Acetic Anhydride React With Water

At the carbonyl carbon that belongs to the bigger half.

A general anhydride looks similar this:

The leaving group is highlighted, assuming #R'# is longer than #R#.

A general dominion of thumb is that the longer the alkyl chain of the #R# group, the higher the pKa of the conjugate acid of the proposed leaving group and the amend the leaving group information technology is. Y'all can run into this when comparing methanoic acid (pKa = #3.75#, one carbon) and pentanoic acid (pKa = #four.84#, five carbons).

Since we are assuming that #R'# is longer than #R#, let'due south attack the carbonyl right now and remove the #R'# carboxylate group.

An anhydride literally lacks h2o. (anhydrous = without water.)

Permit's add water.

1. H2o attacks the carbonyl carbon. Which one? Pick the one that makes the carboxylate with the #R'# leave, considering that's what we're assuming volition happen. This is reversible , and actually is slightly favored in the contrary management, so we should add together quite a bit of water to push the equilibrium forward.
2. Proton transfer via a suitable base. Naturally, the water we added. But this is even so an equilibrium.
3. Something has to leave to stabilize this tetrahedral intermediate. We don't want the h2o nosotros just added to gain a proton and then get out. That'southward the contrary reaction. And of course, #"O"^(two-)# is a much stronger base than #"R"'"OO"^(-)# merely by looking at its charge. So, the #\mathbf(R"'")# carboxylate grouping must leave. At this point information technology's irreversible.
4. The carboxylate would want a proton. Why? The pKa of a typical carboxylic acid is about #v#, but the pKa of hydronium is about #-ane.vii#. The weaker acrid exists in its acidic class predominantly. Hence, hydronium gives upwardly its proton nicely.

You tin at present see that two carboxylic acids form.

Challenge: Can y'all describe the mechanism for the aridity of two carboxylic acids into 1 anhydride? Do yous call back it requires heat? How nearly acid?

Acetic Anhydride React With Water,

Source: https://socratic.org/questions/how-do-anhydrides-react-with-water

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