Tautomerism

Transcript

0:00Introduction

Hello everyone! Here is a small puzzle. In a bottle of pure propanone, a few molecules keep turning into a different compound, with an O H on a carbon carbon double bond, and then turning back. So, is that bottle really pure? Something is strange here, right? And another surprise: in propanone, far fewer than one molecule in a thousand takes this other form, yet phenol is essentially all in that form. That is tautomerism, it comes up often in JEE, so let's begin.

0:33Tautomers: a hydrogen moves, a double bond follows

So, what are tautomers? Structural isomers that are readily interconverted, so they keep changing into each other. On the right, propanone, the keto form, and its partner, the enol form. The two half arrows mean an equilibrium running both ways. And under the keto form, the label says more than ninety nine point nine percent, with the greater than sign, so almost every molecule stays keto. Okay? Now compare them. In the keto form, an oxygen is double bonded to the middle carbon, with a C H three on each side; the small numbers written low are subscripts. In the enol form, an O H sits on top, and the middle carbon is double bonded to a C H two. So, what has moved? Yes, exactly, one hydrogen moved from a C H three to the oxygen, and a double bond moved with it! That is the usual pattern of tautomerism. Clear? In keto enol tautomerism, that hydrogen comes from the alpha carbon. Alpha is this Greek letter, and the alpha carbon is the one right next to the C double bond O, okay? In propanone, both C H three carbons are alpha. With me? Here is the general picture. The dashes are single bonds, the front one leading to the rest of the molecule, and the equals signs are double bonds. The hydrogen on the C H moves to the oxygen, and on the right we get the enol, C double bond C, O H, okay? The words in brackets say acids and bases catalyse this equilibrium. Got it?

2:09Who can show keto–enol tautomerism?

So, not every carbonyl compound can do this. It needs at least one alpha hydrogen, on an s p three carbon. The three written high is a superscript, and s p three is the hybridisation. Okay? Now a quick test. Methanal is HCHO, and it has two hydrogens. So, can methanal form an enol? It cannot, and here is why. Both hydrogens sit on the carbonyl carbon itself, and there is no carbon next to it. So no alpha carbon, no alpha hydrogen, and no keto enol tautomerism. Remember this for Example one. Right?

2:49Keto or enol: which form is more stable?

Now the big question. For a simple aldehyde or ketone, which form is more stable, the keto form or the enol form? It is the keto form, and by a long way. For simple aldehydes and ketones, the keto form is far more stable. Okay? The reason is the bonds: the carbon oxygen double bond is stronger than the carbon carbon double bond. So propanone has much less than zero point one percent enol, with the percent sign you see. That is far fewer than one in a thousand, as in our puzzle. Clear?

3:24When the enol wins

But in some compounds, the enol content is large: beta diketones like pentane two four dione, and beta keto esters. Beta is this Greek letter. In pentane two four dione, the two C O groups have one carbon between them. Its enol is on the far right. Okay? That enol is stabilised in two ways. First, the dotted line is a hydrogen bond from the O H to the other carbonyl oxygen, closing a ring of six atoms. Second, conjugation: the carbon carbon double bond sits next to the carbon oxygen double bond. With me? And aromaticity pushes even harder. Phenol, an O H on a benzene ring, is itself an enol, stabilised by its aromatic ring, so phenol is essentially one hundred percent enol! That was the second part of our puzzle. Right? So here is the order of enol content; these signs mean less than. Simple ketone, less than beta keto ester, less than beta diketone, less than phenol. Write it down. Got it?

4:24Other tautomer pairs

Keto and enol is not the only pair. Nitro compounds can show an aci form, amides an imidic acid form, and imines an enamine form. Same idea each time: one hydrogen and one double bond move.

4:39Trap: tautomers are not resonance structures

Okay? Now, here is a trap that catches many students. The keto form and the enol form look like two drawings where a double bond has moved around. So they are just resonance structures, right? No, no, careful, they are not! In tautomers an atom really moves, the hydrogen, so tautomers are different compounds in equilibrium. That settles our puzzle: the propanone bottle really holds two compounds, though the enol is a tiny fraction. Okay? Resonance structures are just drawings of one species, where only electrons move, never atoms. So if a hydrogen moved, think tautomers, and if only electrons moved, think resonance. Never confuse the two, right?

5:23Example 1: which carbonyls show keto-enol tautomerism

Now let us do Example one. Which of these show keto enol tautomerism: propanone, benzaldehyde, acetophenone, benzophenone, two two dimethyl propanal, and cyclohexanone? Pause the video and try it first. Done? Okay, let's check: we hunt for an alpha hydrogen. Propanone and acetophenone have a C H three beside the C O, and cyclohexanone has ring C H two groups on both sides of it. All s p three carbons with hydrogens, so these three show tautomerism. Okay? Now benzaldehyde and benzophenone: the carbon next to the C O is a ring carbon with no hydrogen, so no enol. And two two dimethyl propanal, in the box? The C H three in brackets, with subscript three, means three methyl groups, nine hydrogens. So it must have an alpha hydrogen, right? No, no, careful, look where those hydrogens sit! The alpha carbon, joined to the CHO group, carries three methyl groups but no hydrogen; those nine hydrogens are one carbon further away. And the aldehyde hydrogen sits on the carbonyl carbon itself, as in methanal. So no enol here either. Got it? So the answer is propanone, acetophenone and cyclohexanone. One simple check, and the whole question is done.

6:46Example 2: enolisable H in propanone, butanone, a diketone

Now a harder one, Example two. How many enolisable hydrogen atoms are there in, part a, propanone, part b, butanone, and part c, pentane two four dione? Here is a hint: count only the alpha hydrogens. Pause the video and try it first. Done? Okay, let's check. Enolisable hydrogens are just the alpha hydrogens. Propanone has two C H three groups, both next to the carbonyl. Three plus three gives six in part a, okay? Now butanone, with eight hydrogens in total. So, are all eight of them enolisable? No, no, careful, only five of them are! The formula is C H three, C O, C H two, C H three. The alpha carbons are the first C H three and the C H two, both touching the carbonyl. The last C H three is one carbon further away. Three plus two gives five. Clear? And part c, pentane two four dione: C H three, C O, C H two, C O, C H three. Both C H three groups are alpha, and the central C H two is alpha to both carbonyls, but count its hydrogens once, okay? Three plus two plus three gives eight. So the answers are six, five and eight!

8:05Quick revision: alpha H, keto vs enol, not resonance

So, quick revision. In tautomers, a hydrogen and a double bond shift, as in keto and enol. Keto enol needs an alpha hydrogen on an s p three carbon, and usually the keto form wins. Hydrogen bonding, conjugation and aromaticity raise the enol content, and tautomers are not resonance structures. Now go and try the practice problems on this page. All the best!