Phenol, C6 H5 OH, is a stronger acid than methanol, CH3 OH, even though both contain an O-H bond

Phenol, C6 H5 OH, is a stronger acid than methanol, CH3 OH,...

Phenol, $\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{OH}$, is a stronger acid than methanol, $\mathrm{CH}_{3} \mathrm{OH}$, even though both contain an $\mathrm{O}-\mathrm{H}$ bond. Draw the structures of the anions resulting from loss of $\mathrm{H}^{+}$ from phenol and methanol, and use resonance structures to explain the difference in acidity.

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Key Concepts

Influence of Molecular Structure and Electron Distribution

The molecular structure, including the presence of substituents, influences the distribution of electrons in a molecule. In phenol, the aromatic ring allows the negative charge from the deprotonated oxygen to be delocalized throughout the ring. In contrast, methanol’s methyl group does not facilitate such delocalization and can even donate electron density slightly, thereby destabilizing the negative charge on its conjugate base.

Resonance Delocalization

Resonance is the delocalization of electrons across multiple atoms or bonds that share a network of overlapping p orbitals. In the context of acid strength, when the negative charge on a conjugate base can be spread over several atoms through resonance, it stabilizes the base. This concept explains why phenol, which forms a phenoxide ion capable of resonance stabilization with the aromatic ring, is more acidic than methanol.

Acid Strength and Conjugate Base Stability

The acidity of a compound is largely determined by the stability of its conjugate base. When an acid loses a proton, forming its conjugate base, the more stable this anion, the stronger the original acid. Stability of the conjugate base can be enhanced by factors such as delocalization of charge, which reduce the energy of the anion.

Transcript

00:01 They tell us that phenol is a stronger acid than methanol, even though both contain an oh bond.

00:08 And they want us to draw the anions for each after we do an acid -based reaction and use the resonance structures for both phenol and methanol to explain the difference in acidity.

00:23 So let's say we just have some base come along.

00:30 And it steals this hydrogen right here.

00:35 So it's going to come.

00:36 Still this hydrogen.

00:37 That's going to pop up.

00:39 And in doing that, we would get...

00:44 So we have our benzene ring here.

00:49 And then we have this alcohol.

00:51 So before we remember, it had two lone pairs, even though they don't draw them in.

00:55 So now it should have three lone pairs and a negative charge.

00:58 Now, if we were to do that same reaction down here, the space comes along.

01:03 Sills that hydrogen.