Why do branched alkanes have a higher melting point than straight alkanes?

[FeralisExtremum]

I was reading an explanation Chad gave to another student and he stated:

"...if you recall from orgo, when comparing 2 alkanes with the same total number of carbons, if one is unbranched and the other is highly branched and symmetrical, the highly branched and symmetrical one will have a higher melting/freezing pt because, due to its shape, it packs into a crystal better. "

This seems to contradict with what we know about saturated vs unsaturated fats: saturated fats can pack more densely, and thus are more likely to remain in a solid state, whereas unsaturated fats have bends in the chain that don't let them pack as tightly and thus tend to be liquid.

Branched alkanes vs straight alkanes should (at least hypothetically) act the same: straight alkanes should pack more densely and have a higher MP than branched alkanes, but according to him this is not the case (he also mentions symmetry, and I'm not sure how that plays a part). Can someone explain this to me?

UPDATE: Found a post from Chad addressing this:

I actually hate this topic. But at the end of the day they're both right (and both wrong). There's no absolute trend that is always correct for the melting point. A significant degree of branching when accompanied by symmetry does indeed usually result in a higher melting point than the corresponding straight-chain alkane as it fits into a crystal structure better (as destroyer teaches it). This is how the majority of texts present it as well. But the truth is that the straight-chain alkane often has a higher melting point than the corresponding alkane that only has a slight degree of branching (as Kaplan said). So there's 2 sides to the truth here but if you have to lean one way or the other I think you're more likely to see the extremes (lots of branching vs straight-chains) and would therefore give preference to Destroyer's explanation.

Hope this helps and sorry this isn't so clear cut!

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[awaitsimpatient]

Steric hindrance and shift arrangement can occur!! Hope that helps

 

[Troyvdg]

Why not just memorize it? Looking too far into is more hassle than its worth
What he said was true.
Usually More branching = higher mp, lower bp

 

[FeralisExtremum]

Why not just memorize it? Looking too far into is more hassle than its worth
What he said was true.
Usually More branching = higher mp, lower bp

I usually like to probe these things with depth to make sure that there aren't any holes in the fundamental concepts I may need to know, but I'm going to defer to your experience/DAT score on this one.

Thanks for the help guys

 

[NewBee12]

If they are sufficiently branched, they have high degree of symmetry, therefore they can fit into a crystal tighter, hence a higher melting point.
Hope this helps

 

[ClemsonFirst]

"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 pack well in the solid phase..."

Straight from wikipedia.

So it can be higher or lower.

To answer your question, unsaturated alkanes are not branched. They have a double bond which decreases the ability for the lipid tail to pack tightly in comparison to the saturated lipid tails, which pack because of the straight lipid chain. These are two straight chains compared based on hydrogen saturation of the lipid tails (aka double bonds). Chad is speaking about branched versus straight chain alkanes - a totally different comparison.

 

[FeralisExtremum]

It's not ENTIRELY different, both follow the same principle of packing based on structure. The principle is the same between saturated vs unsaturated carbon chains and branched vs straight carbon chains, to an extent.

 

[aqz]

I'm no authority but this has always worked for me:

Branched alkanes are more STABLE than unbranched alkanes because if you imagine a carbon with carbon chains branching off of it, it will be more SUBSTITUTED, therefore more STABLE.

Sorry, no citations other than my brain, but even if the reason is wrong, the answer is right. Use it for memorization, not understanding #DATBiology