Problem 16 - Basic Principles of Organic Chemistry

Question

Explain why the absorption band at \(227.3 \mathrm{~nm}\) for trimethylamine, \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{~N}\), disappears in acid solution.

Step-by-Step Solution

Answer

The 227.3 nm absorption band disappears because protonation removes the non-bonding electrons needed for the transition.

Step 1: Understanding the Absorption Band

The absorption band at \(227.3 \mathrm{~nm}\) for trimethylamine corresponds to a \(\pi \to \pi^*\) or \(n \to \pi^*\) transition involving non-bonding electrons on the nitrogen in the molecule.

Step 2: Effect of Acid on Trimethylamine

When trimethylamine is introduced into an acid solution, it reacts with protons (\(\text{H}^+\)) to form the trimethylammonium ion, \((\mathrm{CH}_3)_3\mathrm{NH}^+\).

Step 3: Understanding the Change in Absorption

In the trimethylammonium ion, the non-bonding electrons on the nitrogen are used to form a bond with the proton, thus they are no longer available for \(n \to \pi^*\) transitions. This eliminates the absorption band at \(227.3 \mathrm{~nm}\).

Key Concepts

Electronic Transitions

In absorption spectroscopy, electronic transitions refer to the movement of electrons between different energy levels in a molecule when it absorbs light. Electrons can move from a lower energy orbital to a higher one, such as from a non-bonding orbital ( ) to an anti-bonding orbital ( *). For trimethylamine, the absorption band at is due to these electronic transitions, specifically the or transitions. For electronic transitions to occur, specific conditions must be met, such as having available electrons in the correct orbitals that can absorb light energy. When trimethylamine absorbs light at , it excites electrons from the non-bonding orbitals on nitrogen to the higher energy antibonding orbitals. This process is highly selective and sensitive to changes in the molecule's environment or structure.

Protonation

Protonation is the addition of a proton ( ) to a molecule, thereby converting it into a charged species. This process is especially relevant in solutions where acids are present. In the case of trimethylamine, when it encounters an acid in solution, it accepts a proton on its nitrogen atom. This results in the formation of the trimethylammonium ion, ( ) This newly formed ion has different electronic properties compared to the original base form of trimethylamine. The critical change here is that the protonation of nitrogen uses up the non-bonding electrons. These electrons were previously available for electronic transitions such as transitions. However, after protonation, they are now involved in the bonding with the added hydrogen ion.

Chemical Reactivity

Chemical reactivity describes how readily a molecule undergoes chemical reactions. It depends on molecular characteristics such as bonds, functional groups, and electronic configurations. Trimethylamine is a base due to its nitrogen atom possessing a lone pair of electrons. This lone pair can easily participate in reactions, especially with acids. When trimethylamine is placed in an acidic environment, it reacts to form trimethylammonium ion. This reaction is called protonation, significantly altering the chemical reactivity of the molecule. The nitrogen atom, now protonated, loses its lone pair, making it less reactive in certain aspects but changing how it can participate in different kinds of interactions and transitions.

Molecular Orbitals

Molecular orbitals are regions where the probability of finding electrons is high within a molecule. They result from the combination of atomic orbitals when atoms bond together. These orbitals can be bonding, non-bonding, or anti-bonding. In the case of trimethylamine, the non-bonding orbital on nitrogen contains a lone pair of electrons. These electrons are crucial in various processes such as hydrogen bonding and electronic transitions. When trimethylamine is protonated, these non-bonding electrons form a new bond with the proton, converting the molecular orbital from non-bonding to bonding in the context of the new ion. This change in the molecular orbital configuration affects how the molecule interacts with light and consequently alters its absorption spectrum.