Dipole moment of CH2Cl2 and CHCl3 [duplicate]








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[Difference of dipole moments of dichloromethane and trichloromethane?](/questions/8977/difference-of-dipole-moments-of-dichloromethane-and-trichloromethane)

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Closed 6 years ago.



Why does $\ce{CH2Cl2}$ have a higher dipole moment than $\ce{CHCl3}$?


My argument being that the $\ce{CCl3-}$ ion being rather stable, can generate a good amount of polarity in the bond, moreover, shouldnt the dipoles of both $\ce{Cl}$ atoms cancel out?




As the dipolar moment is a vectorial property, you have to take into account *both* the number of chlorine atoms around the carbon atom, as well as their relative arrangement -- among them, as well in respect to the carbon atom. 


Lacking access to experimentally determined data about the molecular geometries,you may sketch the molecules in question, for example with [Avogadro](https://avogadro.cc/), and optimize the geometry (here, MMFF94 was deployed). You are literally able "to walk around" the molecule and its dipolar moment.


[![enter image description here](https://i.stack.imgur.com/EZre1.png)](https://i.stack.imgur.com/EZre1.png)


You easily will recognize a feature in common -- the similar direction of the resulting dipolar moment. Simultaneously, you will recognize the different angle defined by Cl-C-Cl, too. However, beside the angle, distances like C-Cl and C-H are affected by the different steric demand and electronic properties, too; equally influencing the *overall* dipole moment. Hence the molecule of dichloromethane is more polar than the one of chloroform.


Sidenote: a force field like MMFF94 is a computationally cheap approach to optimize molecular structures. There are better methods, but more costly; for the level of the question addressed, however, I thought it already provides the insight necessary here.


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In $\ce{CHCl3}$ the dipole moment of the $\ce{C-Cl}$ bond is towards $\ce{Cl}$. Since it has a tetrahedral geometry and the dipole moment is a vector quantity, the vector sum of all dipole moments would *try to* cancel out. As they are in the outward direction, they will cancel to *some extent*.
While in $\ce{CH2Cl2}$, the $\ce{C-H}$ bond has the dipole towards $\ce{C}$ and there are only two $\ce{C-Cl}$ bonds. So they wouldn't cancel out.


And talking about the $\ce{CCl3-}$ ion. The carbanion would form only in certain (extreme) conditions.


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