Structure 3.2.2 – Functional Groups and Classification of Organic Compounds

Functional groups are atoms or groups of atoms that define the characteristic chemical and physical properties of organic compounds. Compounds are categorized based on the functional groups they contain.

Each homologous series contains compounds with the same functional group, general formula, and similar chemical behavior.

Functional Groups – Definitions and Structures

Functional groups are specific groups of atoms within molecules responsible for the characteristic reactions of those molecules. Below is a detailed explanation of each required functional group with its name, general structure, and one representative example.

1. Halogeno Group

General structure: –X (where X = Cl, Br, or I)

Description: A halogen atom bonded to a carbon chain. Halogenoalkanes are reactive due to the polar C–X bond.

Example: Chloromethane – \( \text{CH}_3\text{Cl} \)

2. Hydroxyl Group

General structure: –OH

Description: Found in alcohols. The oxygen is bonded to a hydrogen and a carbon. This group is polar and can form hydrogen bonds.

Example: Methanol – \( \text{CH}_3\text{OH} \)

3. Carbonyl Group

General structure: C=O

Description: A double bond between carbon and oxygen. It appears in two main forms:

• • Aldehyde: The carbonyl is at the end of the carbon chain, written as –CHO.
  • Ketone: The carbonyl is within the chain, bonded to two carbon atoms.

Examples: Ethanal – \( \text{CH}_3\text{CHO} \) (aldehyde), Propanone – \( \text{CH}_3\text{COCH}_3 \) (ketone)

4. Carboxyl Group

General structure: –COOH

Description: This group defines carboxylic acids. It contains a carbonyl and a hydroxyl group on the same carbon. It is acidic and can donate a proton.

Example: Ethanoic acid – \( \text{CH}_3\text{COOH} \)

5. Alkoxy Group

General structure: –OR (R = alkyl group)

Description: Found in ethers. An oxygen is bonded to two carbon atoms. These compounds are generally unreactive and used as solvents.

Example: Methoxyethane – \( \text{CH}_3\text{OCH}_2\text{CH}_3 \)

6. Amino Group

General structure: –NH₂

Description: Found in amines. A nitrogen atom is bonded to two hydrogen atoms and one carbon chain. It can act as a weak base.

Example: Aminoethane – \( \text{CH}_3\text{CH}_2\text{NH}_2 \)

7. Amido Group

General structure: –CONH₂

Description: Found in amides. It consists of a carbonyl group attached to a nitrogen. It is a product of the reaction between carboxylic acids and amines.

Example: Ethanamide – \( \text{CH}_3\text{CONH}_2 \)

8. Ester Group

General structure: –COO–

Description: Formed by the reaction between carboxylic acids and alcohols. Esters are known for their sweet/fruity smells.

Example: Methyl ethanoate – \( \text{CH}_3\text{COOCH}_3 \)

9. Phenyl Group

General structure: –C₆H₅

Description: Derived from benzene by removing one hydrogen atom. It represents an aromatic ring attached to a carbon chain.

Example: Phenol – \( \text{C}_6\text{H}_5\text{OH} \)

Saturated vs Unsaturated Compounds

Saturated compounds: Contain only single bonds between carbon atoms (e.g., alkanes). They are relatively stable and less reactive.

• The hydrocarbons (i.e., the carbon and hydrogen containing compounds) which have only single bonds between all the carbon atoms are known as saturated hydrocarbons.
• They are known as alkanes.
• Due to the presence of only single bonds, they are less reactive and more stable.
• They have a higher melting and boiling point because of the close packing of molecules.
• The hydrogen atoms present in these compounds can be replaced by some other atoms which is known as a substitution reaction.
• They undergo complete combustion on burning and produce non-sooty, blue flame.
• Eg: Butane

Unsaturated compounds: Contain one or more double or triple bonds (e.g., alkenes, alkynes). These compounds are generally more reactive.

• In unsaturated hydrocarbons, apart from single bonds, double and triple covalent bonds are also present between carbon atoms.
• The hydrocarbons having C=C double bonds are called alkenes and the ones with C≡C triple bonds are called alkynes.
• They are comparatively more reactive and less stable.
• They have a lower melting point because of the presence of double and triple bonds due to which close packing is not possible.
• These compounds undergo addition reactions wherein addition of atoms takes place over double bond.

             Reaction of Ethylene with Bromine

• They undergo incomplete combustion on burning and produce yellow, sooty flame.
• Eg: Butene, Butyne