How can one compare the $\ce{C-Cl}$ bond lengths in chloromethane ($\ce{CH3Cl}$) and chlorotrifluoromethane ($\ce{CF3Cl}$)?

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?


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The paragraph you cite does not define Bent's rule (see What is Bent's rule?, Utility of Bent's Rule - What can Bent's rule explain that other qualitative considerations cannot?).

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.

Note of caution: I have read many questions about the concepts introduced in J. D. Lee's Concise Inorganic Chemistry; many of which revolve around outdated theories. I do not have a copy of that book, but from what I have experienced so far, I do not think it is a good book. Alas, in this instance it appears to be correct though.

  • $\begingroup$ Thanks for the explanation! Regarding the note of caution: Even worse, mine is an adapted and highly modified version of the original book. I'm using it just because it is advised by my teachers for JEE (Joint Entrance Exam) preparation. $\endgroup$ Oct 30, 2019 at 16:39
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    $\begingroup$ @SatvikPopli As a former JEE aspirant,I can tell you one thing,that many times the stuff we employ for JEE purposes in chemistry is either not the complete truth,or is a mutiliated version of the original explanation so that you can solve a question. So best to accept that people on this platform know better :) $\endgroup$ Nov 25, 2019 at 18:20

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