Question

Among the halides of the elements of group-13 the one which is most acidic is (a) \(\mathrm{AlCl}_{3}\) (b) \(\mathrm{BBr}_{3}\) (c) \(\mathrm{BF}_{3}\) (d) \(\mathrm{BCl}_{3}\)

Short answer

(b) \(\mathrm{BBr}_{3}\) is the most acidic halide.

Step-by-step solution

Understanding the Concept of Acidity

Acidity in the case of halides generally refers to the Lewis acidity, which is the ability of the compound to accept an electron pair. In group-13 halides, the central atom (B, Al, etc.) typically have empty p-orbitals, making them Lewis acids.

Evaluating Boron Halides and Aluminum Halide

Among the given options, three are boron halides (
0 extrm{BBr_3}
1,  extrm{BF_3}
1,  extrm{BCl_3}
1) and one is an aluminum halide ( extrm{AlCl_3}
1). Boron halides tend to be more acidic than aluminum halides because boron is more electronegative and can better stabilize the negative charge that results from accepting an electron pair.

Comparing Halides of Boron

 extrm{BF_3}
1 is less acidic compared to  extrm{BCl_3}
1 and  extrm{BBr_3}
1 because fluorine, being highly electronegative, engages in strong  ext{p}
1- ext{p}
1 back bonding with boron, reducing its ability to accept electron pairs. Between  extrm{BCl_3}
1 and  extrm{BBr_3}
1, the size and lesser electronegativity of bromine make  extrm{BBr_3}
1 a stronger Lewis acid due to less effective back bonding.

Determining the Most Acidic Halide

Based on the ability to accept electrons (Lewis acidity),  extrm{BBr_3}
1 is the most acidic among the halides listed. It has the least effective back bonding, which makes it a better electron pair acceptor than the others.

Key concepts

Lewis Acidity

Understanding Lewis acidity is crucial in chemistry, especially for compounds such as halides of group 13 elements. Lewis acidity refers to the ability of a compound to accept an electron pair. Many molecules show Lewis acidity by having an empty orbital that can accommodate a pair of electrons from another substance. This often involves the central atom seeking to complete its electron octet. In the context of group 13 elements, compounds like boron halides often demonstrate strong Lewis acidity because their central atom typically lacks a full electron octet and has an empty p-orbital. Here, both the electronic configuration and the spatial availability of their empty orbitals contribute significantly to their acidic characteristics. This makes them capable of forming stable complexes with electron pair donors.

Halides of Group 13

Halides are compounds that contain one or more halogen atoms combined with another element. Group 13 halides include compounds such as aluminum halides and boron halides. These halides often exhibit interesting chemical behaviors due to their electron-deficient nature. The chemistry of group 13 halides largely revolves around their behavior as Lewis acids. These halides can attract electron pairs due to the presence of an empty p-orbital in the central atom. These elements, especially boron, show strong tendencies to form halides that have high Lewis acidity. Group 13 halides are important in various industrial applications. They are utilized in catalysis and materials science. Understanding their electron-accepting nature helps in manipulating reactions for desired outcomes.

Boron Halides

Boron halides such as \( ext{BBr}_3 \) , \( ext{BCl}_3 \) , and \( ext{BF}_3 \) are excellent examples of Lewis acids. Boron, due to its smaller size and greater electronegativity, forms halides that are more acidic than their aluminum counterparts. Among them:

• \( ext{BBr}_3 \) : This compound is less involved in p-p back bonding due to the larger size and less electronegative nature of bromine, making it the most acidic.
• \( ext{BCl}_3 \) : Chlorine has moderate electronegativity, leading to some p-p back bonding, which slightly reduces its acidity compared to bromo compounds.
• \( ext{BF}_3 \) : Fluorine is highly electronegative, resulting in strong p-p back bonding, which means less availability for electron pair acceptance, thus reducing acidity.

Boron halides are utilized in organic synthesis and other fields due to their ability to accept electron pairs and act as strong Lewis acids.

Electron Pair Acceptors

Electron pair acceptors are chemical entities capable of accepting a pair of electrons to form a coordinate bond. These are often Lewis acids, such as the halides of group 13, which play an essential role in various chemical reactions by rearranging electrons.The ability of a compound to act as an electron pair acceptor depends on several factors, including the electronegativity of the atoms involved, the size of halogen atoms, and the presence of empty orbitals. For example, the larger size and lesser electronegativity of bromine in \( ext{BBr}_3 \) leads to less effective back bonding, making it a better electron pair acceptor than other boron halides.Understanding how these acceptors operate allows chemists to predict reaction outcomes and design new materials with specific properties. Their wide usage in catalysis, synthesis, and material science highlights their importance in both academic and industrial processes.