mp/bp in orgo

[plumerias]

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I used to have a good understanding of this but I think I'm on the brink of forgetting the basics, so please tell me if this is right..

EDIT:*this post has been corrected*

ionic bonding > hydrogen bonding > dipole/dipole > van der waals

When comparing molecules with IMF, the one with the highest IMF will have highest BP AND MP.
When comparing molecules that ONLY have van der waals, the longest chain length will have the highest boiling point and highest melting point.
The more branched it is, the lower the boiling point and higher the melting point (in general, for small symmetrical branching).

I've searched the forums but I keep finding contradicting answers and I'm getting confused. Thanks!

 

[FeralisExtremum]

What you have listed is mostly correct, except:

"When comparing molecules that ONLY have van der waals, the shortest chain length will have the highest boiling point and highest (?) melting point."

If a molecule only has Van Der Waals forces, those forces will be greater as the size of the molecule increases. So it is the longest chain length that will have the highest BP and MP.

Branching can be a little more complicated, but what you wrote is generally correct for small/symmetrical branching.

 

[plumerias]

What you have listed is mostly correct, except:

"When comparing molecules that ONLY have van der waals, the shortest chain length will have the highest boiling point and highest (?) melting point."

If a molecule only has Van Der Waals forces, those forces will be greater as the size of the molecule increases. So it is the longest chain length that will have the highest BP and MP.

Branching can be a little more complicated, but what you wrote is generally correct for small/symmetrical branching.

Makes perfect sense. I've corrected my original post in attempt not to confuse other people lol. Thank you!

 

[DAT Destroyer]

I used to have a good understanding of this but I think I'm on the brink of forgetting the basics, so please tell me if this is right..

EDIT:*this post has been corrected*

ionic bonding > hydrogen bonding > dipole/dipole > van der waals

When comparing molecules with IMF, the one with the highest IMF will have highest BP AND MP.
When comparing molecules that ONLY have van der waals, the longest chain length will have the highest boiling point and highest melting point.
The more branched it is, the lower the boiling point and higher the melting point (in general, for small symmetrical branching).

I've searched the forums but I keep finding contradicting answers and I'm getting confused. Thanks!

Now let us consider Hexane,,,,,,the mp is -95 Celsius......now lets make a branch....2-methylpentane .....mp is now - 154 Celsius......clearly branching didn't increase it,,,since high symmetry was NOT created. Only when high symmetry is created would you expect a drastic increase in melting points.

I hope this clears this up for you

Dr. Romano