Organic Chemistry - Double Bonds and Effects

[sciencebooks]

Advertisement - Members don't see this ad

So I was looking through the Kaplan flashcards and wasn't quite sure for the reasoning behind the following:

Which form of an alkene has a higher melting point (cis or trans)?
Answer: Trans
Reasoning: Due to its symmetry

Which form of an alkene has a higher boiling point (cis or trans)?
Answers: Cis
Reasining: Due to its polarity



Basically, I understand the reasoning for the first question as it allows for stacking, but don't understand why this doesn't apply to the second question as well.

Thanks.

(Also, sorry, I'm on a details-and-filling-in-any-last-gaps content kick right now which explains my extra posts these days.)

 

[shaboobly]

double bonds put kinks in the structure which doesnt allow for effective packing and thus a lower melting point. however, due to the potential polarity of an alkene (in cis conformation) it will have a higher boiling point than the alkane. remember strong intermolecular interactions between molecules will have a relatively high bp and molecules with a conformation which allows them to be close (in solid phase) will have high mp.

reread your q and realized i didnt answer. stacking doesnt effect bp because the molecules are already apart and only is determined by the forces in between molecules

 

[sciencebooks]

double bonds put kinks in the structure which doesnt allow for effective packing and thus a lower melting point. however, due to the potential polarity of an alkene (in cis conformation) it will have a higher boiling point than the alkane. remember strong intermolecular interactions between molecules will have a relatively high bp and molecules with a conformation which allows them to be close (in solid phase) will have high mp.

reread your q and realized i didnt answer. stacking doesnt effect bp because the molecules are already apart and only is determined by the forces in between molecules

So stacking is more important between solids and gases because they're closer together at this point?

 

[Jengreef]

Sorry if I'm stealing this topic, but could someone explain what stacking is exactly? Oh and answers to the OP's question would also be appreciated.

 

[sciencebooks]

Sorry if I'm stealing this topic, but could someone explain what stacking is exactly? Oh and answers to the OP's question would also be appreciated.

"Stacking" is just when things pile on top of each other and there's greater intermolecular force, usually more so when symmetry is present. Imagine a straight chain carbon molecule. These individuals molecules can stack on top of each other better and increase melting point. Now imagine a branched carbon molecule. These individual molecules cannot stack on top of each other as easily and subsequently decrease melting point.

I'm not even sure if "stacking" is the official term or just how I happened to have been taught it, lol.

 

[shaboobly]

So stacking is more important between solids and gases because they're closer together at this point?

melting point (freezing point) is when a solid changed phase to a liquid or vice versa. stacking taken into account (not really the same as forces between molecules).
boiling point (vaporization, condensation) is when a liquid changes phase to a gas or vice versa. intermolecular forces taken into account (polarity among the many).

 

[NonTraditional3]

Trans would have a lower BP - the dipole is reduced as compared with cis, and so it has a reduced VDW effect.