Boron Halides
Although fluorine is more electronegative than chlorine, BF3 is a weaker Lewis acid than BCl3. The best reason is:
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About This Question
- Subject
- chemistry
- Chapter
- p-block elements
- Topic
- boron halides
- Difficulty
- Medium
- Year
- 2025
Solution
Correct Answer:
Effective p pi-p pi back bonding from F to B is stronger in BF3
In BF3, lone pair donation from fluorine (2p) into vacant 2p orbital of boron is very effective due to good size match. This back bonding reduces electron deficiency at boron and decreases Lewis acidity. In BCl3, 3p(Cl)-2p(B) overlap is poorer, so boron remains more electron-deficient and more acidic.
This medium difficulty chemistry question is from the chapter p-block elements, covering the topic of boron halides. It appeared in the 2025 exam.
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