Question

The strongest Bronsted base among the following ions is: (a) \(\mathrm{CH}_{3} \mathrm{O}^{-}\) (b) \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{O}^{-}\) (c) \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CHO}^{-}\) (d) \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{CO}^{-}\)

Short answer

The strongest Bronsted base is methoxide ion \\(\text{CH}_3\text{O}^{-}\\) due to least steric hindrance.

Step-by-step solution

Understand the Concept of Bronsted Base

A Bronsted base is a substance that can accept a proton. In this context, the Bronsted base strength depends on its ability to accept a proton. The stronger the base, the more readily it will accept a proton.

Analyze the Molecular Structure

Each ion in the options has an alkoxide ion form, \(\text{R--O}^{-}\), where R represents the alkyl group. The basicity typically decreases with the increase of the steric hindrance around the oxygen, which impedes proton acceptance.

Determine the Effect of Steric Hindrance

Compare the structures: (a) \(\text{CH}_3\text{O}^{-}\), \(\text{C}_2\text{H}_5\text{O}^{-}\), (b) \(\text{(CH}_3\text{)}_2\text{CHO}^{-}\), and (c) \(\text{(CH}_3\text{)}_3\text{CO}^{-}\).

Key concepts

Proton Acceptance

In the realm of Bronsted bases, proton acceptance is the key mechanism. A Bronsted base is defined by its ability to accept a proton, typically denoted as \( ext{H}^+ \). The strength of a base is gauged by how readily it can seize a proton from its surroundings. The more eager the base is to accept this proton, the stronger it is classified.

• Proton acceptance is directly linked to the availability of negative charge. This is why ions with higher negative charge density tend to be stronger bases.
• A base's ability to accept a proton is influenced by its structural characteristics, particularly the presence of electron pairs that can bond with a proton.

A strong Bronsted base often features an anionic site, like an oxygen ion, that is only minimally hindered, allowing it to effectively attract and bond with protons.

Steric Hindrance

Steric hindrance refers to the physical obstruction to reaction at a specific site within a molecule, caused by the size of constituent groups. In the context of Bronsted bases and their ability to accept protons, steric hindrance plays a significant role. When bulky groups surround the reactive site, such as an oxygen ion, they can diminish the base's ability to interact with a proton.

• The larger the surrounding groups, the more difficult it becomes for the base to accept a proton because the space for the proton to approach is restricted.
• Bases with less steric hindrance, like those with smaller alkyl groups, can accept protons more easily, leading to increased basicity.

For example, when comparing ions such as \( \text{CH}_3\text{O}^{-} \) and \( \text{(CH}_3\text{)}_3\text{CO}^{-} \), the former has less steric hindrance and thus a higher capability to accept protons.

Alkoxide Ions

Alkoxide ions are a specific type of ion characterized by the formula \( \text{R--O}^{-} \), where R represents an alkyl group. These ions are frequently involved in acid-base reactions, particularly as bases. The basicity of alkoxide ions is a function of both the electron-withdrawing ability of the oxygen and the electronic and steric characteristics of the alkyl group.

• Smaller alkyl groups in alkoxide ions, such as methyl in \( \text{CH}_3\text{O}^{-} \), lead to stronger base behavior because there's less steric hindrance to proton acceptance.
• Larger alkyl groups create more steric hindrance, which in turn reduces the ion's ability to act as a base. This makes them weaker in terms of proton acceptance.

Understanding the structure of alkoxide ions helps in predicting their behavior as bases, specifically in identifying which will most effectively accept protons and thus be the strongest Bronsted base.