Organic Chemistry Practice Problems and Problem Sets
enols
Carbonyls are in equilibrium with their enol forms. An enolate is the deprotonated form of an enol.
Enolates are formed from carbonyls under basic conditions.
Let's go through this equilibrium under basic conditions. Draw a mechanism using curved arrows for each reaction below.
Remember that under basic conditions, most species are either neutral or negatively charged, and rarely positively charged. So your structures will contain either ROH or RO-, but not ROH2+.
a) Carbonyl to Enolate (basic)
b) Enolate to Carbonyl (basic)
These mechanisms are similar to those in problem 738, only under basic conditions.
a) Carbonyl to Enolate (basic)
In the mechanism below the carbonyl was deprotonated at the alpha position, and the enolate was then drawn as a resonance form. You can also go straight from carbonyl to enolate in one reaction arrow. (rip off the proton, form the new double bond, and send the carbonyl "up" to form a negative charged oxygen).
b) Enolate to Carbonyl (basic) "DOWN"
Like a), this reaction can be done in one reaction arrow (negative charge comes "down" and double bond attacks a proton)
It's important that you become familiar with the mechanisms for enol and enolate formation in both acid (Q738) and in base (this problem), as well as carbonyl hydrate formation in both acid (Q706) and base (Q705).
If you can draw these four mechanisms you will be able to figure most of the reactions in second semester organic chemistry!
MendelSet practice problem # 739 submitted by Matt on July 27, 2011.
Carbonyls are in equilibrium with their enol forms. This process is called keto-enol tautomerization.
This equilibrium happens in both acid and base.
Let's go through this equilibrium under acidic conditions. Draw a mechanism using curved arrows for each reaction below.
Remember that under acidic conditions, most species are either neutral or positively charged, and rarely negatively charged. So your structures will contain either ROH or ROH2+, but not RO-.
a) Carbonyl to Enol (acidic)
b) Enol to Carbonyl (acidic)
Enol and enolate mechanisms always involve the alpha position of the carbonyl (one away from the carbonyl carbon). This the alpha hydrogen is acidic, and so can be deprotonated.
To form an enol (or enolate) from a carbonyl, the alpha position of the carbonyl must be deprotonated, and the double bond (pi electrons) travels "goes up" to the oxygen to become a lone pair.
To form a carbonyl from an enol (or enolate), the alpha position of the carbonyl must be protonated, and a lone pair on the oxygen "comes down" to reform the carbonyl.
This "up and down" mechanism is similar to the Nucleophilic Acyl Addition/Substitution mechanisms discussed in problems 705 and 706, with the difference being that the carbon carbonyl is not where bond formation takes place. In enol/enolate mechanisms, new bond formation occurs at the alpha position.
a) Carbonyl to Enol (acidic) "UP"
Because this mechanism takes place under acidic conditions, the carbonyl oxygen must be protonated before its double bond "goes up" to form a lone pair.
b) Enol to Carbonyl (acidic) "DOWN"
Most people remember that a lone pair from the enol oxygen "comes down." But many forget that you have to add something back to the alpha position! In this case, it's just a hydrogen- you reprotonate the alpha position to form the carbonyl.
MendelSet practice problem # 738 submitted by Matt on July 26, 2011.