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If $\ce{CFCl3}$ has a higher molar mass, how come $\ce{CHCl3}$ has a higher boiling point?

$$ \begin{align} \mathrm{bp}(\ce{CFCl3}) &= \pu{23.77 °C} \\ \mathrm{bp}(\ce{CHCl3}) &= \pu{61.15 °C} \end{align} $$

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In addition to the poor polarizability of $\ce{C-F}$ bonds described here, we may consider hydrogen bonding. $\ce{CHCl3}$ has this to a greater extent than most chlorinated organic compounds because the electronegative $\ce{CCl3}$ function draws off the electron density in the bonding orbitals, leaving more of the unoccupied antibonding orbitals on hydrogen. We then have relatively good molecular orbital overlap between hydrogen in one molecule and chlorine in another. $\ce{CFCl3}$ does not offer this possibility.

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The main reason acting here is Dipole Moment. The higher the dipole moment of a molecule, the greater will be the intermolecular attractive forces and the higher will its boiling point be.

Dipole moment is the displacement of electron density in a molecule and it is a vector quantity. The net dipole moment of a molecule is the vector summation of all the individual dipole moment of 2 bonded atoms created by the electronegativity difference between them.

So, at first, let's get the structure of the molecules. Both of them are sp3 hybridised and have a tetrahedral structure.

the dipole moments of CHCl3 and CFCl3

  1. For CHCl3, if you consider the bond between carbon and chlorine, its highly polar. Due to the high electronegativity of chlorine, there will be a net negative pole on the bottom part caused by the three Carbon chlorine bonds. And Hydrogen being less electronegative,however, will not attract the electrons to a great extent. Hence, there will be a pretty significant net dipole moment in CHCl3.

  2. But in the case of CFCl3, yes, there will be a charge separation by the chlorine atoms, but on the upper portion lies the F atom which is very electro-negative. As a result, it will also pull its bond pair of electrons towards itself. Since, the net dipole moment is the vector summation of the individual dipoles, the charge separation of C-F, being in the opposite direction of the charge separation by the 3 chlorine atoms, will reduce the net dipole moment of the molecule.

For this reason, the dipole moment of CHCl3 is more than that of CFCl3 and so, CHCl3 has a higher boiling point.

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