acid derivatives reactivity

[superduper12]

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Reactivity:
acyl chloride > acid anhydride > carboxylic acid > ester > amide

EK says that since a strong base makes a poor leaving group, equilibrium will favor the formation of compound whose leaving group is a strong base.

poor leaving group => strong base = >more stable => less reactive
good leaving group => weak base => less stable => more reactive

why is acid anhydride less reactive than acid chloride? what makes Cl- a better leaving group than O2CR?

 

[Quantum Chem]

Chlorine is a good leaving group. Think about the pKa of the these acids: HCl and HOOCR. HCl has a lower pKa than HOOCR which means that Cl- would be more compared to -OOCR. Since Cl would be more 'happy' to be an anion than the anhydride, its going to be more reactive than the anhydride.

What makes Chlorine a better leaving group is based on a few things. First, the bond on C-O is shorter than the bond between C-Cl. Lower energy in the bond takes less energy for the bond to be broken.

Also, the two oxygens have resonance which makes a normal C-OR bond stronger. While Cl- has free electrons, it does not form resonance as easily because the orbitals in which the valence electrons are in are much larger than those on the carbon, thus there is less probability that there will be overlap that forms the resonance bonds.