Question

Explain the change in bonding when \(\mathrm{Al}_{2} \mathrm{Cl}_{6}\) dissociates to form \(\mathrm{AlCl}_{3}\) in the gas phase.

Short answer

Dissociation breaks coordinate covalent bonds in \(\mathrm{Al}_{2}\mathrm{Cl}_{6}\), forming individual \(\mathrm{AlCl}_{3}\) units with normal covalent bonds.

Step-by-step solution

Understanding the Compound

Aluminium chloride (\(\mathrm{AlCl}_{3}\)) can exist as a dimer, \(\mathrm{Al}_{2}\mathrm{Cl}_{6}\), in which two \(\mathrm{AlCl}_{3}\) units are linked. At room temperature and in the solid state, it often exists as \(\mathrm{Al}_{2}\mathrm{Cl}_{6}\), which is stabilized by coordinate covalent bonds.

Analyzing the Structural Change

When \(\mathrm{Al}_{2}\mathrm{Cl}_{6}\) is heated to produce gaseous \(\mathrm{AlCl}_{3}\), the dimer dissociates into two \(\mathrm{AlCl}_{3}\) molecules. This involves breaking the coordinate covalent bonds that link the two \(\mathrm{AlCl}_{3}\) units in \(\mathrm{Al}_{2}\mathrm{Cl}_{6}\).

Describing Bonding Changes

In the gaseous state, the coordinate covalent bonds in \(\mathrm{Al}_{2}\mathrm{Cl}_{6}\) that contributed to the dimer formation no longer exist. The result is two individual \(\mathrm{AlCl}_{3}\) molecules, each with three standard covalent bonds where the aluminum atom shares electrons with each of the three chlorine atoms.

Concluding the Effect of Dissociation

The dissociation alters the nature of the bonding. The transition from dimer to monomer leaves aluminum in each \(\mathrm{AlCl}_{3}\) molecule three-coordinated with chlorine through typical covalent bonds rather than the additional bonds seen in the dimeric state.

Key concepts

Aluminium Chloride

Aluminium chloride is a chemical compound with the formula \( \mathrm{AlCl}_{3} \). It is an important industrial chemical, commonly encountered in its anhydrous form, where it appears as a white crystalline solid. This compound showcases interesting behavior.

• At room temperature, aluminium chloride can exist as a dimer \( \mathrm{Al}_{2}\mathrm{Cl}_{6} \).
• These dimers are created when two \( \mathrm{AlCl}_{3} \) units join together.
• It's widely used in various chemical reactions like Friedel-Crafts reactions due to its role as a Lewis acid.

Understanding the nature of \( \mathrm{AlCl}_{3} \) helps appreciate its behavior, especially in different states such as solid and gaseous forms.

Dimer Dissociation

Dimer dissociation refers to the process where a dimeric molecule, like \( \mathrm{Al}_{2}\mathrm{Cl}_{6} \), breaks down into two identical molecules, in this case, \( \mathrm{AlCl}_{3} \).

• In \( \mathrm{Al}_{2}\mathrm{Cl}_{6} \), coordinate covalent bonds stabilize the dimer structure.
• Upon heating, these bonds break, leading the compound to dissociate.
• This results in two separate \( \mathrm{AlCl}_{3} \) molecules in the gas phase.

This transformation emphasizes the difference in bonding structures which impact the behavior and properties of the compound.

Coordinate Covalent Bond

A coordinate covalent bond is a type of bonding where both electrons in a bond pair come from one atom. This is different from a regular covalent bond where each atom contributes an electron.

• In \( \mathrm{Al}_{2}\mathrm{Cl}_{6} \), coordinate covalent bonds help link two \( \mathrm{AlCl}_{3} \) units together.
• The aluminium atom in one \( \mathrm{AlCl}_{3} \) unit donates a pair of electrons to form a bond with the chloride from another unit.
• Such bonds are crucial for the stability of dimeric structures.

Understanding coordinate covalent bonds is important because they provide insight into how complex structures like dimers form and stabilize.

Covalent Bond

Covalent bonds are fundamental types of chemical bonds where atoms share pairs of electrons. This sharing allows atoms to achieve a more stable electronic configuration.

• In \( \mathrm{AlCl}_{3} \), each aluminium atom forms three covalent bonds with three chlorine atoms.
• Each bond involves sharing one electron from aluminium and one from chlorine, creating a stable electronic arrangement.
• Covalent bonds are essential for forming stable molecules in most organic and inorganic compounds.

Understanding covalent bonds helps explain the stability and formation of molecular structures and their changes during chemical reactions.