Due to the presence of the polarized carbonyl (C=O) bond, aldehydes and ketones are useful for nucleophilic addition reactions at the C=O bond and for reactions at adjacent positions; Aldehydes are also useful for their ability to be easily oxidized.
1.Nucleophilic additions at the C=O bond
Due to the electron-poor nature of the carbon in the C=O bond present in aldehydes and ketones, nucleophiles (molecules/ions with free electrons to donate) react at this carbon.
Hemiacetals are characterized by a hydroxyl group (alcohol, -OH) using its electron-rich oxygen to attack the carbon on an aldehyde or ketone. The electron flow from the hydroxyl oxygen to the aldehyde/ketone carbon causes a buildup of electrons at the aldehyde/ketone oxygen, turning the double bond between carbon and oxygen into a single bond. The hydrogen from the hydroxyl then rearranges to move from the hydroxyl oxygen to the aldehyde/ketone oxygen, forming a hemiacetal (-O-C-OH).
Acetals undergo the same mechanism. The difference is that hemiacetals form under 1 equivalent of hydroxyl groups (alcohol groups) and acetals form under 2 equivalents of hydroxyl groups. Under this circumstance, rather than forming a -OH substituent as seen in a hemiacetal, the aldehyde/ketone oxygen forms a single O-X bond with whatever is at the end of the hydroxyl group, resulting in an acetal (-O-C-O-).
Imines and Enamines are formed by the same nucleophilic addition reaction seen in acetals and hemiacetals. Imines are formed by the addition of primary amines (-NH2), forming (-N=C) and enamines are formed by the addition of secondary amines (-NH), forming (-N-C=C). In these cases, the oxygen on the aldehyde/ketone reacts with additional water and leaves.
Hydride reagents such as LiAl4 and NaBH4 participate in nucleophilic addition with aldehydes/ketones, reducing the oxygen in the C=O bond to a hydroxyl (OH) and attaching a proton to the functional group, resulting in going from (H2-C=O) to (H3-C-OH).
a cyano group (-C≡N) undergoes nucleophilic addition and reduces the carbonyl in the aldehyde/ketone to a hydroxyl group. The cyano group attaches to the aldehyde/ketone carbon.
2.Oxidation of Aldehydes
Due to the free hydrogen seen in aldehydes, aldehydes can be oxidized to transform the carbonyl group (C=O) to a carboxylic acid group (-COOH).
- Ketones and Aldehydes are reacted at the carbon in their carbonyl group in nucleophilic additions.
- Aldehydes can be oxidized to carboxylic acids; ketones cannot.
- Primary amines: Nitrogen with two hydrogens (-NH2)
- Secondary amines: Nitrogen with one hydrogen (-NH)
- Carboxylic acid: (-COOH)
- Reduction: Gain of electrons
- Oxidations: Loss of electrons