Properties and Uses of Aldehydes and Ketones

Physical properties :

1. HCHO is a gas with irritating smell. It is sold as 40% aqueous solution (with 8% CH₃OH) under the name formalin.

2. Formalin is used as preservative for biological specimen.

3. Acetone has pleasant smell and is used as an excellent organic solvent, nail polish remover etc.

4. C₆H₅CHO has bitter almond smell. It is also known as oil of bitter almond.

5. C₆H₅ COCH₃ is steam volatile solid

6. HCHO, CH₃CHO and CH₃COCH₃ are 100% soluble in water

7. Melting points and boiling points increases with increase in C - atoms

8. Branching decreases boiling point and melting points but the spherical symmetry of the molecule increases the melting point.

9. Ketones have slightly high m.p than isomeric aldehydes.

Chemical Properties :

b) Aldol condensation :

Migration of α-H atom from one carbonyl molecule to another, in the presence of dilute (10%) alkali (or) HCl_(g) at 10°C gives β-hydroxy carbonyl compound known as aldol which on strong heating eliminates water molecule.

(C) Crossed Aldol Condensation :

(i) Two different aldehydes, both having α-H-atom, produce four products. Major product is formed by migration of more acidic α-H atom.
(ii) For same acidic nature of α-H atom migration occurs to that group where C-atom has higher positive charge.

(iii) When acetophenone (Hypnone) is heated with C₂H₅ONa (or) aluminium tert-butoxide DYPNONE is formed.
(iv) If one carbonyl compound has no α-H-atom the reaction is called as Claisen Schmidt (as Claisen reaction).
(v) Intra molecular aldol condensation : It forms five (or) six membered rings.

D) Cannizzaro Reaction : In the presence of 50% aqueous (or) alcoholic NaOH aldehydes having no α-H- atom show auto oxidation and reduction i.e., disproportionation to give alcohol and acid salt at room temperature.

(iii) Simple α-mono alkylated aldehydes having α-H-atom also show cannizzaro reaction at about 300°C. These of course also show aldol condensation

(iv) CCl₃CHO does not contain α-H-atom. On heating with 50% aqueous NaOH, it forms CCl₃CH₂OH and CCl₃COONa mainly and a very little of CHCl₃

(v)  Crossed cannizzaro reaction: when two different aldehydes having no α-H-atom react; the lower one gives its acid salt because of higher positive charge an C-atom group of  lower aldehyde, attack of OH⁻ is easy on C^δ+.

C₆H₅ − CHO + HCHO + NaOH → C₆H₅CH₂OH + HCOONa

(vii) Internal cannizzaro reaction : It is related to a single molecule having two (or) more CHO groups (better for O - positions and weaker for m - positions) but no α-H-atom