Acidity of Aldehydes and Ketones Enolate Ions

The p/ a values of aldehyde and ketone a-hydrogens range from 16 to 21, much lower than the p/ a values of ethene (44) or ethyne (25), but comparable with those of alcohols (15-18). Strong bases can therefore remove an a-hydrogen. The anions that result are known as enolate ions or simply enolates. 

Identify the most acidic hydrogens in each of the following molecules. Give the structure of the enolate ion arising from deprotonation, (a) Acetaldehyde (b) propanal (c) acetone (d) 4-heptanone (e) cyclopentanone. 

Deprotonation of an unsytnmetrical ketone such as 2-methylcyclopentanone may lead to two isomeric species, a more substituted and a less substituted enolate. The former, featuring the more substituted double bond, is more stable than the latter (Section 11-5). As in the elimination reaction of a haloalkane via the E2 mechanism (Section 7-7), the choice of base and reaction conditions determines which one is formed. For example, addition of 2-methylcyclopentanone to a cold solution of LDA in THF gives predominantly the less substituted, less stable enolate. The reason is that LDA is a bulky base and prefers to remove a hydrogen from the less-hindered a-carbon, generating the less stable anion, termed the kinetic enolate. Under these conditions, namely, the absence of a proton source and at low tanperatures, equilibration with the more stable enolate does not occur, and the kinetic enolate can be used as such in practical further transformations. 

In contrast, at room temperature and in the presence of a proton source such as a slight excess of ketone starting material, equihbration of the kinetic with the more stable thermodynamic enolate takes place. It occurs by reversible proton exchange between the unreacted ketone and the respective regioisomeric enolates, furnishing the latter in a ratio that reflects their relative thermodynamic stabilities. In this way, selective reactions, such as alkylations with haloaUcanes (Section 18-4), can be executed with the more substituted enolate. 

S% excess T Reversible conditions equilibrium on the enolate side y Minor (kinetic) Reversible / conditions equilibrium on the ketone side  

---

Acidity of Aldehydes and Ketones Enolate Ions CHAPTER 18... 

---