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Fact or Myth? Temporary Dipole-Dipole Interactions are Weaker than Permanent Dipole-Dipole Interactions

posted Oct 25, 2016, 12:00 AM by Grace Ong   [ updated Oct 25, 2016, 12:03 AM ]
The following post was first posted on Blogger on Thursday, 12 June 2014.



A student sent me a message one evening asking me why phosphoryl chloride has a much lower melting point than phosphorus pentachloride.

Structure of Phosphoryl Chloride
(Source: Wikimedia Commons)

Structure of Phosphorus Pentachloride
phosphoryl chloride (POCl3) phophorus pentachloride (PCl5)
melting point = 1.25 °C melting point = 166.8 °C
Mr = 153.3 Mr = 208.2
dipole moment, μ = 2.54 D dipole moment, μ = 0 D

Since POCl3 has a large molecular dipole moment while PCl5, being a symmetrical molecule, has no net dipole moment, he reasoned that the permanent dipole-dipole interactions between POCl3 molecules would be stronger than the weak temporary dipole-dipole interactions between PCl5. As such, he expected the melting point of POCl3 to be higher than that of PCl5.

His deduction was unfortunately premised on a common misconception that temporary dipole-dipole interactions are much weaker than permanent dipole-dipole interactions. Therefore, in comparing the melting or boiling points of a polar and a non-polar substance, it is always the strength of permanent dipole-dipole interactions that predominates; temporary dipole-dipole interactions are too insignificant to be worthy of consideration.

But this is so untrue!

Let me now attempt to deconstruct this ‘myth’ by going back to where it all begins ...

  1. What is a dipole?

    A dipole simply refers to the separation of charges within a molecule.

    In this regard, other than the fact that one is of a permanent nature while the other is temporary, there is no intrinsic difference between a permanent dipole and a temporary dipole.

  2. How is a permanent molecular dipole produced?

    A permanently polar bond is created when two atoms bonded together have different electronegativities.

    For example, the P–Cl bonds in POCl3 and PCl5, and the P=O bond in POCl3 are all polar, since chlorine and oxygen are more electronegative than phosphorus.

    Possessing polar bonds, however, do not necessarily result in a polar molecule.

    The PCl5 molecule is non-polar as it has all its five polar P–Cl bonds symmetrically directed towards the corners of a trigonal bipyramid, thereby cancelling out opposing dipole moments.

    On the other hand, POCl3 is polar. Although POCl3 has one P=O bond and three P–Cl bonds directed symmetrically towards the corners of a tetrahedron, the polarity of the P=O bond is not the same as that of the P–Cl bond. Thus the molecule has a non-uniform distribution of electrons and a net molecular dipole moment results.

  3. What gives rise to permanent dipole-dipole interactions?

    Permanent dipole-dipole interactions, a.k.a. Keesom forces, are electrostatic forces of attractions between oppositely charged ends of polar molecules.

    These interactions therefore exist between POCl3 molecules but not between PCl5 molecules.

  4. How does molecular polarity affects the strength of permanent dipole-dipole interactions?

    Stronger permanent dipole-dipole attractions arise between molecules with larger dipole moments.

    (Based on arguments so far, it would seem that stronger intermolecular interactions exist between POCl3 molecules that those between PCl5 molecules. Do read on to find out why this is not the case!)

  5. How is a temporary molecular dipole produced?

    In theory, a symmetrical molecule like PCl5 should not have a distortion in electronic distribution that results in molecular polarity.

    PCl5 molecule with no polarity

    In reality, this is only true on average. If we were to take an instantaneous snapshot of the electronic distribution in a PCl5 molecule, you will likely find a δ+ charge developed on one end and a δ– on the other.

    molecule with dipole

    This is because electrons are not stationary and are constantly shifting about in the molecule. At any instant, the molecule may have more electrons on one end than the other, creating a temporary charge separation. An instantaneous dipole results.

    Note:
    Temporary dipoles do not just appear in molecules. They occur even in monatomic noble gases like helium and argon!

  6. What gives rise to temporary dipole-dipole interactions?

    instantaneous dipole-induced dipole interactions

    Molecule I, with its temporarily formed dipole, attracts electrons of adjacent Molecule II with its δ+ end, and repels electrons of Molecule III with its δ− end. This creates induced dipoles in both Molecules II and III.

    Electrostatic forces of attraction between these temporary dipoles give rise to intermolecular interactions that we call London dispersion forces.

  7. How does molecular size affects the size of temporary dipoles and hence the strength of temporary dipole-dipole interactions?

    With larger molecules come larger temporary dipoles. This is because larger molecules have more electrons than smaller ones. Moreover, electrons in a larger molecule have more space in which they can move about.

    And larger dipoles beget stronger intermolecular attractions.

    interactions between small and large dipoles

  8. When do permanent dipole-dipole interactions predominate over temporary dipole-dipole interactions?

    When we compare molecules of similar sizes, i.e. when they have similar temporary dipole-dipole attractions, the strength of permanent dipole-dipole interactions become the predominant factor in determining the melting and boiling points of these substances.

    For example,

    structure of trimethylamine

    structure of 2-methylpropane

    trimethylamine [(CH3)3N] 2-methylpropane [CH(CH3)3]
    Mr = 58.0 Mr = 59.0
    μ = 0.61 D μ = 0.132 D
    Boiling point = 2.9 °C Boiling point = −11.7 °C

  9. When do temporary dipole-dipole interactions predominate over permanent dipole-dipole interactions?

    When we compare molecules of different sizes, contributions from temporary dipole-dipole interactions become predominant in determining the relative strength of intermolecular interactions. This is evident in melting or boiling point trends which run counter to the trend in dipole moments.

    For example,

    Substance Mr μ / D Boiling Point / oC
    CH3Cl 50.5 1.87 −24.2
    CH3Br 94.9 1.80 3.56
    CH3I 141.9 1.60 42.4

    In these cases, the increase in strength of temporary dipole-dipole interactions as molecular size increases far outweighs the decrease in strength of permanent dipole-dipole interactions.

Going back to our original example of why POCl3 has a much lower melting point than PCl5, the explanation is as follows:
  • Permanent dipole-dipole interactions (in addition to temporary dipole-dipole interactions) exist between polar POCl3 molecules (μ = 2.54 D).
  • Only temporary dipole-dipole interactions exist between non-polar PCl5 molecules (μ = 0 D).
  • However POCl3 (Mr = 153.3) is a smaller molecule than PCl5 (Mr = 208.2), and thus possesses fewer electrons than the latter.
  • Larger dipoles created in PCl5 molecules give rise to temporary dipole-dipole interactions which are stronger than the permanent (and temporary) dipole-dipole interactions between POCl3 molecules.
  • Strength of temporary dipole-dipole interactions becomes the predominant factor in determining the relative melting points of POCl3 and PCl5.
  • Thus POCl3 has a lower melting point that PCl5.
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