2
$\begingroup$

The bond angle $\ce{\angle(F-C-F)}$ of $\ce{CHF_3 > CH_2F_2}$ whereas that of $\ce{\angle(Cl-C-Cl)}$ is $\ce{CH_2Cl_2 > CHCl_3}$ why is this the case? Some explanations I read suggest the Bent's rule but I am not able to understand the idea well.

$\endgroup$
3
6
$\begingroup$

According to NIST CCCBDB, the $\angle(\ce{F-C-F})$ bond angles of $\ce{CH2F2}$ and $\ce{CHF3}$ are ${108.421 ^\circ}$ and $108.099 ^\circ$ respectively as calculated using CCSD=FULL/aug-cc-pVTZ.

Similarly, the $\angle(\ce{Cl-C-Cl})$ bond angles of $\ce{CH2Cl2}$ and $\ce{CHCl3}$ are ${112.329 ^\circ}$ and $109.992 ^\circ$ respectively as calculated using CCSD/aug-cc-pVTZ.

Both show a similar trend for bond angles unlike as mentioned in the question.

Now, this trend can be explained using Bent's rule which states:

Atomic s character concentrates in orbitals directed toward electropositive substituents

As the number of electronegative substituents increases, the p-character in each bonding orbital between $\ce{C-Cl}$ increases. This increase in p character leads to a decrease of the bond angle as observed here.

There is a similar decrease for $\ce{C-F}$ as seen above. The reason for the chloromethanes having a higher bond angle is due to steric effects. Chlorine has an A-value of 0.43 where fluorine has an A-value of 0.15. Therefore, the steric effect is more pronounced in the chloromethanes.

$\endgroup$
2
  • $\begingroup$ The book mentions $108^\circ6'$ for CHF3 and $108^\circ3'$ for CH2F2 whereas it is $111^\circ3'$ for CHCL3 and $111^\circ8'$ for CH2CL2 which is just so off that given on the website. Not sure where they might have taken the data from $\endgroup$ – Ashish Jun 9 at 7:44
  • 1
    $\begingroup$ @Ashish Convert the degrees into decimal or vice versa and you will find that both are nothing but equal. $\endgroup$ – Nisarg Bhavsar Jun 9 at 11:53

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.