They have very similar structure except for a central atom, but the difference in boiling points is very large $(\pu{186 ^\circ C}$ for DMSO and $\pu{56 ^\circ C}$ for acetone).

How is central atom impacting the boiling point?

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    $\begingroup$ Checkout the geometry of both molecules and also identify the polar bonds. Now figure out how dipole moment and boiling points relate. $\endgroup$ – Robin Singh Sep 7 '20 at 12:48
  • $\begingroup$ thanks @RobinSingh for the help $\endgroup$ – Maxwell Sep 7 '20 at 13:33

Firstly, sulfur has a larger atomic radius than carbon, so we would expect DMSO (Ddimethyl sulfoxide) to have a much larger and thus polarisable electron cloud than acetone. The London dispersion forces between DMSO are thus stronger, causing DMSO to have a higher boiling point.

Secondly, if we compare the dipole moments of the $\ce{S=O}$ and $\ce{C=O}$ bonds, the $\ce{S=O}$ bond is much more polarised. This is because the $\mathrm{3p-2p}$ overlap between $\ce{S}$ and $\ce{O}$ is much less favourable than the $\mathrm{2p-2p}$ overlap between $\ce{C}$ and $\ce{O}$. As such, the resonance structure on the right is actually the more significant resonance structure. This makes the $\ce{S=O}$ bond much more polarised as compared to the $\ce{C=O}$ bond, causing the permanent dipole-permanent dipole interactions between DMSO to be much stronger than the same interactions between acetone molecules.

Resonance of DMSO

Lastly, we would realise that DMSO has a trigonal pyramidal shape while acetone is planar. Due to this, the net dipole moment in DMSO is likely to be stronger due to it being a more asymmetric molecule.

  • $\begingroup$ Sulphur has higher EN than Carbon $\endgroup$ – Robin Singh Sep 7 '20 at 13:17
  • $\begingroup$ ooops, thanks for the heads up $\endgroup$ – user85426 Sep 7 '20 at 13:18
  • $\begingroup$ thanks @user85426 for the help $\endgroup$ – Maxwell Sep 7 '20 at 13:32
  • $\begingroup$ @user85426: There is another point you forgot, the boiling point is also depends on the molar mass of the compound. Difference in MM: $78.13$ versus $\pu{58.08 gmol-1}$. $\endgroup$ – Mathew Mahindaratne Sep 7 '20 at 18:59
  • $\begingroup$ I have mentioned the point about it having a larger atomic radius i.e. an electronic cloud. A higher molar mass is often used to indirectly justify the larger electron cloud and hence, I feel that it is more apt to directly compare electron cloud rather than molar mass. $\endgroup$ – user85426 Sep 8 '20 at 15:40

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