Question

Which would be considered a stronger Lewis acid: (a) \(\mathrm{BF}_{3}\) or \(\mathrm{BCl}_{3},\) (b) \(\mathrm{Fe}^{2+}\) or \(\mathrm{Fe}^{3+}\) ? Explain.

Short answer

The stronger Lewis acid between \(\mathrm{BF}_{3}\) and \(\mathrm{BCl}_{3}\) is \(\mathrm{BF}_{3}\). Similarly, \(\mathrm{Fe}^{3+}\) is the stronger Lewis acid compared to \(\mathrm{Fe}^{2+}\).

Step-by-step solution

Comparison of \(\mathrm{BF}_{3}\) and \(\mathrm{BCl}_{3}\)

Start by analyzing the pair of molecules \(\mathrm{BF}_{3}\) and \(\mathrm{BCl}_{3}\). These molecules are similar in structure as they both belong to the group 3 elements. The difference lies in their electronegativity. Boron in \(\mathrm{BF}_{3}\) is less electronegative compared to Boron in \(\mathrm{BCl}_{3}\) since Fluorine is more electronegative than Chlorine. This means that Fluorine pulls more electrons away from Boron making it more electron-deficient and hence a better Lewis acid. So, \(\mathrm{BF}_{3}\) is a stronger Lewis acid than \(\mathrm{BCl}_{3}\).

Comparison of \(\mathrm{Fe}^{2+}\) and \(\mathrm{Fe}^{3+}\)

Next, analyze the pair \(\mathrm{Fe}^{2+}\) and \(\mathrm{Fe}^{3+}\). Here, the comparison is based on the charge of the ions. Lewis acid strength increases with an increase in positive charge. So, \(\mathrm{Fe}^{3+}\) with a charge of +3 is a stronger Lewis acid when compared to \(\mathrm{Fe}^{2+}\) with a lesser charge of +2.

Key concepts

Lewis Acid and Base

In the realm of chemistry, the terms 'Lewis acid' and 'base' provide a framework for understanding reactions involving electron pair transfers. A Lewis acid is defined as a substance that can accept an electron pair, while a Lewis base can donate an electron pair. This perspective is broader than classical definitions, as it encompasses a wider range of reactions and substances.

For instance, in the exercise, both \(\mathrm{BF}_{3}\) and \(\mathrm{BCl}_{3}\) are potential Lewis acids as they are capable of accepting electron pairs due to the electron deficiency on the central Boron atom. The strength of a Lewis acid is dependent on how eagerly it accepts electrons, which can be influenced by multiple factors such as the central atom's electronegativity, the molecule's overall charge, and the stability of the molecule upon accepting an electron pair.

Electronegativity and Lewis Acidity

Electronegativity plays a significant role in determining the strength of a Lewis acid. It is a measure of an atom’s ability to attract and hold onto electrons in a chemical bond. When assessing Lewis acidity, we look at the central atom's electronegativity and the surrounding atoms or groups.

In the provided exercise, the comparison of \(\mathrm{BF}_{3}\) and \(\mathrm{BCl}_{3}\) involves examining the electronegativities of Fluorine and Chlorine. Fluorine, being more electronegative than Chlorine, draws electrons more strongly towards itself. This creates a greater electron deficiency on the central Boron atom of \(\mathrm{BF}_{3}\), making it a stronger Lewis acid. The stronger pull exerted by the highly electronegative Fluorine leaves the Boron atom more eager to accept electron pairs, intensifying its Lewis acid character.

Metal Ion Lewis Acids

Metal ions can act as Lewis acids as well. Their Lewis acidity is particularly influenced by their charge; a higher positive charge typically correlates with a stronger Lewis acid. This is because positive charges on metal ions create an electrostatic attraction for electron pairs from Lewis bases.

The exercise illustrates this with \(\mathrm{Fe}^{2+}\) and \(\mathrm{Fe}^{3+}\). The \(\mathrm{Fe}^{3+}\) ion, possessing a higher positive charge than \(\mathrm{Fe}^{2+}\), has a stronger pull on electron pairs, making it a more powerful Lewis acid. The increased positive charge enhances the metal ion's ability to attract and accept electrons from a Lewis base, thus making \(\mathrm{Fe}^{3+}\) the stronger Lewis acid when compared to \(\mathrm{Fe}^{2+}\). Understanding these concepts explains why charge and electron configuration play pivotal roles in Lewis acidity for metal ions.