The unexpected order of acidic strength of the trihalides of boron
can best be explained by.

Options

A. `p pi-p pi` back bonding
B. Hybridisation
C. Trigonal planar structure
D. None of the above

Answer

To explain the unexpected order of acidic strength of the trihalides of boron, we need to analyze the factors that influence the acidity of these compounds. The trihalides of boron include BF3, BCl3, BBr3, and BI3. The order of acidic strength is observed as BI3 > BBr3 > BCl3 > BF3. Here’s a step-by-step explanation:

### Step 1: Understanding Acidic Strength
Acidic strength in this context refers to the ability of the compound to donate a proton (H+) or accept an electron pair. In Lewis acid terms, a stronger Lewis acid can accept electron pairs more readily.

### Step 2: Role of Back Bonding
The unexpected order of acidic strength can be explained by the concept of back donation or back bonding. In trihalides of boron, the halogen atoms can donate their lone pairs of electrons to the empty p-orbital of boron through pπ-pπ bonding.

### Step 3: Analyzing the Halogen Atoms
The ability of halogen atoms to back donate their lone pairs varies:
- **Fluorine (F)** has a small size and a high electronegativity, which allows it to effectively overlap with boron’s p-orbital. However, this strong back donation reduces the electron deficiency of boron, making BF3 a weaker Lewis acid.
- **Chlorine (Cl)**, **Bromine (Br)**, and **Iodine (I)** have larger atomic sizes, which decreases the extent of overlap with boron’s p-orbital. As the size increases from Cl to I, the effectiveness of back donation decreases, leading to an increase in the electron deficiency of boron.

### Step 4: Order of Acidity
As a result of the above factors:
- **BI3** is the strongest Lewis acid because the larger Iodine atom has less effective back donation, leading to a higher electron deficiency in boron.
- **BBr3** follows, as Bromine is larger than Chlorine but smaller than Iodine.
- **BCl3** is next, as Chlorine still has some back donation but less than Bromine.
- **BF3** is the weakest Lewis acid due to significant back donation from the small and highly electronegative Fluorine atoms.

### Conclusion
The unexpected order of acidic strength of boron trihalides can be best explained by the tendency of halogen atoms to back donate their lone pairs to boron through pπ-pπ bonding. As the size of the halogen increases, the back donation decreases, leading to an increase in the acidity of the boron trihalides.