Thursday, February 14, 2019

acid base - Which molecule is more acidic: CHF3 or CHBr3?


I know that fluorine is more electronegative than bromine. However, because of the size of bromine, it is more stable with a negative charge. In the case of HF vs. HBr, to me, HBr is without a doubt the stronger acid. When it comes to comparing CHFX3 with CHBrX3, we are supposed to compare the stability of their conjugate bases.


In CFX3X, the fluorine can hold the electronegative charge quite well. It also exerts a strong inductive effect.


In CBrX3X, the bromine is less electronegative than the fluorine but is more polarizable, so I think it can hold the negative charge more efficiently.


With this understanding, I am concluding that CBrX3X is more stable which would mean CHBrX3 is the stronger acid. However, I've been told otherwise.


I'm aware that the CBr bond length is longer than the CF bond length. I'm wondering if that short bond length causes CFX3X to be more stable than CBrX3X. If this is true, is it okay for me to think about CFX3X dissociating vs. CBrX3X dissociating? In that scenario, I believe CBrX3X dissociates more easily than CFX3X, meaning CFX3X is more stable.



Note: This logic wouldn't apply to the instance of HOX vs. HSX because HSX is more stable than HOX (even though HO bond length is shorter than HS bond length).




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