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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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 SinghCommented Sep 7, 2020 at 12:48
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1 Answer
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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.
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.
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$\begingroup$ Sulphur has higher EN than Carbon $\endgroup$ Commented Sep 7, 2020 at 13:17
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1$\begingroup$ @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 $\pu{58.08 gmol-1}$. $\endgroup$ Commented Sep 7, 2020 at 18:59
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$\begingroup$ 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. $\endgroup$ Commented Sep 8, 2020 at 15:40
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1$\begingroup$ @MathewMahindaratne Sorry for off topic, due to the very poor pi overlap of 3p-2p as large different in size and energy, can I say for methyl phosphate, the predominant resonance structure has 4 formal charges, which 3 oxygens have -1 charge, and Phosphorus has +1 charge, explaining the why P is tetrahedral (sp3 hybridized)? $\endgroup$– QuestionCommented Sep 9, 2021 at 8:23
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$\begingroup$ @Question: I think it's better you post your question as a new one. In that way, it draws the attention of others who are more qualified to answer based on molecular orbital theory. $\endgroup$ Commented Sep 9, 2021 at 16:35