a. Perkin's condensation: The reaction between an aromatic aldehyde and the anhydride of an aliphatic acid in the presence of sodium salt of the same acid, leading to the formation of an alpha, beta unsaturated acid is called Perkin's reaction. For example, benzaldehyde and acetic anhydride give cinnamic acid.

benzaldehyde + acetic anhydride --CH₃COONA (455K)----> Cinnamic acid + CH₃COOH

b. Aldol Condensation: Aldehydes and Ketones having at least one alpha-hydrogen atom in the presence of dilute alkali (dil. NaOH, Na₂CO₃, Ba(OH)₂₂ etc.) undergo rearrangement to give beta-hydroxy aldehyde or beta- hydroxy ketone. This reaction is called aldol condensation. Eg:

Ethanal + Ethanal --OH^-----> 3-hydrocybutanal (aldol)

c. Cannizzaro's reaction: The aldehydes which do not contain any alpha-hydrogen atom in the presence of concentrated solution of an alkali undergo self oxidation-reduction to give a mixture of alcohol and a salt of carboxylic acid is called Cannizzaro reaction. Eg: Formaldehyde gives this reaction with the formation of methyl alcohol and sodium formate.  

2 molecules of methanal (Formaldehyde)----NaOH(conc.)---> Methanol +                                                                                                                             sodiummethanoate (sodium                                                                                                                                    formate)

d. Benzoin condensation: Aromatic aldehydes eg: benzaldehyde when heated with ethanolic solution of KCN undergo condensation to form benzoin. SO, the formation of benzoin by the condensation of two aromatic aldehyde molecules in the presence of hot ethanolic KCN solution is called benzoin condensation. eg:

2 molecules of Benzaldehyde --- KCN(alc.)/heat-------> benzoin

e. Wolff-Kishner Reduction: When hydrazones (or semicarbazones) are heated with sodium ethoxide at 180⁰ C, nitrogen is eliminated and a hydrocarbon is obtained. This is called wolff-Kishner reduction. eg:

f. Carbylaminereaction: When a primary amine is heated with alcoholic caustic potash, and chloroform an offensive smelling compound called carbylamine (isocyanide) is formed. This reaction is used as a test for chloroform or for a primary amine.