3
$\begingroup$

Recently, while solvent optimization of a reaction, I found DMSO froze in an ice-bath but DMF didn't.

From Wikipedia, I found out that DMSO (dimethyl sulfoxide) has a freezing point of 19 °C (ref. 1) while DMF (dimethylformamide) has a freezing point of -61 °C (ref. 2)!

Both DMSO and DMF have similar molecular weights (78.13 g/mol (ref. 1) and 73.09 g/mol respectively) and similar boiling points (189 °C (ref. 1) and 153 °C (ref. 2) respectively).

Also, both molecules are known as polar aprotic solvents (only hydrogen acceptor site ( =O) present ~ no hydrogen bond donor site) with similar dipole moments, 3.96 D (ref. 1) and 3.86 D (ref. 2) respectively.

With these key features being the same, I am not able to understand, why the freezing points are starkly different? (one (DMSO) freezes at a much higher temperature than the freezing point of ice while the other (DMF) freezes in the presence of dry-ice)

References:

  1. DMSO DMSO on Wwikipedia
  2. DMF DMF on Wikipedia
Improve this question
$\endgroup$
4
  • 3
    $\begingroup$ There is a tag for melting point (that is what the question is about). It is better to use kelvins for comparing temperatures. One key feature affecting melting temperatures is the shape of the molecules, even though it is hard to imagine. $\endgroup$
    – Paul Kolk
    Commented Jul 3 at 19:47
  • $\begingroup$ chemistry.stackexchange.com/questions/63898/… $\endgroup$
    – Mithoron
    Commented Jul 3 at 21:03
  • $\begingroup$ chemistry.stackexchange.com/questions/139893/… $\endgroup$
    – Mithoron
    Commented Jul 3 at 21:29
  • $\begingroup$ Seem that the melting point for DMSO is very imprecise. I've seen various numbers ranging from 18 to 19C. It's possible its due to its high hygroscopicity, making measurements difficult. $\endgroup$
    – not2qubit
    Commented Nov 5 at 22:56

0

Reset to default

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.