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An acid is strong when its conjugate base is stable, and resonance stabilizes a base by spreading its charge — over two oxygens in a carboxylate, into the ring in a phenoxide, and further still when a substituent extends the delocalization.
Paper packet. Every task here also exists on screen, where it is checked automatically; answers written on paper are not assessed by Nydus. When you are back at a device, enter your answers there.
You will explain an acidity difference by the conjugate base, count the atoms sharing a base's charge, rank acids by base stability, and predict how a substituent that extends resonance changes pKa.
You can read pKa, and you can write resonance contributors and count the atoms that share a charge. This lesson connects them: the acid's strength is decided by its conjugate base.
The conjugate base is the acid minus its proton. A charge is delocalized when resonance spreads it over several atoms. A carboxylate is the base of a carboxylic acid; a phenoxide, of a phenol.
Giving up a proton is an equilibrium, and it lies further towards the products when the products are more stable. So to compare two acids, compare their conjugate bases.
The rule: the more atoms share the charge, and the more electronegative they are, the more stable the base and the stronger the acid.
Another way: steps
To compare acids by resonance:
A group that gives the charge somewhere else to go makes the acid stronger. In 4-nitrophenol the phenoxide's charge reaches the carbon opposite the oxygen and passes on into the nitro group, onto two more oxygens: pKa 7, a thousand times more acidic than phenol. The same reasoning explains why a C–H next to a C=O is surprisingly acidic (propanone, pKa 20, against ethane, 50): its conjugate base, the enolate, shares its charge with the oxygen.
Resonance makes an acid donate every proton completely. It makes the acid stronger, not strong.
Compare the acids. Compare the conjugate bases.
More hydrogens means more acidic. Only the one that leaves matters.
Any spreading is as good as spreading onto oxygen. Oxygen holds a negative charge better than carbon.
Methanoate, HCOO−, shares its charge over two oxygens.
Resonance in the base.
Methoxide, CH3O−, keeps it on one oxygen.
No resonance.
So methanoic acid is far more acidic than methanol, with a pKa about 4 against 16.
Stable base, strong acid.
Remove a C–H next to the C=O. Where can the charge go?
Onto the oxygen, through the C=O.
And in propane?
Nowhere: it stays on carbon, so propanone is the far stronger acid.
Ethanoic acid (pKa about 5) is far more acidic than ethanol (pKa about 16), though both lose the hydrogen of an O–H bond. Why?
Across its resonance contributors, how many atoms of the carbonate ion, CO3 2− carry its negative charge in at least one contributor?
Answer:
Rank these acids from strongest to weakest by thinking about their conjugate bases: ethanol, ethanoic acid, phenol.
Number the steps in order (write the number in the box):
Match each acid to the reason its conjugate base is, or is not, stabilized by resonance.
| charge shared by two oxygens | charge spread into a benzene ring | charge shared by a carbon and an oxygen | no resonance: charge on one oxygen | |
|---|---|---|---|---|
| ethanoic acid | ||||
| phenol | ||||
| propanone, at the C–H next to C=O | ||||
| methanol |
4-nitrophenol, with a –NO2 group on the ring opposite the –OH, has a pKa of about 7, compared with phenol's 10. Why is it a thousand times more acidic?
Lesson test: one question per skill, one attempt each, no hints. Your answers are checked when you submit.
For each conjugate base, give the number of atoms that share its negative charge across its resonance contributors.
| atoms sharing the charge | |
|---|---|
| the methoxide ion, CH3O− | |
| the methanoate ion, HCOO− | |
| the phenoxide ion, C6H5O− |
You can explain acidity by where the base's charge goes. Tell someone why ethanoic acid is so much stronger than ethanol. Next: charges pulled on through sigma bonds — induction.
8. Your turn: propanone against propane, step 3