MCAT Organic Chemistry Review

Aldehyde and Ketones II: Enolates

7.1 General Principles

In the previous chapter, we focused on how the electronegativity of the oxygen atom in a carbonyl pulls electrons away from the carbonyl carbon, making it partially positively charged. In this chapter, we take the electron-withdrawing characteristics of oxygen one step further, focusing on the α-carbon in an aldehyde or ketone.

ACIDITY OF α-HYDROGENS

An α-carbon is adjacent to the carbonyl carbon, and the hydrogens connected to the α-carbon are termed α-hydrogens. Through induction, oxygen pulls some of the electron density out of these C–H bonds, weakening them. This makes it relatively easy to deprotonate the α-carbon of an aldehyde or ketone, as shown in Figure 7.1. The acidity of α-hydrogens is augmented by resonance stabilization of the conjugate base. Specifically, when the α-hydrogen is removed, the extra electrons that remain can resonate between the α-carbon, the carbonyl carbon, and the carbonyl oxygen. This increases the stability of this enolate intermediate, shown in the next section. Through this resonance, the negative charge can be distributed to the more electronegative oxygen atom. The electron-withdrawing oxygen atom thereby helps stabilize the carbanion (a molecule with a negatively charged carbon atom). When in basic solutions, α-hydrogens will easily deprotonate.

Figure 7.1. Deprotonation of an α-Carbon, Forming a Carbanion

The α-hydrogens of ketones tend to be slightly less acidic than those of aldehydes due to the electron-donating properties of the additional alkyl group in a ketone. This property is the same reason that alkyl groups help to stabilize carbocations—but in this case, they destabilize the anionic deprotonated form.

KEY CONCEPT

Electron-withdrawing groups like oxygen stabilize organic anions. Electron-donating groups like alkyl groups destabilize organic anions.

STERIC HINDRANCE

In reactions, aldehydes are slightly more reactive to nucleophiles than ketones. This is due in part to steric hindrance in the ketone, which arises from the additional alkyl group that ketones contain. When the nucleophile approaches the ketone or aldehyde in order to react, the additional alkyl groups on the ketone are in the way, more so than the single hydrogen of the aldehyde. This makes for a higher-energy, more crowded intermediate step. Remember, higher-energy intermediates mean that the reaction is less likely to proceed.

KEY CONCEPT

Ketones are slightly less likely to react with nucleophiles than aldehydes because the extra alkyl group destabilizes the carbanion and increases steric hindrance.

MCAT Concept Check 7.1:

Before you move on, assess your understanding of the material with these questions.

1.    Why are the α-hydrogens of aldehydes and ketones acidic?

2.    Are the α-hydrogens of aldehydes or ketones more acidic? Why?

3.    How does steric hindrance affect the relative reactivity of aldehydes and ketones?