an alkene.

Analyzing Option (b)

Option (b) is \( \mathrm{RNCO} \). This is an isocyanate group which contains a carbonyl functional group and can potentially form an alkene when reacted in a Wittig reaction.

Analyzing Option (c)

Option (c) is \( \mathrm{RNC} \), representing an isocyanide group. Neither a carbonyl nor a structure capable of forming a double bond with \( \mathrm{PPh}_{3}=\mathrm{CH}_{2} \). Hence, it is unlikely to form an alkene.

Analyzing Option (d)

Option (d) is \( \mathrm{R}_{2} \mathrm{C}=\mathrm{NR} \), an imine structure. Imines can resemble the carbonyl group for reactions and potentially undergo the Wittig reaction to form an alkene.

Conclusion

Among the choices, the one not fitting a carbonyl-like structure for the Wittig reaction is \( \mathrm{RNC} \). Therefore, option (c) will not give an alkene with \( \mathrm{Ph}_{3} \mathrm{P}=\mathrm{CH}_{2} \).

Key concepts

Phosphorous Ylides

Phosphorous ylides are remarkable compounds in organic chemistry, especially due to their role in the Wittig reaction. These compounds generally have the form of R₃P=CHR, where R can be various organic groups. In the context of the Wittig reaction, the ylide acts as a nucleophile that targets carbonyl compounds.
Phosphorous ylides are particularly known for facilitating the conversion of carbonyl groups into alkenes. This conversion is highly valuable in synthesis because it allows for the precise control of double bond formation.

• The structure of phosphorous ylides includes a positively charged phosphorous atom and a negatively charged carbon atom.
• This unique structure enables the ylide to react with carbonyls, exchanging the oxygen atom for a carbon-carbon double bond.

Understanding their structure and reactivity is key to predicting outcomes in reactions like the one described in the exercise.

Carbonyl Compounds

Carbonyl compounds are a broad category of organic molecules that are characterized by a carbon-oxygen double bond, written as C=O. This grouping includes aldehydes and ketones. The carbon in carbonyl groups is electron-deficient, making it reactive, particularly towards nucleophiles like phosphorous ylides.
The Wittig reaction leverages this reactivity to transform carbonyl compounds into alkenes. This is accomplished when the carbonyl oxygen atom is replaced by the carbon atom from the ylide.
In the exercise's context, we explore which substrates resemble carbonyl compounds. Options like (a) and (b) include carbonyl-like structures, making them candidates for alkene formation through this reaction. The readiness of carbonyl compounds to participate in such reactions marks them as versatile intermediates in synthetic chemistry.

Alkene Formation

Alkene formation is a central goal in many organic transformations, including the Wittig reaction. Alkenes are hydrocarbons featuring carbon-carbon double bonds, and they serve as key building blocks in the synthesis of complex molecules.

• The transformation starts with the nucleophilic attack of the ylide’s carbanion on the electrophilic carbonyl carbon.
• This results in the formation of an intermediate called a betaine, which eventually collapses to form the desired alkene.

Alkene formation via the Wittig reaction is highly valued because it can provide stereochemically defined products, which are essential for the synthesis of many pharmaceuticals and agrochemicals. Understanding the nuances of this formation helps in designing reactions where the geometry of the resultant alkene is crucial. In the given exercise, options identified as capable of forming alkenes include carbonyl-like or reactive groupings.

Isocyanates

Isocyanates represent compounds that include the functional group -N=C=O. They are versatile intermediates in organic chemistry, often employed in the synthesis of ureas, carbamates, and other nitrogen-containing compounds. Notably, isocyanates include a carbonyl functional group, akin to those involved in Wittig reactions.
This resemblance suggests that under the right conditions, isocyanates can undergo transformations similar to those observed with typical carbonyl compounds. In the exercise, option (b), representing an isocyanate, is considered capable of forming alkenes through the Wittig reaction.

• Isocyanates are known for their high reactivity, particularly with nucleophiles.
• This reactivity is crucial in the broad application of isocyanates across different industrial processes.

Recognizing the potential of isocyanates to behave like carbonyl compounds can be a powerful tool in strategic chemical syntheses.

Imines

Imines comprise organic compounds with a carbon-nitrogen double bond, denoted as R₂C=NR. They are often formed through the condensation of primary amines and carbonyl compounds.
In reactions resembling those with carbonyl compounds, imines can participate in transformations such as the Wittig reaction to produce alkenes.
In the exercise example, option (d) contains an imine structure, hinting at its capacity to engage in Wittig-like chemistry.

• Imines, like carbonyls, manifest reactive electrophilic carbon atoms.
• The presence of a carbon-nitrogen double bond offers a unique landscape for nucleophilic attack.

Understanding the dual potential of imines to either be stable intermediates or reactive partners extends their utility in synthetic applications, including the production of alkenes through reactions that traditional carbonyls undergo.