Aldehydes are organic compounds that contain a carbonyl group (C=O) bonded to at least one hydrogen atom. They are widely used as reagents for chemical synthesis because they can undergo various reactions with different nucleophiles, such as alcohols, amines, hydrides, organometallics and enolates. Some of the common reactions of aldehydes include:

• Reduction: Aldehydes can be reduced to primary alcohols by using reducing agents such as sodium borohydride (NaBH₄) or lithium aluminium hydride (LiAlH₄).
• Oxidation: Aldehydes can be oxidized to carboxylic acids by using oxidizing agents such as potassium permanganate (KMnO₄) or chromic acid (H₂CrO₄).
• Nucleophilic addition: Aldehydes can react with nucleophiles to form addition products such as hemiacetals, acetals, imines and cyanohydrins.
• Condensation: Aldehydes can react with other aldehydes or ketones to form condensation products such as aldols and crossed aldols.
• Cannizzaro reaction: Aldehydes that do not have alpha-hydrogens can undergo disproportionation reaction in the presence of a strong base to form a carboxylic acid and an alcohol.

These reactions allow aldehydes to be converted into various functional groups and intermediates for further synthesis. Therefore, aldehydes are versatile and useful reagents for chemical synthesis.