Aldehydes and Ketones
Carbonyls: 10 key concepts (Part 2)
Last updated: May 24th, 2021 |
If you’re on the typical college cycle, chances are you’re taking Org II right now. As you are by now well aware [more aware than you wish you were, I can hear some of you say] one of the main focii of Org II is on the many different facets of carbonyl chemistry. Following on the heels of the previous post, here are five further important points to keep in mind when studying the chemistry of these species. [Note – these will be included in the second carbonyl summary sheet, currently in preparation]. Without further ado…
6. Carbonyls make adjacent alkyl groups more acidic.
How much more acidic? Consider ethane. For all practical purposes, ethane is inert to base: the pKa of its hydrogens is 50. But when you exchange one of the protons of ethane for a carbonyl group (say, -COOCH3) something phenomenal happens. The acidity changes by a factor of 10 ^25. That is an incredible number to wrap your head around. The difference in chemical reactivity between different species can be incredibly, mind-bogglingly vast. We are not talking about the difference in quarterbacking ability between Peyton Manning and Dan Marino here, or even the comparative basketball skill set of LeBron James and Verne Troyer. The differences in reactivity* are truly cosmic: like comparing the width of your armspan to the length of the Milky Way galaxy.
What’s going on here? Simply put, the carbonyl π system provides a “sink” for the carbanion to donate electron density, setting up a sharing of negative charge between the α-carbon and the carbonyl oxygen. The key structural feature is not so much the resonance, although that is a factor – [the lower pKa of propene (42) is a good example of resonance stabilization] The reason why the carbonyl stabilizes the carbanion so much is that the negative charge becomes localized on a much more electronegative atom (oxygen), which is better able to stabilize negative charge.
*just to be clear: reactivity = the position of the equilibrium, between acid and conjugate base – not relative energies. To get the relative energies, you’d have to solve ΔG = –RT(lnK) for each and compare.
7. The more electrophilic the carbonyl, the more acidic its α-protons.
The stabilization afforded by the carbonyl group depends on how electron-poor it is. Placing a highly electronegative grouping like CF3 adjacent to the carbonyl makes the carbonyl much more electrophilic, which makes it better able to stabilize negative charge. On the other hand, carbonyls with electron-donating groups attached (like amides) do not stabilize negative charge nearly as well. This translates to lowered acidity of the alpha proton.
These effects are additive, by the way. With this principle in mind, you should be able to identify rank the following dicarbonyl compounds on the basis of acidity.
8. Carbonyls also activate alkenes toward nucleophilic attack.
Ethene (ethylene) reacts well with electrophiles like Br2 and mCPBA, but when combined with a nucleophile like diethylamine, no reaction happens. This is in spite of the fact that the reaction would actually be thermodynamically favorable: you’re trading a C-C π bond and N-H single bond for a new C-H single bond and a C-N single bond. The reason for this lack of reactivity, just as it was for #6, is that the resulting carbanion that would result from nucleophilic attack is highly unstable.
Replace a hydrogen with a carbonyl derivative, however, and suddenly the alkene becomes a lot more frisky. Again, the reason is the ability of the electrophilic carbonyl group to stabilize the newly generated negative charge. The immediate product of conjugate addition is an enolate, which can then be quickly protonated, depending on reaction conditions. The key point is that the carbonyl stabilizes the intermediate of the reaction, which allows it to proceed.
9. The reactivity of an α,β-unsaturated carbonyl is proportional to the stability of its enolate.
Let’s say you’re asked to rank the reactivity of the following α,β-unsaturated carbonyl derivatives toward nucleophiles:
By knowing the relative donation ability of each substituent at the carbonyl (e.g. NEt2 > OEt > CH3 > CF3) you can rank the stability of the enolates that will be generated through conjugate addition. Since nucleophilic addition is the rate-limiting step of this reaction, you will therefore know the relative rate of addition.
The intersection between α,β-unsaturated carbonyls and enolates has led to all kinds of innovative discoveries in chemistry. [Not on the exam: The Baylis-Hillman reaction].
Just remember: addition to an α,β-unsaturated carbonyl gives you an enolate intermediate.
[FYI, there is another, subtle effect with α,β-unsaturated carbonyls that I don’t have time to get into right now: the gamma protons are acidic. ]
10. Enolates are nucleophiles.
They come up in reactions again and again. By understanding their relative stability, you also understand their relative reactivity.
My 3rd year organic chemistry teacher, Prof. Szarek, had us say the following out loud in class, as a group, several times a lesson, until we finally got this fact drilled into our heads:
“The enolate is a nucleophile.”
If you’re studying Org II right now, you can hopefully think of 3 or 4 examples off the top of your head where you see the enolate… adding to different electrophiles.
– the Aldol reaction
– the Claisen condensation
– the Michael reaction
– enolate alkylation
What about relative reactivity? Recall the order of carbonyl α-proton acidity:
The reactivity of a nucleophile is inversely proportional to its stability. Makes sense, right? The less stabilized it is, the more reactive it should be. Another way of putting it: the higher the pKa of the carbonyl compound, the more reactive the resulting enolate will be.
Say it loud, say it proud: the enolate is a nucleophile.