Problem 106 - Introductory Chemistry Atoms First

Question

Show how a carboxylic acid, \(\mathrm{R}-\mathrm{COOH}\), reacts with a tertiary amine, \(\mathrm{NR}_{3}\).

Step-by-Step Solution

Answer

A carboxylic acid (R-COOH) reacts with a tertiary amine (NR3) in a Brønsted-Lowry acid-base reaction. The carboxylic acid donates a proton (H+) to the tertiary amine, forming a carboxylate anion (R-COO⁻) and an ammonium cation (NR4⁺). The balanced reaction equation is: \[ R-COOH+\mathrm{NR}_3 \rightarrow R-COO^-+\mathrm{NR}_4^+ \] Thus, a carboxylate salt is formed as the product.

Step 1: Identify the acidic and basic components

In this reaction, the carboxylic acid (R-COOH) acts as the acidic component in the reaction because it donates a proton (H+). The tertiary amine (NR3) acts as the basic component as it accepts the proton.

Step 2: Draw the Lewis structures of the reactants

To better understand the reaction, it's helpful to draw the Lewis structures of carboxylic acid (R-COOH) and tertiary Amine (NR3). Carboxylic acid (R-COOH): ``` O || R--C--O--H ``` Tertiary amine (NR3): ``` R1 | R2--N--R3 ```

Step 3: Reaction between the reactants

The nitrogen in the tertiary amine (NR3) has a lone pair of electrons that can act as a nucleophile. The proton in the carboxylic acid (R-COOH) can form a bond with the nucleophile. This results in the transfer of a proton from the carboxylic acid to the tertiary amine.

Step 4: Form the products

After the proton transfer, the R-COOH molecule becomes R-COO⁻ and the NR3 molecule becomes NR4⁺. The charge on both the carboxylate anion and the ammonium cation is now balanced. Carboxylate anion (R-COO⁻): ``` O || R--C--O⁻ ``` Ammonium cation (NR4⁺): ``` R1 | R2--N--R3 | H ```

Step 5: Final reaction equation

Combining all the steps above, we can now write the full balanced reaction equation between a carboxylic acid (R-COOH) and a tertiary amine (NR3): \[ R-COOH+\mathrm{NR}_3 \rightarrow R-COO^-+\mathrm{NR}_4^+ \] In conclusion, a carboxylic acid reacts with a tertiary amine to form a carboxylate anion and an ammonium cation, which together form a carboxylate salt.

Key Concepts

Carboxylic Acid

Carboxylic acids are organic compounds with the formula \( R-COOH \). They contain a carboxyl group, which is characterized by \(-COOH\).
This group is acidic due to its ability to donate a hydrogen ion (H⁺) in reactions.

The carbon is double-bonded to an oxygen and single-bonded to a hydroxyl group \(-OH\). This structure makes it capable of donating a proton easily, which is why it's considered acidic. In the reaction we're looking at, the carboxylic acid donates its proton to interact with the base.

Tertiary Amine

Tertiary amines are a type of amine with the formula \( R_3N \), where the nitrogen atom is bonded to three carbon-containing groups.
They are basic due to the lone pair of electrons on the nitrogen, which can accept protons.

In our reaction, this lone pair makes the tertiary amine a nucleophile (a molecule that donates an electron pair). This nucleophilic behavior is central to their role in forming bonded compounds by accepting protons, such as those from carboxylic acids.

Carboxylate Anion

The carboxylate anion \( R-COO^- \) is formed when a carboxylic acid loses its proton (H⁺).
This creates a negatively charged ion because the oxygen atoms hold more electrons than before.

This ion is more stable than its neutral carboxylic acid form due to resonance. Resonance allows the negative charge to be delocalized across the structure, stabilizing the anion. The carboxylate anion plays an essential role in forming salts in the acid-base reaction with amines.

Ammonium Cation

Ammonium cations, \( R_4N^+ \), are formed when a tertiary amine accepts a proton from a donor like a carboxylic acid.
The nitrogen atom in the amine gains a proton, resulting in a positively charged ion.

This charge arises because adding an extra hydrogen results in four substituents around the nitrogen, making it positively charged. The formation of ammonium cations allows them to pair with negatively charged ions, like carboxylates, to create stable salts.