Bent's rule for bond length

  • #1
Titan97
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Main Question or Discussion Point

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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  • #2
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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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  • #3
Titan97
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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 dont know "where" i am going wrong)
 
  • #4
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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.
 
  • #5
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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.
 
  • #6
Titan97
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So what about:
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.
 
  • #7
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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.
 
  • #8
Titan97
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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.
 
  • #9
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"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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  • #10
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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
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Did you understand @Titan 97?
 
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  • #12
Titan97
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I understand now. Thank you.
 
  • #13
Titan97
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I understand now. Thank you.
 

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