Many times extra stability is characteristic of a molecule or ion in which resonance is possible

Many times extra stability is characteristic of a molecule...

Key Concepts

Resonance Stabilization

Resonance stabilization is the phenomenon where the electron density or charge in a molecule is delocalized over two or more atoms via resonance structures. This delocalization leads to a lower overall energy and increased stability, which is particularly important in understanding the strength of acids. When the conjugate base of an acid is stabilized by resonance, it is lower in energy, making the deprotonation process more favorable.

Conjugate Base Stability

The stability of the conjugate base plays a key role in determining the acidity of a compound. A conjugate base that is stabilized by factors such as resonance, inductive effects, or aromaticity can better accommodate the negative charge after deprotonation. The more stable the conjugate base, the stronger the acid, as the loss of a proton is more readily achieved.

Aromatic Resonance

Aromatic systems, like a benzene ring, can participate in resonance stabilization by delocalizing electron density across the ring. When a substituent on the aromatic ring allows for the dispersion of a negative charge into the aromatic system, the conjugate base is particularly stabilized. This additional resonance stabilization provided by the aromatic ring can significantly enhance the acidity of the compound.

Transcript

00:01 So in this video we're going to go over question 164 from chapter 8, which says many times extra stability is characteristic of a molecular ion in which resonance is possible.

00:12 How could this be used to explain the acidities of the following compounds? the acidic hydrogen is marked by an asterisk.

00:18 Part c shows resonance in the c6 h5 ring.

00:22 So in a, we're given this molecule here.

00:27 And this is the acidic hydrogen.

00:30 So if we were to get rid of the hydrogen, like acids tend to do.

00:35 Acids like to donate hydrogen atoms, or protons specifically.

00:39 They like to donate protons.

00:41 Then we would end up with this molecule or this ion here.

00:46 This ion is our conjugate base.

00:48 So if the conjugate base of our acid is particularly stable, then we're more likely to form that conjugate base.

00:56 It's more favorable to form that conjugate base.

01:00 So we would call this molecule very acidic.

01:04 Because very acidic molecules are ones where k .a.

01:09 Is very large.

01:10 And k .a.

01:11 Is basically the equilibrium constant for this reaction.

01:14 So if we can draw resonant structures for this ion, then that would account for the acidity of this molecule.

01:22 So let's try drawing resonant structures for this.

01:26 So if we have our negative charge on this oxygen atom, we have our oxygen singly bonded and it has three pairs of electrons on it.

01:36 That's what gives it its negative charge.

01:38 If we were to take one of those electron pairs and move it into the bond and then take the electrons in this double bond and move them back out to the oxygen atom, then we have this resonant structure here.

01:51 So we have these two equivalent resonant structures.

01:55 That are stabilizing the ion.

01:59 So the true structure is kind of a hybrid between these two.

02:03 So in b, we're given this molecule here, and we're told that this is our acidic proton right here.

02:10 So if we remove that proton, then we end up with this ion.

02:16 So resonance would account for the stability.

02:19 If we're able to draw resonant structures for this ion, then that would account for the acidity of this, because a very acidic molecule would have a very stable conjugate base and resonance would stabilize.

02:37 So let's draw our molecule.

02:39 So we have our methyl and our carbon double bonded to oxygen, bonded to carbon, double bonded to another carbon with an oxygen on it...

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