7.1 General Principles

LEARNING GOALS

After   Chapter 7.1,    you will    be  able    to:

Explain the acidic  nature  of  α-hydrogens on  aldehydes   and ketones 

Compare the acidity of  the α-hydrogens of  aldehydes   to  those   of  ketones 

Describe    the relationship    between steric  hindrance   and reactivity

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 bond 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, described 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.