JEE MAIN - Chemistry (2008 - No. 27)
The bond dissociation energy of B - F in BF3 is 646 kJ mol-1 whereas that of C - F in CF4 is 515 kj mol-1. The correct reason for higher B - F bond dissociation energy as compared to that of C - F is
stronger $$\sigma$$ bond between B and F in BF3 as compared to that between C and F in CF4
significant $$p\pi - p\pi$$ interaction between B and F in BF3 whereas there is no possibility of such interaction between C and F in CF4
lower degree of $$p\pi - p\pi$$ interaction between B and F in BF3 than that between C and F in CF4
smaller size of B - atom as compared to that of C- atom.
Explanation
In BF3, B is sp2
hybridised and by Back Bonding method strong p$$\pi $$-p$$\pi $$ bond is created between filled p-orbital of F and vacant p-orbital of
B which leads to shortening of B–F bond length which results in higher bond dissociation energy of
the B–F bond. However in CF4, C does not have any vacant
p-orbitals to undergo $$\pi $$-bonding.
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