Question

Carboxylic acids having a second carbonyl group two atoms away lose \(\mathrm{CO}_{2}\) ( decarboxylate) through an intermediate enolate ion when treated with base. Write the mechanism of this decarboxylation reaction using curved arrows to show the electron flow in each step.

Short answer

The carboxylic acid decarboxylates through deprotonation, forming an enolate ion, followed by the loss of CO_2.

Step-by-step solution

Deprotonation of the Carboxylic Acid

A base (e.g., OH^−) abstracts a proton (H^+) from the hydroxyl group of the carboxylic acid, forming a carboxylate anion and a water molecule. This deprotonation step increases the nucleophilicity of the carboxylic acid anion, preparing it for further reaction steps.

Formation of the Enolate Ion

The electron-rich carboxylate anion undergoes a keto-enol tautomerization, where the alpha-carbon (the carbon next to the carbonyl group) adjacent to the carboxylate is deprotonated by the base. This results in the formation of the enolate ion with the negative charge on the oxygen of the enol form (or on the alpha carbon).

Decarboxylation through Enolate Ion

In the enolate form, the carboxylate anion undergoes decarboxylation. The electron pair from the C=O bond migrates to form a C=C bond while a pair of electrons on the alpha hydrogen form a pi-bond with the carbonyl carbon, causing the loss of the carbon dioxide molecule (CO_{2}). Curved arrows should be used to show this electron flow through the enolate intermediate.

Tautomerization to the Keto Form

The reaction concludes with the conversion of the resulting enol form of the compound back to the more stable keto form. This typically involves the migration of a proton from one oxygen to the other, and the double bond relocating to form the carbonyl group.

Key concepts

Carboxylic Acids

Carboxylic acids are organic compounds characterized by the presence of a carboxyl group, \(-COOH\). They are one of the most common and versatile functional groups in organic chemistry. The carboxyl group can donate a proton (H^+) to form a carboxylate anion, \(-COO^-\).
This property makes them acidic.
One key feature of carboxylic acids is their ability to participate in various chemical reactions, especially when they are in the form of a carboxylate ion.
These reactions often involve the breaking and forming of bonds within the carboxyl group.

• They can lose \( \mathrm{CO}_{2} \) under certain conditions, such as in the presence of a base, through a process known as decarboxylation.
• Decarboxylation often involves the presence of a carbonyl group two carbon atoms away from the carboxyl group, enhancing the reaction's feasibility.

Enolate Ion Formation

Enolate ion formation is a crucial step in many organic reactions, particularly in decarboxylation processes. It involves deprotonation at the alpha position, which is the carbon adjacent to a carbonyl group.
The alpha carbon is typically more acidic because of the electron-withdrawing effect of the carbonyl group.
When a base abstracts a proton from this carbon, an enolate ion is formed.

• This ion is resonance-stabilized, meaning its negative charge can be delocalized between the carbon and the oxygen atom.
• This stabilization makes enolates excellent intermediates for further chemical reactions.

Understanding enolate ion formation helps explain why decarboxylation often involves the creation of such intermediates.
These intermediates are crucial for efficient electron flow during the reaction.

Keto-Enol Tautomerization

Keto-enol tautomerization is a chemical equilibrium between two forms: the keto form and the enol form.
This process is significant in understanding the converting of intermediates during reactions, such as the decarboxylation of carboxylic acids.
In the keto form, the molecule has a typical carbonyl group (C=O), while in the enol form, it has an alcohol (-OH) adjacent to a double bond.

• The conversion between these forms is driven by the migration of a proton and the shifting of a pi-bond.
• This tautomerization is essential because it often leads to the more stable keto form as a product, as seen at the end of many reactions.

During some reaction mechanisms, such as the one involved in the formation of the enolate ion, keto-enol tautomerization allows for free movement of electrons, facilitating bond rearrangements necessary for reaction progression.

Electron Flow in Reactions

Understanding electron flow is critical in organic chemistry as it dictates how reactions progress. In decarboxylation and many other reactions, electron flow is illustrated using curved arrows in mechanism diagrams.
These arrows show the movement of electron pairs from one atom to another or from one bond to the next.
Knowing the direction of electron flow helps predict the transformations of molecules during a reaction.

• For example, in the decarboxylation of a carboxylic acid, electron flow from the C=O bond facilitates the formation of a C=C bond, leading to \( \mathrm{CO}_{2} \) being released.
• The enolate ion plays a central role by stabilizing the extra electron density until the electrons can rearrange into new bonds.

Electron flow is also essential in the final tautomerization step, stabilizing the final keto form by reorganizing electron density.