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

Click For Summary

Discussion Overview

The discussion revolves around the bond lengths of C-Cl in CF3Cl and CH3Cl, specifically examining why the bond length in CF3Cl is shorter than in CH3Cl despite the implications of Bent's rule. The scope includes theoretical reasoning and conceptual clarification regarding hybridization and electronegativity effects.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants reference Bent's rule, suggesting that more electronegative atoms increase p-character in their attached orbitals, which should lead to longer bond lengths.
  • One participant asserts that since CH3Cl has only chlorine as the electronegative atom, the C-Cl bond should have more p-character and thus a longer bond length compared to CF3Cl.
  • Another participant questions the relevance of the C-F bond's p-character to the C-Cl bond length, proposing that the partial positive charge on carbon in CF3Cl may affect the bond length differently.
  • Some participants argue that the increased electronegativity of fluorine in CF3Cl leads to a concentration of p-character in the C-F bond, thereby reducing the p-character in the C-Cl bond.
  • One participant clarifies that in CF3Cl, the p-character is decreased in the C-Cl bond due to the presence of more electronegative fluorine, which increases s-character in the C-Cl bond.
  • Confusion arises regarding the implications of fluorine's electronegativity on the bond lengths, with participants expressing uncertainty about how it affects the C-Cl bond in CF3Cl versus CH3Cl.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between electronegativity, p-character, and bond lengths, leading to an unresolved discussion with multiple competing perspectives on the topic.

Contextual Notes

Participants highlight limitations in their understanding of how p-character and electronegativity interact, with some acknowledging confusion about the implications of Bent's rule in this context.

Who May Find This Useful

This discussion may be of interest to those studying molecular chemistry, hybridization, and the effects of electronegativity on bond lengths.

Titan97
Gold Member
Messages
450
Reaction score
18
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?
 
Chemistry news on Phys.org
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.
 
  • Like
Likes   Reactions: vishaltr55 and Qwertywerty
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.
 
  • Like
Likes   Reactions: Titan97
  • #10
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 .
 
  • Like
Likes   Reactions: vishaltr55, Titan97 and gracy
  • #11
Did you understand @Titan 97?
 
  • Like
Likes   Reactions: Bystander and Titan97
  • #12
I understand now. Thank you.
 
  • #13
I understand now. Thank you.
 

Similar threads

Does Bond Length Always Equal Bond Strength?
  • · Replies 2 ·
Replies
2
Views
15K
Graduate Why quantum mechanics follows octet rule?
  • · Replies 15 ·
Replies
15
Views
11K