alkane alkene boiling/melting pt

[manutdfan]

how do the boiling/melting pts compare between equivalent alkane alkene alkynes?

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[linkin06]

if i remember correctly, alkane and alkene are comparable. alkynes are slightly higher.

branching for alkanes reduces boiling point, but if it's symmetrical, it is less reduced, since it can be packed together.

for alkenes, a trans alkene has an elevated melting point because it can be packed together. a cis alkene has an elevated boiling point because of the dipole moment.

this is after two weeks after my mcat, so i may be wrong, but that's what i memorized.

 

[lDanny]

I got confused on this topic.

Here are the reasoning. TPR says, branching will make it harder to stack so increase in bp and decrease in mp. But in Wade, branching will lead to a more compact 3D shape therefore, decrease in bp and increase in mp. Can someone help clear this up? Thanks.

 

[PiBond]

I got confused on this topic.

Here are the reasoning. TPR says, branching will make it harder to stack so increase in bp and decrease in mp. But in Wade, branching will lead to a more compact 3D shape therefore, decrease in bp and increase in mp. Can someone help clear this up? Thanks.

The more branching, the lower the boiling boiling point and melting point (not sure if there was a typo in TPR or what). The reason that boiling point decreases in terms of Wade's explanation is because the compact shape leads to less available surface area for van der Waals forces to interact. Therefore, less intermolecular forces equate to a lower boiling point.

does that make sense?

 

[lDanny]

The more branching, the lower the boiling boiling point and melting point (not sure if there was a typo in TPR or what). The reason that boiling point decreases in terms of Wade's explanation is because the compact shape leads to less available surface area for van der Waals forces to interact. Therefore, less intermolecular forces equate to a lower boiling point.

does that make sense?

Thanks, that makes sense. There was a typo in what I put. It should have read, "TPR, branching: lowers bp and mp" because of the decreased surface area, like what you said.

But it Wade, it says that the compact/packing shape will cause a higher mp. So will the mp increase or decrease? It makes sense that since there is less interaction, less energy is needed to break the bonds so lower mp. But it also makes sense that if they are can pack into a solid easier, the melting point would be higher (Wade).

Thanks

 

[PiBond]

Thanks, that makes sense. There was a typo in what I put. It should have read, "TPR, branching: lowers bp and mp" because of the decreased surface area, like what you said.

But it Wade, it says that the compact/packing shape will cause a higher mp. So will the mp increase or decrease? It makes sense that since there is less interaction, less energy is needed to break the bonds so lower mp. But it also makes sense that if they are can pack into a solid easier, the melting point would be higher (Wade).

Thanks

MP is decreased as well. The reasoning you said about mp decreasing is correct

 

[lDanny]

MP is decreased as well. The reasoning you said about mp decreasing is correct

This is from Wade.

 

[PiBond]

This is from Wade.

i think that book is in error, but that middle example in the middle of the page is one caveat to be aware of...however it is way beyond the scope of the MCAT. sometimes symmetrical alkanes exhibit great packing properties which shows how the MP trends are much harder to predict than BP trends (more clean).

overall just know, increased branching decreases both MP and BP

 

[PiBond]

this will help explain further: http://forums.studentdoctor.net/showpost.php?p=2877597&postcount=3

 

[lDanny]

this will help explain further: http://forums.studentdoctor.net/showpost.php?p=2877597&postcount=3

thanks for the link, I also found this post. http://forums.studentdoctor.net/showpost.php?p=1287140&postcount=17

 

[PiBond]

thanks for the link, I also found this post. http://forums.studentdoctor.net/showpost.php?p=1287140&postcount=17

Oh the power of the internet...the information super highway

 

[boaz]

how do the boiling/melting pts compare between equivalent alkane alkene alkynes?

"The boiling point of each alkene is very similar to that of the alkane with the same number of carbon atoms. Ethene, propene and the various butenes are gases at room temperature. All the rest that you are likely to come across are liquids.

In each case, the alkene has a boiling point which is a small number of degrees lower than the corresponding alkane. The only attractions involved are Van der Waals dispersion forces, and these depend on the shape of the molecule and the number of electrons it contains. Each alkene has 2 fewer electrons than the alkane with the same number of carbons."

http://www.chemguide.co.uk/organicprops/alkenes/background.html

They mean that if there are more electrons there would be more Van Der Waals interactions.

Apparently, this is not always the case: compare isopentane with 2-methyl-2-butene. Whereas propane and propene do obey that rule. So do n-octane and 1-octene.

So it's not so simple to answer...