AlCl₃ — JEE/NEET Facts

  1. Lewis acid
    • Al has only 6 electrons around it in monomeric AlCl₃.
    • Therefore, AlCl₃ is an electron-deficient compound and acts as a Lewis acid.
  2. Dimerisation
    • AlCl₃ exists as Al₂Cl₆ (especially in vapour/organic solvents under suitable conditions).Two AlCl₃ units are joined through two bridging Cl atoms.
    AlCl₃ ⇌ Al₂Cl₆
  3. Hybridisation in AlCl₃
    • Monomeric AlCl₃ → sp², trigonal planar.
    • Al₂Cl₆ → each Al is approximately sp³ because of the two bridging Cl bonds

Most common JEE question

Q. The geometry and hybridisation of Al in monomeric AlCl₃ are:

A. Linear, sp
B. Trigonal planar, sp²
C. Tetrahedral, sp³
D. Pyramidal, sp³

Answer: B — Trigonal planar, sp².

Shortcut:
AlCl₃ = electron deficient → Lewis acid → dimerises to Al₂Cl₆.

so in di bridge structure two cordinate bonds

Yes. In the dimer Al2​Cl6​, there are two bridging Cl atoms, and the bonding is commonly represented with two coordinate (dative) bonds.

Al2​Cl6​​ structure

  • Each AlCl₃ initially has 3 Al–Cl bonds.
  • One Cl atom from each AlCl₃ donates a lone pair to the electron-deficient Al of the other unit.
  • Thus, 2 bridging Cl atoms → 2 coordinate bonds.

Important: The two coordinate bonds are often drawn as Al←Cl. After formation, the Al–Cl bridging bonds can have considerable covalent character; the arrow mainly indicates the origin of the electron pair.

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