Which of the following is the correct order of the strength...

Which of the following is the correct order of the strength of Lewis acid? (1) $\mathrm{BBr}_{3}>\mathrm{BCl}_{3}>\mathrm{BF}_{3}$ (2) $\mathrm{BCl}_{3}>\mathrm{BBr}_{3}>\mathrm{BF}_{3}$ (3) $\mathrm{BF}_{3}>\mathrm{BCl}_{3}>\mathrm{BBr}_{3}$ (4) $\mathrm{BCl}_{3}>\mathrm{BF}_{3}>\mathrm{BBr}_{3}$

Which of the following is the correct order of the strength of Lewis acid?
(1) $\mathrm{BBr}_{3}>\mathrm{BCl}_{3}>\mathrm{BF}_{3}$
(2) $\mathrm{BCl}_{3}>\mathrm{BBr}_{3}>\mathrm{BF}_{3}$
(3) $\mathrm{BF}_{3}>\mathrm{BCl}_{3}>\mathrm{BBr}_{3}$
(4) $\mathrm{BCl}_{3}>\mathrm{BF}_{3}>\mathrm{BBr}_{3}$

Key Concepts

Electronegativity

Electronegativity is a measure of an atom’s ability to attract electrons towards itself. In the context of boron trihalides, the differing electronegativities of the halogen atoms affect both the electron-withdrawing capacity and the extent of back bonding, which in turn modulates the overall Lewis acidity of the compound.

Lewis Acid-Base Theory

This concept involves the definition of Lewis acids as electron pair acceptors and Lewis bases as electron pair donors, providing a framework to understand interactions that do not involve hydrogen atoms as in Brønsted acid-base theory. It is crucial for analyzing the reactivity and strength of acids and bases in various chemical reactions.

Boron Trihalides

Boron trihalides are compounds where a boron atom is bonded to three halogen atoms. Their molecular properties, including electron deficiency at the boron center and the influence of the halogen atoms, play a significant role in determining their behavior as Lewis acids.

Back Bonding

Back bonding refers to the donation of electron density from the filled orbitals of a halogen atom into the empty p orbital of the boron atom. This interaction reduces the electron deficiency of the boron center, thereby decreasing its Lewis acidity. The effectiveness of back bonding varies with the size and electronegativity of the halogen, influencing the acid strength across boron trihalides.

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