# Comparison of C-Cl bond length in CH3Cl and CF3Cl

Topic: Chemical Bonding (Application of Bent's Rule)

Bent's rule is also consistent with Gillespie's VSEPR model, and may provide alternative rationalisation for effect of electronegativity. So it is restated as: 'more electronegative atom not only prefers to stay in the orbital having more $$\mathrm{p}$$ character but also can increase the $$\mathrm{p}$$ character in its attached orbital from the central atom depending on the circumstance.'

With increase in $$\mathrm{p}$$ character in an orbital, bond length will increase while with increase in $$\mathrm{s}$$ character in an orbital, bond length will decrease. For example, $$d_\ce{(C-Cl)}$$ in $$\ce{CH3Cl}$$ ($$\pu{1.78 \unicode{xC5}}$$) $$\gt d_\ce{(C-Cl)}$$ in $$\ce{CF3Cl}$$ ($$\pu{1.75 \unicode{xC5}}$$).

Source: Concise Inorganic Chemistry for JEE (by J.D. Lee and Sudarshan Guha)

I understand this alternative statement to Bent's rule. Can anybody explain how have the $$\ce{C-Cl}$$ bond lengths been compared here? I don't get this. I understand that fluorine is more electronegative than hydrogen and might induce small positive charge on the carbon atom. Also, the $$\ce{C-F}$$ bond will have more $$\mathrm{p}$$ character than the $$\ce{C-H}$$ bond. How do I proceed further to reach the conclusion?

If you compare $$\ce{H3CCl}$$ and $$\ce{F3CCl}$$, then the $$\ce{C-H}$$ bond has less carbon-p character than an idealised sp³ orbital, while the $$\ce{C-F}$$ bond will have more carbon-p character than the idealised sp³ orbital. This in turn means that in $$\ce{H3CCl}$$ the $$\ce{C-Cl}$$ bond will have more carbon-p character as an idealised sp³ orbital and is therefore longer, while in $$\ce{F3CCl}$$ the $$\ce{C-Cl}$$ bond will have less carbon-p character and will therefore be shorter.