Question

Which is the more strongly basic, an aqueous solution of trimethylamine or an aqueous solution of tetramethy1ammonium hydroxide? Why?

Short answer

An aqueous solution of tetramethylammonium hydroxide is more strongly basic than an aqueous solution of trimethylamine. This is because tetramethylammonium hydroxide directly releases hydroxide ions \( (OH-) \) into the solution, whereas trimethylamine forms a less stable conjugate acid \( (CH3)3NH+ \) upon accepting a proton from water.

Step-by-step solution

Understand the structure of trimethylamine and tetramethylammonium hydroxide

First, let's examine the structures of the two compounds. Trimethylamine (CH3)3N is a tertiary amine with the nitrogen atom bonded to three methyl groups (CH3). Tetramethylammonium hydroxide [(CH3)4N]OH is a quaternary ammonium salt consisting of ammonium ion (NH4+) where all four hydrogen atoms are replaced by four methyl groups (CH3), thus forming a positively charged ion, and a hydroxide ion (OH-).

Evaluate the basicity in water

When dissolved in water, both compounds can potentially increase the basicity of the solution. But their behavior in water is different. Trimethylamine (CH3)3N can accept a proton from water (H2O) to form a conjugate acid, trimethylammonium ion (CH3)3NH+, and release hydroxide ions (OH-) in the solution as follows: \( (CH3)3N + H2O \rightleftharpoons (CH3)3NH+ + OH- \) In the case of tetramethylammonium hydroxide [(CH3)4N]OH, when dissolved in water, the OH- ions are directly released into the solution since it is an ionic compound. No proton transfer occurs: \( (CH3)4N+ OH- \rightarrow (CH3)4N+ + OH- \)

Compare the basicity of both compounds

Now, let's compare the basicity of both compounds. In general, the stronger the base, the weaker the conjugate acid formed upon accepting a proton, and vice versa. Trimethylamine (CH3)3N has a lone pair of electrons on the nitrogen atom that can accept a proton. Consequently, trimethylamine acts as a Bronsted-Lowry base in water. However, the resulting conjugate acid, trimethylammonium ion (CH3)3NH+, is unstable due to the positive charge on the nitrogen atom, which diminishes the basic strength of trimethylamine in water. On the other hand, tetramethylammonium hydroxide [(CH3)4N]OH releases hydroxide ions directly into the solution without any proton transfer. The presence of hydroxide ions in the solution increases the basicity instantaneously.

Determine the stronger base

Considering the behavior of both compounds in water and based on the comparison made in previous steps, we can conclude that: An aqueous solution of tetramethylammonium hydroxide is more strongly basic than an aqueous solution of trimethylamine because it directly releases hydroxide ions into the solution, whereas trimethylamine forms a less stable conjugate acid upon accepting a proton from water.

Key concepts

Trimethylamine

Trimethylamine, represented chemically as (CH₃)₃N, is a type of molecule known as a tertiary amine. It is characterized by a nitrogen atom bonded to three methyl groups (CH₃). This structure is significant because the nitrogen atom has a pair of electrons that are not engaged in bonding, making it available to bond with an additional proton (H+). This capability to accept a proton is what confers basicity to the molecule. In an aqueous solution, trimethylamine can react with water to accept a proton and release hydroxide ions, which increases the solution's pH.

The reaction is reversible, and when trimethylamine acts as a Bronsted-Lowry base by accepting a proton, it forms its conjugate acid, the trimethylammonium ion ((CH₃)₃NH+). However, the positive charge on the nitrogen makes the conjugate acid less stable, which in turn makes trimethylamine a weaker base compared to compounds that form more stable conjugate acids.

Tetramethylammonium Hydroxide

Tetramethylammonium hydroxide, with the formula [(CH₃)₄N]⁺OH^-, differs from trimethylamine primarily because it is a quaternary ammonium salt. It contains a positively charged tetramethylammonium ion ( [(CH₃)₄N]⁺ ) and a hydroxide ion (OH^-). Some of its unique characteristics include its inability to accept more protons because the nitrogen atom is already fully substituted with no free electron pair.

When dissolved in water, this compound dissociates completely to release hydroxide ions into the solution, which directly contributes to the solution's basicity. Unlike trimethylamine, no equilibrium or proton transfer is involved in the process of increasing pH, hence tetramethylammonium hydroxide more promptly raises the solution’s basicity level.

Bronsted-Lowry Base

The Bronsted-Lowry theory defines a base as a substance that can accept protons (H+ ions) — a perspective that allows us to explain why substances like trimethylamine act as bases. This proton acceptance shifts the equilibrium between base and conjugate acid in aqueous solutions.

A Bronsted-Lowry base must have an atom capable of donating an electron pair to the hydrogen ion. The stability of the resulting conjugate acid — created when the base accepts a proton — significantly influences the basicity of the original molecule. For instance, a less stable conjugate acid suggests a stronger base, as the equilibrium will favor the base over its conjugate acid to maintain stability.

Conjugate Acid

A conjugate acid forms when a Bronsted-Lowry base gains a proton. This concept helps in understanding base strength, as a base is stronger if it forms a weaker conjugate acid after accepting a proton.

In the case of trimethylamine, the nitrogen’s lone pair of electrons binds to a proton to form trimethylammonium ion ((CH₃)₃NH⁺). Here, the positive charge on the nitrogen implies a higher energy and less stability, which in turn impacts the compound’s basicity. By contrast, tetramethylammonium hydroxide doesn’t form a conjugate acid upon dissolution but simply dissociates to release OH^- ions, hence the comparison of conjugate acid stability is not applicable in this instance.

Hydroxide Ions

Hydroxide ions (OH^-) are powerfully basic and are responsible for increasing the pH of solutions, making them more basic or alkaline. These negatively charged ions are the counterpart to protons (H⁺) in acid-base chemistry.

When a substance like tetramethylammonium hydroxide ([(CH₃)₄N]⁺OH^-) is dissolved in water, the hydroxide ions are directly released, leading to an immediate increase in the solution's basicity. Their presence is a key factor in evaluating the strength of bases in aqueous solutions, as their concentration directly correlates with the basicity of the solution.