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In this answer to the question "Why is the dipole moment of chloromethane larger than the dipole moment of fluoromethane?", the order of the dipole moments was given as $\ce{CH3Cl > CH3F > CH3Br > CH3I}$, with an explanation that bond length is also a contributing factor; specifically, electronegativity dominates for $\ce{Br/I}$ and bond length/order dominates for $\ce{F/Cl}$.

  • Can more detail be given as to why there is this division between $\ce{F/Cl}$ and $\ce{Br/I}$?
  • In general, is there a method to determine which effect (electronegativity or bond length/order) is more important for comparing the magnitude of dipole moments?
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  • $\begingroup$ These anomalies usually happen in compounds having period 2 elements. $\endgroup$
    – Bastardane
    Mar 11, 2018 at 15:43
  • $\begingroup$ Are you referring to introduction of d-orbitals? If yes, how does that help here? $\endgroup$
    – Yusuf Hasan
    Mar 11, 2018 at 16:40
  • $\begingroup$ Not d-orbitals in this case, but the small size of flourine atom causing interelectronic repulsions among its own electrons. $\endgroup$
    – Bastardane
    Mar 14, 2018 at 18:27

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I need help understanding, in that trend specifically, as to when the length factor dominates, and when the electronegativity factor does?

There is nothing much to understand here. All you need to understand is that this is basically an experimental observation.

We are trying to use qualitative arguments (higher bond length/more electronegativity) to explain the actually calculated values. When we observed a contradiction in our answer for fluoromethane and chloromethane, we corrected our reasoning, by saying that the bond length dominated over the electronegativity difference in this case. And that is true, because this correction is based on the calculated values only.

So, we first try to predict the order by our own qualitative arguments (logically reasoned, of course). If they work, great! If they don't, we look into the factors that we originally deemed to be minor instead. We calculate their values, and see what effect they had. Then, we justify those minor factors by saying that they dominated over the other in the particular case.

Another such interesting case is the order of the basicity of ammonia derivatives in water. It is $\ce{NEt2>NEt3>NEt1}$ (for the ethyl derivatives) and $\ce{NMe2>NMe1>NMe3}$ (for the methyl derivatives). Here, three factors are operating simultaneously! The solvation effect, the steric hindrance of alkyl groups, and the positive inductive effect of alkyl groups - all either reinforcing or hindering each other. In such edge cases, it becomes extremely tough to predict any order solely based on these factors, and hence, computated values are much preferred.

If you prefer to read the theory of those experiments (be warned it's scary), here is one such paper discussing the detailed calculation of the dipole moments of various fluoromethane derivatives.

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  • $\begingroup$ So is there no logical way to explain this? $\endgroup$
    – Yusuf Hasan
    Mar 12, 2018 at 7:51
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    $\begingroup$ @YUSUFHASAN The point of my answer was that there are logical qualitative arguments, but they cannot predict the order of the calculated values with 100% accuracy. You can only guess which factors would be minor and which major beforehand. If the calculated data doesn't agree with your major factor, you would have to say that the minor factor dominated here instead. And that is true, if you would consult the values, you would find that the minor factor did dominate. $\endgroup$
    – Gaurang Tandon
    Mar 12, 2018 at 7:54

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