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?

  • 1
    $\begingroup$ 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 at 12:48
  • $\begingroup$ thanks @RobinSingh for the help $\endgroup$ – Maxwell Sep 7 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.

| improve this answer | |
  • $\begingroup$ Sulphur has higher EN than Carbon $\endgroup$ – Robin Singh Sep 7 at 13:17
  • $\begingroup$ ooops, thanks for the heads up $\endgroup$ – user85426 Sep 7 at 13:18
  • $\begingroup$ thanks @user85426 for the help $\endgroup$ – Maxwell Sep 7 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 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 at 15:40

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.