Melting Points and Boiling Points

[drzakisadiq]

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Can someone explain to me the trends to look for in these two? I know boiling points are high when hydrogen bonding takes places, H bonds on the FON, but other than that, I'm a little lost. Thanks for the help

 

[osimsDDS]

when were talking about alkanes, it goes like this:

Bp:
-increases with chain length due to dispersion forces
-decreases with branching

MP:
-increases with branching because the molecules can pack tightly
-increases with chain length againn due to dispersian forces

so basically the only difference is that melting point likes branching because the molecules can pack more tightly...

 

[Mstoothlady2012]

also for the branching trans have higher melting point because they can "pack" better than cis and have lower boiling point because they are less polar.

 

[Lazerous]

Wait, doesn't branching mean they CAN'T pack more tightly?

 

[PreDent08]

For both BP and MP for alkanes:
as chain length increases BP and MP increases.
as branching increases, both BP and MP decreases.

internal alkenes have higher BP than terminal alkenes.
cis has higher bp than trans due to dipole, and lower mp than trans and vice versa.

 

[winkipink]

Wait, doesn't branching mean they CAN'T pack more tightly?

i thought that too. this was the problem i had between kaplan vs. destroyer. kaplan says branching decreases melting point because they dont pack as well THEN destroyer says branching increases melting point bc branching makes lattices taht are harder to break apart. i was going with kaplan for a long time until i saw the destroyer ochem #152.

nows im alls confuseds 😕

 

[klutzy1987]

Branching creates more symetry and therefore the molecule can pack into a crystal lattice much tighter and easier. This means that branching increases melting point, because that just means that the molecule will form a solid easier.

 

[futuredent22]

wikipedia says branching can either increase or decrease melting point, depending on whether the branching increases or decreases its ability to pack tightly.

"The melting points of branched-chain alkanes can be either higher or lower than those of the corresponding straight-chain alkanes, again depending on the ability of the alkane in question to packing well in the solid phase: This is particularly true for isoalkanes (2-methyl isomers), which often have melting points higher than those of the linear analogues."

http://en.wikipedia.org/wiki/Alkane#Melting_point