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Boiling point of Alkanes vs alkenes vs alkynes

jay47

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How do the boiling points of comparable alkanes, alkenes and alkynes vary? Been searching for like 10 minutes, no luck here. Seems like it varies on a case by case basis...please do explain the concept behind it as well, thanks.
 

fldnstrm

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When comparing alkynes, alkenes, and alkanes of similar molecular weight and structure(unbranched), I am pretty sure alkynes will have the highest boiling point due to the fact of increased electron density from your pi orbitals. It is more "polarizable" so you have an increased dipole-dipole interaction between molecules thus increasing bp. I think that sounds right, what do you think?
 

Sublimation

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it would be really close to call. there isnt much of a diff. all i can think of is to add to post #2 and that cuz the alkyne is planar it will have higher dipole dipole interaction. although the alkene is planar its dipole wont be as strong as that of the alkyne. that would be my reasoning, i could be wrong.
 

jay47

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When comparing alkynes, alkenes, and alkanes of similar molecular weight and structure(unbranched), I am pretty sure alkynes will have the highest boiling point due to the fact of increased electron density from your pi orbitals. It is more "polarizable" so you have an increased dipole-dipole interaction between molecules thus increasing bp. I think that sounds right, what do you think?

I kinda thought so, but after looking for 2 hours on the net, no good answers, so I graphed alkane vs terminal alkene and other stuff.. (looked up the freaking BP manually...grrr....) and these are the conclusions...

1. Boiling boints of cis alkenes are higher than trans because of increased ability to stack and therefore increased symmetry
2. Boiling points of terminal alkenes are LESS than those of their alkane counterparts (like ethene vs ethane)
3. Cyclic counterparts (e.g. cylcohexane) have higher boiling pointes because they can pack together better.
4. Terminal alkenes have lower BP than corresponding alkanes, BUT, nonterminal alkenes have a higher BP slightly (these will be cis and trans, with cis generally being higher)
5. Alkynes will have a higher BP than alkenes or alkanes.

So, to sum it up, here is the order, for corresponding alkanes and -kenes and -kynes...

cycloalkanes>alkynes> cis alkenes>trans alkenes>alkanes>terminal alkenes

I just need to post the graph now...
 
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jay47

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it would be really close to call. there isnt much of a diff. all i can think of is to add to post #2 and that cuz the alkyne is planar it will have higher dipole dipole interaction. although the alkene is planar its dipole wont be as strong as that of the alkyne. that would be my reasoning, i could be wrong.

Yeah, close call indeed, half the damn time there was only like a 1 or .5 degree difference in BP, but I guess there is.....
 

AngusBC

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alkyne>alkane>alkene

London Dispersion Forces are electrical interactions that occur in the absence of dipoles.

Alkanes: the longer the chain the higher the bp/mp, due to increased London dispersion forces, greater interaction and increased stackability/overlap of the molecules.

Alkanes>Alkenes: because alkenes contain 2 less electrons total, decreasing the effects of London dispersion forces. The "Stackability" of cis alkenes makes this difference close to negligible (remember trans fatty acids increase membrane fluidity and LDL, for the same reason: less compact = more gaps = less intermolecular forces).

Alkynes: the electron density created by the triple bond increases London dispersion forces in this case. The bond confers a linear bond angle between the carbons of the alkyne and the adjacent carbons, this increases the surface area for electrical interaction. This combination of density and exposure outweighs the loss of electrons and increases LDFs overall. Remember triple bonds make great nucleophile due to momentary induced dipoles or "high polarizability"? These same induced dipoles are the driving force in London dispersion. Molecules collide and electrons in the triple bond shift away from the collision giving a weak dipole moment. Pi electrons are highly polarizable due to high overlap of orbitals.

These are the weakest intermolecular forces. Throw one dipole in there and you will dramatically increase boiling point.
 
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