Nucleophilic addition at the C=O bond

• These reactions transform a trigonal planar carbonyl that is $sp^{2}$ hybridized to an $sp^{3}$ hybridized tetrahedral carbon.
• Hemiacetals and acetals
  • Hemiacetals are unstable molecules that are created when an aldehyde is dissolved in one equivalent of an alcohol (Note: acid- and base- catalyzed hemiacetal formation is also possible, but alcohol mediated hemiacetal formation is most common). Acetals are the products observed when a hemiacetal is combined with another molar equivalent of an alcohol.

    Figure 47.12: Hemiacetal and acetal formation. Depending on the R' of the initial reactant, an aldehyde (R' = H) or ketone (R' = alkyl) can be used in this reaction.

    

• Imines and enamines
  • Imines are produced by reacting an aldehyde or ketone with a 1$^{o}$ amine while enamines are produced by reacting an aldehyde or ketone with a 2$^{o}$ amine. See Figure 47.13 below.
    

    Figure 47.13: Imines and enamines.