ALDEHYDES (R–CHO)

Definition

Aldehydes are organic compounds that contain the formyl functional group (–CHO), in which a carbon atom is double-bonded to oxygen and single-bonded to hydrogen.

General formula:
R–CHO

where R is an alkyl or aryl group.

Nomenclature (Naming of Aldehydes)

IUPAC Naming Rules

1. Identify the longest carbon chain containing the –CHO group.

2. The carbon of the –CHO group is always numbered carbon 1.

3. Replace the –e ending of the parent alkane with –al.

Examples:

Molecular Formula	IUPAC Name	Common Name
HCHO	Methanal	Formaldehyde
CH₃CHO	Ethanal	Acetaldehyde
C₂H₅CHO	Propanal	Propionaldehyde
C₆H₅CHO	Benzaldehyde	Benzaldehyde

Occurrence of Aldehydes

• Found in natural substances such as vanilla (vanillin) and cinnamon (cinnamaldehyde).

• Present in essential oils, perfumes, and flavorings.

• Produced during oxidation of alcohols.

Preparation of Aldehydes

1. Oxidation of Primary Alcohols

Aldehydes are prepared by controlled oxidation of primary alcohols.

R–CH₂OH + [O] → R–CHO + H₂O

Oxidizing agents:

• Acidified potassium dichromate (VI) (K₂Cr₂O₇/H₂SO₄)

• Acidified potassium permanganate (KMnO₄)

NOTE:  Excess oxidation converts aldehydes to carboxylic acids.

2. Dehydrogenation of Alcohols

Passing alcohol vapour over heated copper (300°C).

R–CH₂OH ---{Cu}--> R–CHO + H₂

3. Hydrolysis of Geminal Dihalides

R–CHX₂ + ---{H₂O}--->R–CHO

Physical Properties of Aldehydes

1. Lower aldehydes are colorless liquids with sharp smells.

2. They are polar compounds.

3. Lower members are soluble in water due to hydrogen bonding.

4. Boiling points are higher than alkanes but lower than alcohols.

5. Formaldehyde exists as a gas at room temperature.

Chemical Properties of Aldehydes

1. Oxidation (Reducing Nature)

Aldehydes are easily oxidized to carboxylic acids.

R–CHO + [O] → R–COOH
 

a. Tollens’ Test (Silver Mirror Test)

• Aldehydes reduce ammoniacal silver nitrate to silver.

• Produces a silver mirror.

✔️ Distinguishes aldehydes from ketones.

b. Fehling’s Solution Test

• Aldehydes reduce Fehling’s solution to a brick-red precipitate (Cu₂O).

2. Reduction

Aldehydes are reduced to primary alcohols.

R–CHO + [H] → R–CH₂OH

Reducing agents:

• NaBH₄

• LiAlH₄

• Hydrogen/Nickel catalyst

3. Addition Reactions

Due to the polar C=O bond, aldehydes undergo nucleophilic addition.

Example:
R–CHO + HCN → R–CH(OH)CN

4. Reaction with Sodium Hydrogen Sulphite

Forms crystalline addition compounds (used for purification).

Uses of Aldehydes

1. Formaldehyde (Methanal)

• Used as formalin for preserving specimens.

• Manufacture of plastics, resins, and disinfectants.

• Used in textile and leather industries.

2. Ethanal (Acetaldehyde)

• Manufacture of acetic acid.

• Used in perfumes and dyes.

3. Benzaldehyde

• Used in flavorings and perfumes (almond smell).

Differences Between Aldehydes and Ketones

Aldehydes	Ketones
Have –CHO group	Have –CO– group
Easily oxidized	Not easily oxidized
Give Tollens’ test	Do not
Carbonyl carbon attached to H	No H attached

Important tip

• Aldehydes are reducing agents.

• Prepared from primary alcohols only.

• Distinguished using Tollens’ and Fehling’s tests.

• General formula: R–CHO

 

OBJECTIVE QUESTIONS

1. The functional group present in aldehydes is
   A. –COOH
   B. –CO–
   C. –CHO
   D. –OH

2. The general formula of aliphatic aldehydes is
   A. R–CO–R
   B. R–COOH
   C. R–CHO
   D. R–OH

3. The IUPAC name of CH₃CHO is
   A. Methanal
   B. Ethanal
   C. Propanal
   D. Acetone

4. Which of the following alcohols on oxidation produces an aldehyde?
   A. Tertiary alcohol
   B. Secondary alcohol
   C. Primary alcohol
   D. Phenol

5. Which reagent gives a silver mirror with aldehydes?
   A. Fehling’s solution
   B. Benedict’s solution
   C. Tollens’ reagent
   D. Acidified KMnO₄

6. Aldehydes can be oxidized to form
   A. Ketones
   B. Alcohols
   C. Carboxylic acids
   D. Esters

7. Which aldehyde exists as a gas at room temperature?
   A. Ethanal
   B. Methanal
   C. Propanal
   D. Benzaldehyde

8. Aldehydes act as reducing agents because they
   A. accept hydrogen easily
   B. are easily oxidized
   C. are acidic
   D. have low boiling points

9. Which of the following does NOT react with aldehydes?
   A. Tollens’ reagent
   B. Fehling’s solution
   C. Sodium hydrogen sulphite
   D. Schiff’s reagent

10. Reduction of an aldehyde produces a
    A. secondary alcohol
    B. tertiary alcohol
    C. primary alcohol
    D. carboxylic acid

11. Which of the following aldehydes has an aromatic ring?
    A. Methanal
    B. Ethanal
    C. Propanal
    D. Benzaldehyde

12. The oxidation of aldehydes to acids involves
    A. loss of hydrogen
    B. gain of hydrogen
    C. loss of oxygen
    D. no chemical change

13. Aldehydes undergo nucleophilic addition mainly due to the
    A. non-polar nature of C=O
    B. polarity of the carbonyl group
    C. presence of –OH group
    D. acidic nature of hydrogen

14. Which of the following can be used to purify aldehydes?
    A. Dilute acids
    B. Sodium hydroxide
    C. Sodium hydrogen sulphite
    D. Calcium chloride

15. The common name of methanal is
    A. Acetaldehyde
    B. Formaldehyde
    C. Propionaldehyde
    D. Benzaldehyde