Problem 112 - Chemistry The Science in Context

Question

When left at room temperature, mixtures of \(\mathrm{BF}_{3}\) and \(\mathrm{BCl}_{3}\) are found to contain significant amounts of \(\mathrm{BF}_{2} \mathrm{Cl}\) and BFCl \(_{2} .\) Using Lewis structures, explain the origin of the latter two compounds..

Step-by-Step Solution

Answer

Question: Explain the origin of the compounds BF2Cl and BFCl2 when mixtures of BF3 and BCl3 are left at room temperature. Answer: When mixtures of BF3 and BCl3 are left at room temperature, ligand exchange reactions occur between the two molecules. One Fluorine (F) ligand from BF3 may exchange with a Chlorine (Cl) ligand from BCl3, leading to the formation of BF2Cl. Similarly, two different Fluorine ligands from BF3 may exchange with two different Chlorine (Cl) ligands from BCl3, forming BFCl2.

Step 1: Drawing Lewis Structures of Reagents

First, draw the Lewis structures for \(\mathrm{BF}_{3}\) and \(\mathrm{BCl}_{3}\). Both elements, boron and fluorine/chlorine, have one Lone pair except fluorine with two lone pairs. For \(\mathrm{BF}_{3}\): Boron (B) is in Group 3, so it has 3 valence electrons. Fluorine (F) is in Group 7, so it has 7 valence electrons. Thus the Lewis structure of \(\mathrm{BF}_{3}\) looks like this: ``` F | B -- F \ F ``` For \(\mathrm{BCl}_{3}\): Chlorine (Cl) is also in Group 7, so it also has 7 valence electrons. Hence, the Lewis structure of \(\mathrm{BCl}_{3}\) is similar to \(\mathrm{BF}_{3}\) but with chlorine instead of fluorine: ``` Cl | B -- Cl \ Cl ```

Step 2: Explanation of BF\(_{2}\)Cl and BFCl\(_{2}\) formation

To explain the formation of the BF\(_{2}\)Cl and BFCl\(_{2}\) molecules, consider a reaction between one molecule of \(\mathrm{BF}_{3}\) and another molecule of \(\mathrm{BCl}_{3}\) in the mixture. During this reaction, a ligand exchange occurs: 1. One Fluorine (F) ligand from \(\mathrm{BF}_{3}\) may exchange with a Chlorine (Cl) ligand from \(\mathrm{BCl}_{3}\), leading to the formation of \(\mathrm{BF}_{2} \mathrm{Cl}\). The Lewis structure for \(\mathrm{BF}_{2} \mathrm{Cl}\) is given by: ``` F | B -- F \ Cl ``` 2. Similarly, one Fluorine (F) ligand from BF\(_{3}\) may exchange with a \(\mathrm{Cl}\) ligand from BCl\(_{3}\), leaving another \(\mathrm{Cl}\) ligand to exchange with a different F ligand from the same \(\mathrm{BF}_{3}\), resulting in the formation of \(\mathrm{BFCl}_{2}\). The Lewis structure for \(\mathrm{BFCl}_{2}\) is given by: ``` F | B -- Cl \ Cl ``` Now, you can see that the BF\(_{2}\)Cl and BFCl\(_{2}\) are formed due to the ligand exchange reactions between the \(\mathrm{BF}_{3}\) and \(\mathrm{BCl}_{3}\).

Key Concepts

Chemical Bonding

Chemical bonding is the force that holds atoms together in molecules, enabling the formation of stable chemical compounds. Bonds arise from the interaction between the outermost electrons of atoms, known as valence electrons. There are several types of chemical bonds, including ionic, covalent, and metallic bonds. In the example given,

Covalent Bonding in BF₃ and BCl₃

the bonds that form in molecules like BF₃ and BCl₃ are covalent. This occurs when atoms share pairs of electrons to fill their outer shells, striving for a more stable electronic configuration. Boron trifluoride (BF₃) and boron trichloride (BCl₃) both feature boron atoms bonding with three halogen atoms. Boron has three valence electrons and forms three covalent bonds, while fluorine and chlorine, with seven valence electrons each, form one covalent bond with boron, filling their octet to achieve stability. The absence of a full octet in boron makes these molecules electron-deficient, which is why they tend to react with other compounds.

Valence Electrons

Valence electrons play a pivotal role in chemical bonding as they are the electrons that interact to form chemical bonds. Located in the outermost shell of an atom, they determine an atom's bonding behavior. When atoms bond, they tend to follow the

Octet Rule

, which dictates that atoms are most stable when they have eight electrons in their valence shell, similar to noble gases.

In the example of BF₃ and BCl₃, drawing Lewis structures allows us to visualize the lone pairs and bonding pairs of electrons, reflecting the number of valence electrons. This is crucial for understanding how these molecules bond and react. Fluorine and chlorine each have seven valence electrons and need only one more to complete their octet, which they achieve by forming a covalent bond with boron. Boron, on the other hand, with three valence electrons, forms three bonds but still does not achieve an octet, making it a good candidate for further reactions.

Ligand Exchange Reactions

Ligand exchange reactions involve the replacement of one ligand (an atom, ion, or molecule that attaches to a central metal atom) with another ligand. These types of reactions are common in coordination chemistry where complex ions are present. In our example concerning BF₃ and BCl₃,

ligand exchange reactions between molecules

result in the formation of BF₂Cl and BFCl₂.

The origin of these compounds can be understood by recognizing that boron's electron deficiency makes it susceptible to reactions where it can attain additional electrons. Ligand exchange, in this case, is driven by the opportunity for BF₃ and BCl₃ to mix and redistribute their ligands (F or Cl) to create new compounds with slightly different electron distributions. The ability of Lewis structures to depict such exchanges graphically makes this concept easier to comprehend for students.