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Fluoromethanes and their boiling points

The fluoromethanes are a class of substances based on methane, CH4. When we replace one, two, three, or all the hydrogen atoms of methane with fluorine, we get mono-, di-, tri-, and tetrafluoromethane.

Methane and tetrafluoromethane have low boiling points

In methane, individual dipoles exist between every hydrogen atom and the central carbon atom due to their electronegativity difference. Still, methane has no net dipole as its geometry causes the dipoles to cancel each other out. The same is the case for tetrafluoromethane, CF4, where all the hydrogen atoms have been replaced with fluorine. As a result, both CH4 and CF4 have low boiling points, as there are no permanent dipoles to cause an attraction between molecules.

Net permanent dipoles increase the boiling point

For CH3F and CH2F2, and CHF3 net molecular dipoles will form. Net dipoles form because partially replacing H with the more electronegative F means that the dipoles will no longer cancel each other out. The presence of these net dipoles will increase the boiling point of these compounds. The boiling point will be highest for CH2F2, because all its individual dipoles contribute significantly to the net dipole.

Two ball and stick molecules are shown. The left image shows a methane molecule, so a central black circle bonded to four, significantly smaller, white circles. One white circle is pointing upwards in the plane of the paper, two others are bonded to the base of the black circle and pointing out of the plane of paper. The final white circle is bonded to the centre of the central circle and is pointing into the plane of the paper. For all four of the bonds, a red arrow points towards the central circle. The right image shows a diflouromethane molecule. A central, black circle is bonded to two, significantly smaller, white circles and two, moderately smaller, yellow circles. The white circles are bonded 180 degrees apart at the base of the central circle, pointing out and into the plane of the paper. The remaining yellow circles are bonded at the top of the central atom, 180 degrees apart, and pointing out and into the plane of the paper. The pair of yellow circles are positioned at a 90 degree tilt from the white circles. For the white circle to black circle bonds short red arrows point to the central circle. While, for the yellow and black circle bonds a slightly long red arrow points towards the yellow circles. A red arrow starting from the bottom white circles and ending at the top, yellow atoms is labelled as the Net dipole.

Figure 1: Methane, CH4, has the lowest boiling point of the fluoromethanes, because it has no net molecular dipole. Difluoromethane, CH2F2, has the highest boiling point of the fluoromethanes, as all the individual dipoles contribute strongly to the net dipole.

CHF3 and CH3F have very similar boiling points. Even though we would expect CHF3 to have a higher boiling point due to stronger individual dipoles, the slightly distorted geometry of the compound places the dipoles antagonistically, which partially cancels them out.

Referred from:

Article
Permanent dipole-dipole interactions