This is it. Section III, our last section on carbonyls. Here we will attempt to tie everything together and show you how what you learned in previous sections is transferable.
To start things off we are going back to aldehydes and ketones. This time we are expanding consideration of reactivity to the alpha carbon, which is the carbon attached to the carbonyl carbon.
This carbon is significant because its protons are acidic. Why?Well that’s where the carbonyl comes in. After removing the proton, the resulting negative charge can be delocalized to the oxygen of the carbonyl through resonance(review Acid Base 6).
The BIG PICTURE of this section, and why organic chemists esteem this as an important class of reactions, is the ability to make a carbon nucleophile. This carbon nucleophile is the focus of the rest of this section.
Under either acidic or basic conditions an aldehyde or ketone can tautomerize to the enol form. Notice, these structures are in equilibrium and NOT resonance structures.
In most cases, equilibrium heavily favors the left side. We actually saw the implications of this equilibrium in the hydration of an alkyne (Triple 3). So why care about the enol if there is so little of it? Well the alpha carbon of the enol is nucleophilic and can attack electrophiles. When this attack happens, the enol is consumed causing equilibrium to shift to the right. Review and practice the mechanisms for tautomerization (under both acidic and basic conditions) in the videos and puzzles below.
Next let’s look at the reactions of enols/enolates beginning with halogenation. When tautomerization occurs under acidic conditions the enol is used as a nucleophile for halogenation of the alpha carbon. Under basic conditions the mechanism goes through an enolate. The key thing to remember in both cases is the alpha carbon is electron rich (aka nucleophilic) as can be more clearly seen in the resonance structures. (Also, you can relate the C=C pi bond to addition reactions where the carbon was electron rich and attacked electrophiles.)
Typically you would use the enol for halogenation because then only one halogen is added. Watch the videos listed for Halogenation to learn why under basic condition more than one halogen is added.
The real star of the show, as you will see below, is the enolate. The enolate a better nucleophile and can even be used in SN2 reactions. Practice halogenation and attempt a substitution reaction using an enolate as the nucleophile in the puzzles listed below.
Enolates are great carbon nucleophiles. Now think back toAldehydes and Ketones. That section was all about how the carbonyl carbon is a great electrophile. Enolates (nucleophiles) are made from aldehydes/ketones(electrophiles). Nucleophile + Electrophile = Reaction. So yes, as the enolate is formed it can and will react with any aldehyde/ketone present in what we call an Aldo reaction.
Finally, we’ll end with a variant of the aldol. Esters can also form enolates. The enolate of an ester can attack the carbonyl of another ester in what is called a Claisen reaction.
To see the mechanisms in action and learn some of the finer details, watch the videos of Aldol and Claisen reactions. Then practice these mechanisms and enjoy not having to draw all the proton transfers on paper :)
Halogenation and Substitution
Ultimate Challenge Problem!