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Author Topic: cis and trans isomers and melting point.  (Read 3958 times)

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fc3stud

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cis and trans isomers and melting point.
« on: May 10, 2006, 11:31:06 AM »

Can you explain why the cis isomer of but-2-ene has a higher mp and bp than the trans isomer?  Surely the trans can pack better?  Or does pi bond repulsion play a role?  Many thanks, Andrew Walker.
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wereworm73

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Re: cis and trans isomers and melting point.
« Reply #1 on: May 10, 2006, 11:53:00 AM »

It's the dipole-dipole attractions that make the difference here.  The dipole moments cancel out in the trans isomer but not in the cis.
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Donaldson Tan

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Re: cis and trans isomers and melting point.
« Reply #2 on: May 11, 2006, 01:34:09 AM »

There is pi bond in both isomers, so if there is a pi bond repulsion, it must affect both isomers in the same fashion.

It is clear that the fact that the cis- exhibit higher mp/bp means that the cis- exhibits stronger intermolecular bonding.

The only way this can happen is due to that the cis-isomer must has an overall polarity, thus constituting for strong intermolecular bonding.  Although the trans-isomer can pack better, both isomers essentially share the same number of intermolecular bonding sites. This means although the trans- has a greater density than the cis-isomer, it has a lower mp/bp than the cis-isomer.
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