Key Principles

What demonstrates significant acidity in aldehydes and ketones?

Figure 47.5: The acidity of hydrogens in aldehydes and ketones: The $\alpha$-hydrogen has considerably more acidity than the $\beta$-hydrogen ($\alpha$-hydrogen p $K_{a} \approx 20$ vs. $\beta$-hydrogen p $K_{a} \approx 50$).

Explain this acidity in terms of resonance.

The electron-withdrawing carbonyl group allows for the removal of the $\alpha$-hydrogen. Subsequent electron delocalization results in a resonance-stabilized anion, see Figure 47.6 below:

Figure 47.6: Role of electron-withdrawing carbonyl in $\alpha$-hydrogen acidity.

Why are organometallics commonly used in aldehyde/ketone reactions?

Organometallics - aka Grignard reagents - are useful because they react well with the carbonyl carbons of aldehydes or ketones. Common organometallics include $R-Li$ or $R-MgBr$.

What are the products observed when organometallics (Grignard reagents) are reacted with methanal, and other aldehydes and ketones?

Table 47.2: Products observed from specific organomerallic reactions.

Methanal	1$^{o}$ alcohols
Longer chain aldehydes	2$^{o}$ alcohols
(i.e. longer than methanal)
Other ketones	3$^{o}$ alcohols

Figure 47.7: Role of electron-withdrawing carbonyl in $\alpha$-hydrogen acidity.

How do carbonyl substituents affect reactivity?

Intiuitively, larger substituents cause more steric hinderance and result in less-reactive reactants. Conversely, smaller substituents create less steric hinderance and result in more reactive reactants.