Question

Describe and give examples of base-catalyzed nucleophilic addition reactions occurring at the carboxy1 carbon.

Short answer

Base-catalyzed nucleophilic addition reactions involve a nucleophile reacting with an electrophile in the presence of a base catalyst. In this case, the electrophile is the carboxyl carbon in a molecule. The carboxyl carbon is the carbon atom within the carboxyl functional group (COOH), which consists of a carbonyl group (C=O) bonded to a hydroxyl group (O-H). In the reaction mechanism, the base catalyst abstracts a proton from the hydroxyl group, and the nucleophile attacks the carboxyl carbon. This results in the addition of the nucleophile to the carboxyl carbon, forming a new molecule. Examples of such reactions include the reaction of acetic acid with an alkoxide ion (\( CH_3COOH + R'O^- \rightarrow CH_3COOR + OH^- \)) to form an ester, and the reaction of a carboxylic acid with ammonia (\[ RCOOH + NH_3 \rightarrow RCONH_2 + H_2O \]) to form an amide.

Step-by-step solution

Define base-catalyzed nucleophilic addition reactions

A base-catalyzed nucleophilic addition reaction is a type of reaction where a nucleophile, which is a species with a pair of electrons, reacts with an electrophile, which is a species deficient in electrons, in the presence of a base catalyst. In this case, the electrophile is the carboxyl carbon in a molecule.

Identify the carboxyl carbon

The carboxyl carbon is the carbon atom present in a carboxyl functional group, written as -COOH. This functional group consists of a carbonyl group (C=O) bonded to a hydroxyl group (O-H). The carboxyl carbon is the carbon atom that is double-bonded to the oxygen atom.

Explain the mechanism of the base-catalyzed nucleophilic addition reaction

In a base-catalyzed nucleophilic addition reaction, the base catalyst first abstracts a proton from the hydroxyl group of the carboxyl functional group, forming a carboxylate anion. The nucleophile then attacks the carboxyl carbon, forming a new bond with it. The double bond between the carboxyl carbon and the oxygen atom is broken, with the electrons becoming a part of the new bond formed with the nucleophile. This ultimately results in the addition of the nucleophile to the carboxyl carbon and the formation of a new molecule.

Provide examples of base-catalyzed nucleophilic addition reactions

Some examples of base-catalyzed nucleophilic addition reactions include: 1. Reaction of Acetic acid with an alkoxide ion: \( CH_3COOH + R'O^- \rightarrow CH_3COOR + OH^- \) In this case, the alkoxide ion \( (R'O^-) \) acts as a nucleophile, attacking the carbonyl carbon in the acetic acid molecule. The base catalyst abstracts a proton from the hydroxyl group of the carboxyl group, forming an acetate ion. The final product is an ester. 2. Reaction of a carboxylic acid with ammonia: \[ RCOOH + NH_3 \rightarrow RCONH_2 + H_2O \] In this reaction, ammonia \( NH_3 \) acts as a nucleophile and attacks the carbonyl carbon in the carboxylic acid (RCOOH) molecule. The base catalyst abstracts a proton from the hydroxyl group of the carboxyl group, forming a carboxylate ion. The final product is an amide.

Key concepts

Carboxyl Carbon

In organic chemistry, the carboxyl carbon is an essential component of the carboxyl functional group, denoted as \(-COOH\). This group is a cornerstone in many chemical reactions due to its dual nature containing both a carbonyl group \((C=O)\) and a hydroxyl group \((O-H)\). The carboxyl carbon is the central figure here—specifically, it is the carbon atom bonded to both the oxygen in the carbonyl process and to the hydroxyl group.

Its position as the electrophilic site makes it highly reactive, especially in base-catalyzed nucleophilic addition reactions. In such reactions, the carboxyl carbon's partial positive charge attracts nucleophiles, which are electron-rich species ready to donate electrons. Understanding its role here is vital when studying further transformations such as ester and amide formation.

Carboxylate Anion

During base-catalyzed reactions, the carboxyl carbon's associated functional group undergoes a crucial transformation into a carboxylate anion. This occurs when the base catalyst abstracts a proton \((H^+)\) from the hydroxyl group. The resulting carboxylate anion is stabilized by resonance, making it an important species in these reactions.

Here's the mechanism: The removal of a proton leads to the formation of a negative charge on the oxygen, which can be delocalized over the carboxylate group. This delocalization stabilizes the anion, increasing its reactivity and readiness for subsequent nucleophilic attack on the carboxyl carbon. The carboxylate anion's existence is a critical step in bypassing the need for an activated complex, simplifying these reactions significantly.

Nucleophile

A nucleophile is a chemical entity characterized by its electron-rich nature, possessing at least one pair of non-bonding electrons. These pairs make nucleophiles eager participants in chemical reactions, particularly those involving electron-poor electrophiles such as the carboxyl carbon.

In a base-catalyzed nucleophilic addition, the presence of a nucleophile is crucial. It attacks the partial positive charge of the carboxyl carbon, forming a new covalent bond. This interaction is the essence of creating new chemical products like esters or amides. For example, in the reactions of esters and amides, the nucleophiles can be alkoxide ions or ammonia, respectively, leading to the functional group shift critical to these processes.

Ester Formation

Ester formation is a fascinating process typically resulting from the reaction between an alcohol and an acid. In the context of base-catalyzed nucleophilic addition, the setup involves a carboxylic acid and a nucleophile, typically an alkoxide ion \((R'O^-)\). Here's how it works:

First, a base abstracts the proton from the acid's hydroxyl group to produce a carboxylate anion. Subsequently, the nucleophile—our alkoxide ion—attacks the carboxyl carbon. This results in a rearrangement where the oxygen of the nucleophile forms a new bond with the carbonyl carbon. The resultant reaction is the formation of an ester, characterized by the replacement of the hydroxyl \((-OH)\) by an alkoxy \((R'O)\) group. Esters are notable for their aromatic properties and are used in fragrances and flavorings.

Amide Synthesis

Amide synthesis, like ester formation, is a process that transforms carboxylic acids into another valuable class of compounds known as amides. Here’s a simplified rundown of what occurs:

When a carboxylic acid reacts with ammonia, facilitated by a base catalyst, an amide is formed. The base first causes the removal of a proton from the carboxylic acid, forming a carboxylate ion. The nucleophile, ammonia \((NH_3)\), then attacks the carboxyl carbon, culminating in the displacement of the hydroxyl group and the formation of an amide bond \((RCONH_2)\).

• Amides are important in both biological systems and industrial applications.
• They are key components of proteins and present in many synthetic materials.

Thus, understanding the synthesis of amides via base-catalyzed reactions is crucial for both biochemical and synthetic organic chemists.