Melting point and boiling point questions for alkanes and alkenes: branching

[510586]

Hi everyone. How come the first hydrocarbon pictured below has a higher melting point? Doesn't it have smaller van der waals forces despite branching since it is a lower mass and has the same forces (dispersion only)? For boiling, when we have a hydrocarbon that is smaller in number of carbon atoms, we do not take into account branching. We only take into account branching when they are equal in number of carbons. Is that only for boiling then? Also, why do we consider stacking/lattice for the first structure? Isn't that only applied to fats?

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[DAT Destroyer]

Hi everyone. How come the first hydrocarbon pictured below has a higher melting point? Doesn't it have smaller van der waals forces despite branching since it is a lower mass and has the same forces (dispersion only)? For boiling, when we have a hydrocarbon that is smaller in number of carbon atoms, we do not take into account branching. We only take into account branching when they are equal in number of carbons. Is that only for boiling then? Also, why do we consider stacking/lattice for the first structure? Isn't that only applied to fats?

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The first hydrocarbon is not branched and is saturated, meaning, it does not have any double bonds. It can therefore form Van Der Waals interactions with another molecule. They will interact via a tight packing as oppose to a double bond. Double bonds (Especially cis) prevent a tight packing interactions, and thus lower melting point. They package in a U-bending shape.

It is different from a boiling point. With boiling point, usually, longer hydrocarbon chain has a higher boiling point.

Hope this helps.

 

[510586]

The first hydrocarbon is not branched and is saturated, meaning, it does not have any double bonds. It can therefore form Van Der Waals interactions with another molecule. They will interact via a tight packing as oppose to a double bond. Double bonds (Especially cis) prevent a tight packing interactions, and thus lower melting point. They package in a U-bending shape.

It is different from a boiling point. With boiling point, usually, longer hydrocarbon chain has a higher boiling point.

Hope this helps.

Thank you! So just to clarify, we would treat any alkane chain similarly to the way we treat saturated vs unsaturated fatty acids? And in hydrocarbons, double bonds are higher priority than chain length/mass for mp and chainlength/mass has priority over branching in boiling points?

 

[DAT Destroyer]

Thank you! So just to clarify, we would treat any alkane chain similarly to the way we treat saturated vs unsaturated fatty acids? And in hydrocarbons, double bonds are higher priority than chain length/mass for mp and chainlength/mass has priority over branching in boiling points?

Yes. Correct. However, remember than Branched alkanes will have HIGHER Melting Point than linear alkanes.....

 

[510586]

Yes. Correct. However, remember than Branched alkanes will have HIGHER Melting Point than linear alkanes.....

Oh yea branching more than linear more than alkenes. Got it thank you!

 

[deleted768895]

Hi Dr Romano,
I'm seeing a lot of contradicting answers as to whether MP increases/decreases for branched alkanes, and its really confusing me! Furthermore, from watching Chad's videos, he mentions that BP decreases for branched alkanes due to decreased surface area, but surely there is increased surface area with branching??! Please could you give me a complete answer and reason behind it?
Thanks so much!