Question

The major product obtained in the following reaction is $$ \mathrm{Ph}_{3} \mathrm{P}=\mathrm{CH}-\mathrm{CH}_{3}+\mathrm{CH}_{3}-\mathrm{CH}=\mathrm{O} \longrightarrow $$ (A) CC=CC (B) CC=CC (C) O=CCCC=O (D) CC=CC=O

Short answer

The major product obtained in the given reaction is (D): CC=CC=O.

Step-by-step solution

Identify the functional groups and possible reactive sites

First, let's identify the functional groups and possible reactive sites in the starting material. The main functional groups present in the molecule are the phosphorus ylide group and the ketone (C=O) group. From these functional groups, the active site for a reaction to occur is the carbon atom that forms a double bond with the phosphorus atom (\(P=C\)) and the carbonyl group (\(C=O\)).

Consider the ylide reacting with the ketone

Now, we need to consider how these reactive sites might interact with each other. The main reaction of phosphorus ylides is with carbonyl compounds, and they typically react through a concerted mechanism called the Wittig reaction. The Wittig reaction forms an alkene and a side product containing a phosphorus-oxygen double bond. The process involves the nucleophilic attack of the ylide carbon at the carbonyl carbon and the transfer of electrons from the carbonyl group to the phosphorus atom, leading to the formation of a phosphorus-oxygen double bond.

Determine the possible products

Based on the Wittig reaction mechanism, we can suggest the possible products formed in the reaction: 1. The alkene product formed by the double bond between the carbon attacked by the nucleophilic ylide carbon and the former carbonyl carbon. 2. The phosphorus compound that has the phosphorus-oxygen double bond. Now, comparing these suggested products with the given options: (A) and (B) are both incorrect because they are both simple alkenes and do not contain any oxygen atoms from the ketone group. (C) is incorrect because it contains a six-membered ring and no double bonds apart from the \(C=O\) groups. The structure of this compound is completely different from the suggested alkene product. (D) matches the suggested alkene product formed in the Wittig reaction. It has a double bond between the carbon attacked by the nucleophilic ylide carbon and the former carbonyl carbon. Hence, (D) is the correct answer.

Result

The major product obtained in the given reaction is (D): CC=CC=O.

Key concepts

Phosphorus Ylide

Phosphorus ylide is a fascinating species in organic chemistry, serving as the linchpin in one of the most useful carbon-carbon double bond forming reactions known as the Wittig reaction. A phosphorus ylide can be understood as a neutral molecule containing a negatively charged carbon atom next to a positively charged phosphorus atom, forming a dipolar bond represented as Ph3P+-CH2. The presence of these opposite charges on adjacent atoms creates a significant region of high electron density, which makes the carbon atom in the ylide nucleophilic—that is, it seeks out positively charged or electron-deficient centers, such as the carbonyl carbon of a ketone.

To improve understanding, one might conceptualize the ylide as a seesaw with electron density, where the carbon end sits heavily laden with electrons ready to swing into action during a reaction. When preparing for a Wittig reaction, the phosphorus ylide is usually synthesized first from a phosphonium salt, involving the deprotonation of a phosphonium ion by a strong base. This produces the ylide, which can then be employed to generate alkenes in a usually stereospecific manner by reaction with carbonyl compounds.

It's crucial for students to grasp that ylides are more than just intermediates: they enable the formation of complex molecules through straightforward and predictable reactions. Recognizing this will deepen their understanding of how ylides behave and react within organic synthesis.

Ketone Reactivity

In discussing ketone reactivity, it's important to understand how the structural components of ketones influence their behavior in reactions. Ketones, characterized by the carbonyl group \(C=O\), are often reactive due to the polar nature of the double bond. This polarity arises because oxygen is more electronegative than carbon, causing it to draw away electron density and leave the carbon slightly positive.

This electrophilic (electron-loving) carbon is therefore a hotspot for nucleophilic attack, a key concept in explaining ketone reactivity. In a Wittig reaction, for instance, this electrophilic carbonyl carbon is precisely what the nucleophilic ylide carbon seeks out and binds to, forming a new carbon-carbon double bond. For students, visualizing the carbonyl group as a beacon emitting an 'electrophilic signal' can simplify the approach to understanding its reactivity.

Furthermore, the presence of adjacent groups can dramatically influence a ketone's reactivity as well. Electron-withdrawing groups increase the carbonyl carbon's electrophilicity, thereby accelerating nucleophilic addition reactions. Conversely, electron-donating groups reduce this reactivity. Recognizing these influencers is essential for students to predict outcomes in reactions involving ketones accurately.

Organic Reaction Mechanisms

Delving into organic reaction mechanisms, we must illuminate the stepwise sequence of events at the molecular level that lead to a chemical reaction. Each step involves the movement of electron pairs, resulting in the breaking and forming of chemical bonds. Understanding these mechanisms is akin to following a detailed map through a complex landscape, where knowing every turn and stop is essential to arrive at the final destination—the product.

A critical skill for students to develop is the ability to visualize these electron movements. One such strategic move is the 'concerted mechanism' occurring in the Wittig reaction. Unlike some multi-step mechanisms where intermediates can be isolated, a concerted mechanism means that bond-breaking and bond-forming happen simultaneously, in a single kinetic step, akin to a well-choreographed dance move.

In the case of the Wittig reaction, it's vital to acknowledge the nucleophilic attack, the transfer of the electron pair from the ylide carbon to the carbonyl carbon, and simultaneously the movement of the carbonyl's electrons toward the phosphorus, forming a new double bond. Grasping these steps is not just about memorizing the pathway; it develops a student's ability to predict the structure of the reaction products and to understand the fundamental principles governing organic reactions.