Problem 124 - General Chemistry: Principles and Modern Applications

Question

Acetone \(\left(\mathrm{CH}_{3}\right) \mathrm{C}=\mathrm{O},\) a ketone, will react with a strong base \((\mathrm{A})\) to produce the enolate anion, \(\mathrm{CH}_{3}(\mathrm{C}=\mathrm{O}) \mathrm{CH}_{2}^{-} .\) Draw the Lewis structure of the enolate anion, and describe the relative contributions of any resonance structures.

Step-by-Step Solution

Answer

The enolate anion Lewis structure consists of a double bond between the central carbon and the oxygen, and a single bond between the central carbon and two carbon atoms. One carbon atom (alpha) has a negative charge and two hydrogen atoms attached. It has two resonance structures where the second has the negative charge on oxygen, representing the delocalized charge.

Step 1: Formation of Enolate Anion

Acetone \(\left(\mathrm{CH}_{3}\right) \mathrm{C}=\mathrm{O}\) reacts with a strong base, donating a proton from the alpha carbon atom (one carbon atom away from the carbonyl group, C=O) to the base. This forms an enolate anion, \(\mathrm{CH}_{3}(\mathrm{C}=\mathrm{O}) \mathrm{CH}_{2}^{-}\), where there's a negative charge on the alpha carbon and a double bond between the central carbon and oxygen.

Step 2: Drawing the Lewis Structure of the Enolate Anion

The Lewis structure starts with the acetone central framework \(\mathrm{H}_{3}\mathrm{C}-\mathrm{C}-\mathrm{CH}_{3}\) where the central carbon is double bonded to the oxygen and single bonded to a carbon atom that also has three hydrogen atoms bonded. The second carbon atom (alpha carbon), aside from being single-bonded to the central carbon, bears a negative charge and is also single bonded to two hydrogen atoms.

Step 3: Resonance Structures Contribution

There are two important resonance structures in the anion enolate since the negative charge on alpha carbon can be delocalized on oxygen atom. In the second structure, the central carbon is single bonded to oxygen, and the oxygen bears the negative charge while the central carbon is double bonded to the alpha carbon. The two structures (canonical forms) contribute to the overall shape of the enolate.

Key Concepts

Resonance Structures

Resonance structures are different ways of arranging electrons in a molecule that help depict a molecule's electron distribution more realistically. For an enolate anion that is derived from acetone, there are two canonical forms that are especially important.
A canonical form refers to one possible structure in the set of resonance structures. These are not physical entities, but rather visualizations to illustrate the concept of electron delocalization.

• In the first resonance structure, the negative charge resides on the alpha carbon, which is adjacent to the carbonyl group (C=O). This helps in illustrating the direct effect post deprotonation by a strong base.
• The second resonance structure shows the negative charge being transferred to the oxygen atom, making the carbon-oxygen bond a single bond. In this case, the alpha carbon develops a double bond with the central carbon.
• These resonance structures allow chemists to represent the concept of electron delocalization, meaning that the electrons are shared between the carbon and oxygen atoms, increasing stability.

By understanding these resonance forms, students can better comprehend how these electrons are distributed across the molecule in reality, rather than just being stuck to a single atom. This delocalization results in increased stability of the enolate anion.

Lewis Structure

The Lewis structure is a way of representing molecules in terms of electrons, showing how they are shared or transferred between atoms. It visualizes how the atoms are bonded within the molecule and accounts for all valence electrons.
For the enolate anion derived from acetone, its Lewis structure demonstrates the negative charge primarily residing on the alpha carbon next to the carbonyl group.

• The central atom is a carbon connected with a double bond to an oxygen, depicting the classic carbonyl group.
• One of the carbons, the alpha carbon, bears the negative formal charge highlighting the loss of a proton (H⁺).
• Bond lines represent shared electron pairs forming covalent bonds while lone pairs are shown as dots, often around the oxygen in resonance structures.

Altogether, this portrayal helps in visualizing the placement of electrons and the resulting charge distribution, which is critical for predicting the behavior of the molecule in various reactions.

Acid-Base Reaction

Acid-base reactions are fundamental in chemistry, involving the transfer of protons (H⁺) between molecules. In the context of the enolate anion formation, this concept is crucial as it involves the deprotonation of acetone.
When acetone, a common ketone, is treated with a strong base, a transfer occurs where an alpha hydrogen (a hydrogen atom bound to the alpha carbon adjacent to the carbonyl group) is donated to the base. This results in the formation of an enolate anion.

• The strong base initially abstracts a proton from the alpha position of acetone, generating the enolate anion while leaving a negative charge on the alpha carbon.
• This negative charge is what facilitates the formation of resonance structures, which stabilize the anion.
• Understanding this proton donation and charge generation is essential, as it underlines why specific carbon atoms become charged and how they contribute to the molecule's reactivity.

This process highlights the importance of acid-base reactions in organic synthesis and transformations, providing essential pathways for molecules like the enolate anion to change and react.