Question

Why do boron and aluminium halides behave as Lewis acids? (a) Both halides \(\left(M X_{3}\right)\) can accept electrons from (a) a donor to complete their octet. (b) Both halides \(\left(M X_{3}\right)\) can donate a pair of electrons. (c) Both halides \(\left(M X_{3}\right)\) are covalent polymeric structures. (d) Both halides \(\left(M X_{3}\right)\) react with water to give hydroxides and \(\mathrm{HCl}\).

Short answer

Boron and aluminium halides behave as Lewis acids because they can accept electron pairs to complete their octet, making the correct option (a).

Step-by-step solution

Understanding Lewis Acids and Bases

Lewis acids are electron pair acceptors while Lewis bases are electron pair donors. To determine why boron and aluminium halides act as Lewis acids, we must evaluate whether these compounds can accept an electron pair.

Analyzing the Electron Configuration of Boron and Aluminium

Boron and aluminium are group 13 elements and have three valence electrons. When they form trihalides (MX3), they share their valence electrons with three halide ions, but their octet is still incomplete as they only have six electrons in the bond formation, leaving room to accept more electrons.

Evaluating the Given Options

Assess each option to see which one aligns with the Lewis acid behavior: (a) accepting electrons to complete the octet, (b) donating a pair of electrons which is characteristic of a Lewis base, (c) being a polymeric structure does not directly relate to Lewis acidity, and (d) the reaction with water illustrates reactivity but not the Lewis acid behavior per se.

Determining the Correct Answer

The option that directly explains the Lewis acid behavior of boron and aluminium halides is that they can accept electrons to complete their octet. Therefore, the correct answer is (a).

Key concepts

Boron Halides

Boron halides, such as boron trifluoride (BF₃), boron trichloride (BCl₃), and boron tribromide (BBr₃), are compounds where a boron atom is bonded to three halogen atoms. The key feature of these molecules is that the boron atom has an incomplete octet; it only has six electrons in its valence shell after forming three covalent bonds with the halogens. This electron deficiency makes boron halides highly electron-pair acceptors, classifying them as Lewis acids.

In chemical reactions, boron halides can readily accept an electron pair from a Lewis base to form a coordinate covalent bond, completing the boron's octet. This tendency to accept electrons is demonstrated in their reactions with substances like ammonia, where the lone pair on the nitrogen atom forms a dative bond with the boron atom, resulting in a stable complex.

Aluminium Halides

Aluminium halides, such as aluminum chloride (AlCl₃), also behave as Lewis acids. Aluminium, like boron, belongs to group 13 and has three valence electrons. When aluminium forms trihalides, it shares its valence electrons with three halide ions and falls short of the stable octet configuration. Unlike boron, aluminium has a larger atomic radius, which makes it less electron-deficient. However, it still seeks to acquire an extra pair of electrons to achieve a full octet.

AlCl₃ and other aluminium halides can react with Lewis bases, such as ethers and amines, to form adducts. The ability to accept electrons makes these compounds useful as catalysts in organic synthesis, such as the Friedel-Crafts alkylation and acylation reactions.

Electron Configuration

The electron configuration of an atom describes the distribution of electrons in its atomic orbitals. For elements like boron (B) and aluminium (Al), their electron configurations in the ground state are pivotal to their chemical reactivity. Boron has the electronic configuration 1s² 2s² 2p¹, and aluminium is 1s² 2s² 2p⁶ 3s² 3p¹. From their configurations, it is clear that both elements have three valence electrons and need three more to complete their second (for boron) or third (for aluminium) shell.

When they form covalent bonds with halogens, they share their valence electrons but do not achieve an octet. The propensity to receive electrons to attain a stable configuration underlies their behavior as Lewis acids. This electron-accepting attribute plays a significant role in many chemical reactions and industrial applications involving these elements.

Octet Rule

The octet rule is a fundamental principle in chemistry stating that atoms tend to bond in such a way that they have eight electrons in their valence shell, giving them the same electronic configuration as a noble gas. This rule explains the stability associated with having a full valence shell. Exceptions to the octet rule occur, particularly with elements that are capable of having fewer (e.g., hydrogen) or more (e.g., sulfur in SF₆) than eight electrons.

Boron and aluminium halides demonstrate an interesting aspect of the octet rule. These compounds form with boron and aluminium only achieving six electrons in their valence shell through bonding with halides. This electron deficiency leads these elements to act as Lewis acids, adhering to the principle that atoms strive to reach an octet configuration when bonding. By accepting electron pairs from Lewis bases, these halides satisfy the octet rule and in turn, stabilize their electronic structure.