Question

Which of the following can be obtained by reaction between a carbonyl compound and phosphorous ylide? (A) Methane (B) Acetylene (C) Ethylene (D) Ethane

Short answer

The product that can be obtained by the reaction between a carbonyl compound and a phosphorus ylide is (C) Ethylene, as it is an alkene synthesized through the Wittig reaction.

Step-by-step solution

Understand the Wittig Reaction

The Wittig reaction is a widely used organic reaction to convert carbonyl compounds (aldehydes and ketones) to alkenes (or olefins) using a phosphorus ylide (or phosphorane) as a reagent. The general reaction can be written as: Carbonyl compound + Phosphorus ylide → Alkene + Phosphine oxide The mechanism of the Wittig reaction involves the nucleophilic attack of the ylide on the carbonyl compound, followed by the elimination of the oxygen as a phosphine oxide, leading to the formation of an alkene.

Evaluate the possible products

Now that we know the general reaction and mechanism, let's evaluate each of the given options: (A) Methane - Methane is an alkane, not an alkene, so it cannot be obtained by the Wittig reaction. (B) Acetylene - Acetylene is an alkyne, not an alkene, so it also cannot be obtained by the Wittig reaction. (C) Ethylene - Ethylene is an alkene, which can be synthesized by the Wittig reaction between a carbonyl compound and a phosphorus ylide. (D) Ethane - Ethane is an alkane, not an alkene, so it cannot be obtained by the Wittig reaction.

Choose the correct answer

Based on our evaluation, the only option that corresponds to a product that can be obtained by the Wittig reaction is (C) Ethylene.

Key concepts

Understanding Carbonyl Compounds

Carbonyl compounds play a central role in organic synthesis and are characterized by a carbon atom double-bonded to an oxygen atom. This functional group is at the heart of a multitude of reactions due to its polarity and reactivity.

Carbonyl compounds are segregated into two main categories: aldehydes and ketones. Aldehydes have at least one hydrogen atom attached to the carbon center, whereas ketones feature two alkyl or aryl groups. It's this carbonyl carbon that serves as an electrophilic site, making these compounds reactive towards nucleophiles, like the phosphorus ylide in the Wittig reaction.

The reactivity of carbonyl compounds makes them versatile intermediates in organic synthesis, allowing chemists to create a diversity of products, including alcohols, carboxylic acids, and most pertinently for the Wittig reaction, alkenes.

Phosphorus Ylide Chemistry

Phosphorus ylides, also known as phosphoranes, are intriguing species in organic chemistry with a unique structure featuring a positively charged phosphorus atom adjacent to a negatively charged carbon atom. This polarization is stabilized by resonance and the presence of substituents that can donate electron density to the central phosphorus.

The most critical feature of phosphorus ylides in organic synthesis is their nucleophilicity. Due to the negatively charged carbon atom, they can effectively attack electrophilic carbon atoms, such as those in carbonyl groups. Their preparation generally involves treating phosphonium salts with strong bases to deprotonate the carbon adjacent to the phosphorus, creating the necessary ylide for the Wittig reaction.

In the context of the Wittig reaction, it's important to note that different types of ylides (stabilized, semi-stabilized, or unstabilized) lead to different outcomes in the stereoselectivity of the resulting alkene. This adaptability makes phosphorus ylides essential for synthesizing a variety of alkenes with desired geometric configurations.

Alkene Synthesis via the Wittig Reaction

The Wittig reaction represents a cornerstone of organic chemistry for the synthesis of alkenes. It distinguishes itself by the use of phosphorus ylides to convert carbonyl groups into C=C double bonds.

The reaction proceeds through a concerted [2+2] cycloaddition process where the ylide forms a four-membered cyclic intermediate with the carbonyl compound. This ring then cleaves to generate the desired alkene and byproduct phosphine oxide. This elegant transformation allows for the formation of alkenes with stereochemical precision, as the geometry of the product alkene can be controlled by the choice of ylide.

It's important for students to recognize that only alkenes can be directly synthesized through this method, as illustrated by the exercise where ethylene (an alkene) is identified as the correct Wittig product. Alkanes and alkynes do not form directly through the Wittig reaction, highlighting its specificity for forming carbon-carbon double bonds.