Why is the Bond Length of CF3Cl Shorter than CH3Cl Despite Bent's Rule?

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SUMMARY

The bond length of C-Cl in CF3Cl (1.75 Å) is shorter than in CH3Cl (1.78 Å) due to the influence of electronegativity and hybridization as described by Bent's rule. In CF3Cl, the sp3 hybridized carbon has increased p-character in the C-F bonds, which decreases the p-character in the C-Cl bond, leading to a shorter bond length. Conversely, in CH3Cl, the chlorine is the only electronegative atom, resulting in a higher p-character in the C-Cl bond, thus increasing its bond length. This relationship between p-character and bond length is crucial for understanding molecular geometry and bond dynamics.

PREREQUISITES
  • Bent's Rule
  • Hybridization concepts (sp3 hybridization)
  • Electronegativity and its effects on bond lengths
  • Understanding of bond character (s-character vs. p-character)
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  • Study the implications of Bent's Rule in molecular geometry
  • Explore the relationship between hybridization and bond lengths in various compounds
  • Investigate the effects of electronegativity on bond character and molecular stability
  • Learn about the differences in bond lengths across halogenated hydrocarbons
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Chemistry students, molecular chemists, and researchers interested in bond dynamics and molecular structure analysis will benefit from this discussion.

Titan97
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Taken from 'Concise Inorganic Chemistry - J.D.Lee': "According to Bent's rule, more electronegative atom not only prefers to stay in the orbital having more p-character but can also increase the p-character in its attached orbital of the central atom depending on the circumstance... With an increase in p-character, bond length increases..".
So shouldn't dC-Cl of CF3Cl > dC-Cl of CH3Cl?
But dC-Cl in CH3Cl=1.78 A° and dC-Cl in CF3Cl = 1.75 A°. Why is it opposite to my conclusion?
 
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Titan97 said:
dC-Cl in CH3Cl=1.78 A° and dC-Cl in CF3Cl = 1.75 A°.
I think it is correct.
Bent's rule is that in a molecule, a central atom bonded to multiple groups will hybridise so that orbitals with more s character are directed towards electropositive groups, while orbitals with more p character will be directed towards groups that are more electronegative.
.
Because in CH3Cl ,C-Cl bond will have more p character than in CF3Cl. Because cl is the only electronegative atom in CH3Cl but in CF3Cl Fluorine is more electronegative.Ans as p character increases bond length also increases.Hence dC-Cl in CH3Cl is more than dC-Cl in CF3Cl.
 
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gracy said:
but in CF3Cl Fluorine is more electronegative
What does the increased p-character in C-F bond have to do with the bond length of C-Cl bond? Does fluorine affect the electronegativity difference between Carbon and Chlorine? What I think is, a partial positive charge appears on carbon in CF3Cl of a graeter magnitude than that in CH3Cl and Electronegativity difference between C and Cl is more in CF3Cl. So p-character is more in CF3Cl and hence bond length is reverse of the calculated bond length.
(I know what I am saying is wrong but i don't know "where" i am going wrong)
 
Titan97 said:
What does the increased p-character in C-F bond have to do with the bond length of C-Cl bond?
It is about C-Cl bond,as data given is of C-Cl bond.
 
As p character in C-Cl is more in CH3Cl than in CF3Cl, dC-Cl in CH3Cl is more than dC-Cl in CF3Cl.
Because p character ∝ bond length.
 
So what about:
Titan97 said:
What I think is, a partial positive charge appears on carbon in CF3Cl of a graeter magnitude than that in CH3Cl and Electronegativity difference between C and Cl is more in CF3Cl. So p-character is more in CF3Cl and hence bond length is reverse of the calculated bond length.
 
It's not about electronegativity difference.It's about electronegativity of a particular atom or group.Greater P character is conferred to the bond of central atom with the most electronegative atom or group of a compound which is C-Cl bond in CH3Cl but not C-Cl bond of CF3Cl.
 
This is from your post: "Fluorine is more electronegative.Ans as p character increases bond length also increases.Hence dC-Cl in CH3Cl is more than dC-Cl in CF3Cl."
You say that "Fluorine is more electronegative...Hence dC-Cl in CH3Cl is more than dC-Cl in CF3Cl". If fluorine is more electronegative, then C-F bond length increases. But you are talking about C-Cl bond using E.N of fluorine.
I am sorry. But I am confused.
 
Titan97 said:
"Fluorine is more electronegative...Hence dC-Cl in CH3Cl is more than dC-Cl in CF3Cl".
Fluorine is more electronegative so p character would be concentrated in C-F bonds so bond C-Cl would not have high percentage of p character as in C-Cl bond of CH3Cl because in CH3Cl Cl was the only electrnegative element therefore p character would be concentrated in C-Cl bond.
 
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Titan97 said:
If fluorine is more electronegative, then C-F bond length increases. But you are talking about C-Cl bond using E.N of fluorine.
I am sorry. But I am confused.
CF3Cl consists of an sp3 hybridised carbon . By Bent's rule , p-character will increse in the F-C bonds , decreasing p-character in Cl-C bond , or , effectively , increasing s-character in Cl-C bond .

In CH3Cl on the other hand , Cl-C bond has a higher p-character . Now , p-character corresponds to longer bond length as compared to s-character . Hence , the latter has greater bond length .

Hope this helps .
 
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  • #11
Did you understand @Titan 97?
 
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I understand now. Thank you.
 
  • #13
I understand now. Thank you.
 

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