Question

The structure of \(Al_{2}Cl_{6}\) is given below. The correct order of bond angles X, Y and Z is

• A. \(X>Y>Z\)
• B. \(Z>X>Y\)
• C. \(Y>X>Z\)
• D. \(Z>Y>X\)

Hint:

In bridged structures, the bridge-angle is always the smallest due to the rigid geometry of the cyclic structure, whereas terminal angles expand to minimize steric interference.

Answer 1

The Correct Option is B

Step 1: Picture the dimer.
Solid and vapour $AlCl_3$ exists as $Al_2Cl_6$. Two aluminium atoms are joined by two bridging chlorine atoms, making a small four membered ring. Each Al also holds terminal chlorines.
Step 2: Find which angle is the bridge angle.
Inside the small $Al_2Cl_2$ ring the angle (call it Y) is squeezed by the ring. A ring forces the angle to be smaller than the normal tetrahedral $109.5^\circ$.
Step 3: Look at the terminal angle.
The terminal Cl-Al-Cl angle (call it Z) is free of ring strain and is pushed wide by lone pair and bond repulsions, so it is the largest.
Step 4: Place the middle angle.
The angle X involving one terminal and one bridge bond lies between the two extremes.
Step 5: Order them.
So the size order is terminal largest, then mixed, then bridge smallest: $Z>X>Y$.
Step 6: State the answer.
\[ \boxed{Z>X>Y} \]