Molecular polarity describes how electrical charge is distributed within a molecule, dictating many of a substance’s physical and chemical properties. A polar molecule has a net separation of positive and negative charge, while a nonpolar molecule has perfectly balanced charges. Determining the polarity of aluminum chloride (\(\text{AlCl}_3\)) requires examining its bonding and molecular geometry. The structure of \(\text{AlCl}_3\) changes depending on its physical state, which complicates its classification.
Understanding Bond Polarity and Molecular Polarity
The analysis of a molecule’s overall polarity begins by considering the individual bonds. A bond’s polarity is determined by the difference in electronegativity between the two bonded atoms. In \(\text{AlCl}_3\), chlorine (electronegativity \(\approx 3.16\)) is significantly more electronegative than aluminum (\(\approx 1.61\)). This difference means the electrons are not shared equally, creating polar covalent \(\text{Al}-\text{Cl}\) bonds.
The chlorine atoms pull electron density toward themselves, acquiring a slight negative partial charge (\(\delta-\)), while the aluminum atom acquires a slight positive partial charge (\(\delta+\)). This unequal sharing results in a bond dipole moment pointing toward the chlorine atom.
The presence of polar bonds does not guarantee a polar molecule. Molecular polarity is determined by the net dipole moment, which is the vector sum of all individual bond dipole moments. If the individual bond dipoles cancel each other out due to the molecule’s symmetry, the net dipole moment is zero, and the molecule is nonpolar.
Geometry and Symmetry: The \(\text{AlCl}_3\) Monomer
The \(\text{AlCl}_3\) monomer typically exists in the gas phase at very high temperatures. Its structure is predicted using Valence Shell Electron Pair Repulsion (VSEPR) theory. Aluminum, the central atom, is bonded to three chlorine atoms and has no lone pairs of electrons, corresponding to an \(\text{AX}_3\) geometry.
The VSEPR model predicts a perfectly symmetrical trigonal planar molecular geometry. In this shape, the three chlorine atoms lie on the same plane as the central aluminum atom, with an ideal bond angle of \(120^\circ\). This high degree of symmetry is crucial for determining polarity.
Since the three \(\text{Al}-\text{Cl}\) bonds are chemically identical, they have equal bond dipole moments. These three equal dipole vectors are oriented \(120^\circ\) apart in a flat plane. When mathematically summed, the vectors perfectly cancel each other out. Therefore, the net dipole moment of the \(\text{AlCl}_3\) monomer is zero, classifying the gaseous molecule as nonpolar.
The Real World: Dimerization and Ionic Character
The \(\text{AlCl}_3\) monomer is nonpolar, but aluminum chloride rarely exists in this form under standard conditions. Its behavior changes drastically depending on its physical state and temperature. At room temperature, solid anhydrous aluminum chloride exists as an ionic-like lattice structure, not as discrete molecules.
In the solid state, aluminum atoms are surrounded by six chlorine atoms in an octahedral coordination geometry, forming a layered crystal structure. This structure has significant ionic character. However, \(\text{AlCl}_3\) sublimes at a relatively low temperature of about \(178^\circ\text{C}\), unlike purely ionic compounds.
Upon melting or vaporization at moderate temperatures, the structure converts into the dimer, \(\text{Al}_2\text{Cl}_6\). The dimer forms because the electron-deficient aluminum atom seeks to complete its octet. It does this by forming coordinate covalent bonds using lone pairs from two chlorine atoms of a neighboring \(\text{AlCl}_3\) unit.
The \(\text{Al}_2\text{Cl}_6\) Dimer
The resulting \(\text{Al}_2\text{Cl}_6\) molecule features two bridging chlorine atoms connecting the two aluminum centers. In this dimeric structure, each aluminum atom is surrounded by four chlorine atoms in a tetrahedral geometry. Although the overall \(\text{Al}_2\text{Cl}_6\) molecule remains nonpolar due to its symmetrical arrangement, its complex nature defies a single classification, existing as an ionic solid, a covalent dimer, or an ionic species in aqueous solution.
Why Polarity Influences Chemical Behavior
A molecule’s polarity significantly influences its chemical behavior, particularly solubility and reactivity. The principle of “like dissolves like” dictates that nonpolar substances dissolve in nonpolar solvents, and polar substances dissolve in polar solvents. The nonpolar nature of the \(\text{AlCl}_3\) monomer and the covalent character of the \(\text{Al}_2\text{Cl}_6\) dimer allow aluminum chloride to dissolve readily in nonpolar organic solvents.
This solubility is crucial for its function as a catalyst in organic synthesis reactions. Anhydrous aluminum chloride is a powerful Lewis acid, meaning it can accept an electron pair. This property is utilized extensively in reactions such as the Friedel-Crafts alkylation and acylation. Its ability to function in nonpolar environments distinguishes it from many simple ionic metal halides, making it a versatile reagent.