Correlating density and melting point in alkenes

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Discussion Overview

The discussion revolves around the correlation between density and melting point in alkenes, specifically focusing on the properties of cis and trans 1,2 dichloroethenes. Participants explore the observed trends and seek explanations for the discrepancies in these properties.

Discussion Character

  • Exploratory, Technical explanation, Homework-related

Main Points Raised

  • One participant notes that cis 1,2 dichloroethenes have a higher density but a lower melting point compared to trans isomers, questioning how this can be explained.
  • Another participant suggests that the discussion may be related to a homework question and asks what other properties differ significantly.
  • A participant points out that boiling points differ as expected, with the more polar cis molecule exhibiting a higher boiling point.
  • One participant identifies dipole moment as a key factor influencing boiling point.
  • However, another participant argues that while dipole moment accounts for the boiling point differences, it does not explain the observed trends in melting point and density.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between dipole moment, boiling point, melting point, and density, indicating that the discussion remains unresolved with multiple competing perspectives.

Contextual Notes

Limitations include potential missing assumptions regarding molecular packing and the influence of polarity on melting point and density trends, which have not been fully explored.

Miffymycat
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Quoted data for cis and trans 1,2 dichloroethenes shows cis has higher density but lower melting point than trans. How can this be explained? Packing arguments are clearly unable to rationalise these observations. Other 1,2 disubstituted ethenes show similar pattern. Thank you.
 
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This homework ? What other property differs strongly ?
 
Boiling points differ as expected - the more polar molecule (cis) is higher
 
Yes, dipole moment is the key.
 
Dipole moment explains boiling point but not the contrary melting point and density trend!
 

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