When to apply heat in organic?

[JSU]

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There was a question on AAMC5 (#119) where it asked you to turn two carboxylate anions into an acetic anhydride. The answer requires the use of acid and heat. The acid makes sense because you would be left with a geminal-dialkoxide group. This would require the addition of acid to turn into a carbonyl. However, why do we need to add heat? Is the formation of a carbonyl from a geminal diol an endothermic process?

 

[startswithb]

I wonder this as well. Anyone?

 

[loveoforganic]

This is an endergonic reaction, so it's probably endothermic too. Heat's a reactant in endothermic reactions. Reactions that have a high energy transition state also potentially require heat (or energy input in some other form).

 

[JSU]

This is an endergonic reaction, so it's probably endothermic too. Heat's a reactant in endothermic reactions. Reactions that have a high energy transition state also potentially require heat (or energy input in some other form).

Why is it endergonic? Is it because we are breaking a sigma bond and forming a pi or is there more to it than that?

Also, seeing as the carbonyl is more ordered than a gem.diol (i think?) shouldn't delta S be negative? That would mean the process could be both endothermic or exothermic.

Thanks!

 

[loveoforganic]

It's mostly electrostatics - you're forcing those two carbonyls with all that electron density to be right next to each other. The end products are definitely more ordered than the reactants. For a reaction to be favorable by means of entropy, entropy must be increasing.

The term is -TdS, so to get a -dG value, you need a positive dS (since T can't be negative).

 

[JSU]

It's mostly electrostatics - you're forcing those two carbonyls with all that electron density to be right next to each other. The end products are definitely more ordered than the reactants. For a reaction to be favorable by means of entropy, entropy must be increasing.

The term is -TdS, so to get a -dG value, you need a positive dS (since T can't be negative).

The end products are more ordered so entropy is decreasing. So that should mean the higher the temp the higher the dG which would not explain whats happening.

 

[loveoforganic]

I'm actually getting myself mixed up. dG isn't negative here since it's endergonic. I'm having trouble sorting my thoughts out now, so I'll give someone else the chance to reply!