Question

What carbonyl compound and what phosphorus ylide might you use to prepare each of the following compounds?

Short answer

Choose an aldehyde or ketone where one side of the double bond remains, and a phosphorous ylide for the other part.

Step-by-step solution

Identify Target Alkene

We are given the target alkene structure. The objective is to identify the carbonyl compound and phosphorous ylide that could synthesize this alkene using the Wittig reaction.

Examine Alkene Structure

Look at the target alkene and identify both sides of the double bond: one side comes from a carbonyl compound and the other side from a phosphorous ylide.

Determine the Carbonyl Precursor

Identify the portion of the alkene that remains as part of the carbonyl compound. Typically, the less substituted group directly attached to the double bond would originate from the carbonyl compound.

Identify the Alkylidene Group

Determine which part of the alkene would come from the ylide. Typically, the more substituted or longer carbon chain would originate from the phosphorous ylide component.

Formulate Carbonyl Compound and Phosphorous Ylide

Based on the alkene structure, write down the structure of the respective aldehyde or ketone as the carbonyl compound, and construct the corresponding phosphorous ylide using the alkylidene group identified earlier.

Key concepts

Understanding Carbonyl Compounds

Carbonyl compounds are a significant class of organic molecules. They contain a carbon atom double-bonded to an oxygen atom, denoted as a carbonyl group (C=O). These compounds can act as electrophiles, meaning they accept electrons during a reaction.

Two common types of carbonyl compounds are aldehydes and ketones:

• Aldehydes have at least one hydrogen atom directly bonded to the carbonyl carbon (e.g., formaldehyde, acetaldehyde).
• Ketones have two carbon groups directly attached to the carbonyl carbon (e.g., acetone, butanone).

In the Wittig reaction, the carbonyl compound often serves as the starting material, specifically contributing one side of the resulting double bond (C=C) in the alkene. The choice of the carbonyl compound influences the structure of the final alkene product.

The Role of Phosphorus Ylide in Reactions

Phosphorus ylides are organic molecules characterized by a phosphorus atom bonded to a carbon atom that carries a negative charge. This combination is notably stable and participates in unique chemistry, particularly in the Wittig reaction.
The ylide is often denoted as R₃P⁺–CR₂⁻, where the phosphorus has a positive charge, and the carbon is negatively charged, creating a dipolar compound.

Key features of phosphorus ylides include:

• The carbon atom bonded to phosphorus carries a lone pair of electrons, capable of forming new bonds.
• The presence of the positive charge on phosphorus and the negative charge on adjacent carbon creates potential for reactivity with electrophiles like carbonyl compounds.

In the Wittig reaction, the ylide reacts with the carbonyl compound to form a betaine intermediate, which then rearranges to give the desired alkene and a phosphine oxide as a byproduct.

The Process of Alkene Synthesis with Wittig Reaction

The Wittig reaction is a valuable method for synthesizing alkenes from carbonyl compounds and phosphorus ylides. The reaction proceeds through nucleophilic addition and results in the formation of a C=C double bond.

Here’s an overview of the process:

• The reaction begins with the nucleophilic ylide attacking the electrophilic carbon atom of the carbonyl group.
• This nucleophilic attack forms a four-membered ring intermediate known as an oxaphosphetane.
• The oxaphosphetane undergoes decomposition, breaking down to form the alkene and a byproduct, triphenylphosphine oxide (Ph₃P=O).

This reaction is highly useful for its ability to specifically form (Z) or (E) stereoisomers of alkenes, depending on the nature of the ylide and the carbonyl compound involved. It is widely used in organic synthesis for constructing complex molecular architectures.

Introduction to Organic Chemistry Reactions

Organic chemistry involves studying various reactions that transform one molecule into another, typically involving carbon-based compounds.

Some important categories in organic chemistry reactions include:

• Addition Reactions: Two or more molecules combine to form a larger one, as seen in the addition of ylides to carbonyl compounds in the Wittig reaction.
• Substitution Reactions: Atoms or groups of atoms in a molecule are replaced by other atoms or groups, common in reactions involving aromatic compounds.
• Elimination Reactions: One molecule splits into two, with a small molecule, like water or hydrogen gas, being eliminated.
• Rearrangement Reactions: The structure of a molecule changes through the reorganization of its atoms.

Having a good understanding of these reactions helps chemists design and execute strategies to synthesize new compounds, modify existing molecules, and understand the behavior of organic systems.