Q.14 The number of 2-center–2-electron bonds in anhydrous AlCl₃ is __________.

Introduction (SEO Optimized)

Questions related to electron-deficient compounds and types of chemical bonds are frequently asked in JEE, NEET, and other competitive chemistry examinations. Aluminium chloride (AlCl₃) is a classic example used to test understanding of Lewis acidity, covalent bonding, and molecular structure.

In this article, we determine the number of 2-center-2-electron (2c–2e) bonds present in anhydrous AlCl₃, using a clear structural and bonding approach suitable for exams.

 Key Concept Used

What is a 2-Center-2-Electron Bond?

• A 2-center-2-electron (2c–2e) bond is a normal covalent bond

• It involves:

  • Two atoms

  • Two shared electrons

• Example: single σ-bonds like C–H, Al–Cl (terminal)

 Structure of Anhydrous AlCl₃

Important Points:

• Aluminium (Al) has only 6 valence electrons after bonding

• Hence, AlCl₃ is electron-deficient

• In the solid or vapor state, anhydrous AlCl₃ exists as a dimer (Al₂Cl₆)

 Bonding in Al₂Cl₆ (Dimeric AlCl₃)

Bonds present:

1. Terminal Al–Cl bonds

   • Normal covalent bonds

   • Each is a 2-center-2-electron bond

2. Bridging Al–Cl–Al bonds

   • These are 3-center-4-electron bonds

   •  NOT counted as 2c–2e bonds

 Step-by-Step Bond Counting

Step 1: Write the structure of Al₂Cl₆

• Two aluminium atoms

• Six chlorine atoms

  • 4 terminal chlorines

  • 2 bridging chlorines

Step 2: Count terminal Al–Cl bonds

• Each terminal chlorine forms one normal covalent bond

• Number of terminal Al–Cl bonds = 4

Step 3: Identify bridging bonds

• Bridging bonds are Al–Cl–Al

• These are 3-center-4-electron bonds

•  Not included

 Final Answer

4​

👉 Number of 2-center-2-electron bonds in anhydrous AlCl₃ = 4

 Common Student Confusion (Exam Insight)

•  Counting all Al–Cl interactions gives 6 → WRONG

•  Bridging bonds are not 2c–2e

• ✔ Only terminal Al–Cl bonds are counted

 Quick Exam Tip

Whenever you see:

• AlCl₃

• Electron-deficient compound

• Anhydrous / dimeric form

 Always draw Al₂Cl₆ and count only terminal bonds for 2c–2e questions.

 Conclusion

Anhydrous aluminium chloride exists as a dimer (Al₂Cl₆) due to electron deficiency. In this structure, only the four terminal Al–Cl bonds are true 2-center-2-electron bonds, while the bridging bonds are multi-center bonds.

Understanding this distinction is crucial for solving bonding and structure questions accurately in competitive exams.