S3.2.3 Homologous series

Homologous Series: Definition and Key Families of Organic Compounds

1. Imagine you’re organizing your bookshelf.
2. You group books by genre—mystery, science fiction, biographies—because books in the same genre often share similar themes or writing styles.

In organic chemistry, we use a similar strategy to classify compounds into "families" based on their structure and properties.

What Is a Homologous Series?

A homologous series is a group of organic compounds that:

1. Share the same general formula.
2. Differ by a single CH₂(methylene) unit between consecutive members.
3. Exhibit similar chemical properties due to the presence of the same functional group.
4. Show gradual changes in physical properties, such as boiling and melting points, as the chain length increases.

Key Features of a Homologous Series

• General Formula: Each series can be represented by a formula that applies to all its members.

• Incremental Change: Moving from one member to the next adds exactly one CH₂ group.

• Functional Group: All members of a homologous series share the same functional group, such as –OH for alcohols.

Why Are Homologous Series Important?

Homologous series simplify the study of organic chemistry by allowing you to:

• Predict properties and reactions of compounds based on their functional group.
• Identify trends in physical properties like boiling points, which increase with chain length due to stronger intermolecular forces.
• Deduce the structure of unknown compounds using their general formula.

Key Homologous Series and Their General Formulas

1.Alkanes ($C_n{H}_{2n+2}$)

• Structure: Saturated hydrocarbons (only single bonds between carbon atoms).
• Example: Methane ($C{H}_4$), ethane ($C_2{H}_6$), propane ($C_3{H}_8$).
• Chemical Properties: Relatively unreactive but can undergo combustion and substitution reactions.

2.Alkenes ($C_n{H}_{2n}$)

• Structure: Unsaturated hydrocarbons with at least one carbon-carbon double bond.
• Example: Ethene ($C_2{H}_4$), propene ($C_3{H}_6$).
• Chemical Properties: Undergo addition reactions due to the reactivity of the double bond.

3.Alkynes ($C_n{H}_{2n-2}$)

• Structure: Unsaturated hydrocarbons with at least one carbon-carbon triple bond.
• Example: Ethyne ($C_2{H}_2$), propyne ($C_3{H}_4$).
• Chemical Properties: Similar to alkenes but more reactive due to the triple bond.

4.Alcohols ($R–OH$)

• Structure: Contain a hydroxyl (–OH) functional group.
• Example: Methanol ($C{H}_{3}OH$), ethanol ($C_2{H}_{5}OH$).
• Chemical Properties: Can undergo combustion, oxidation, and dehydration reactions.

5.Aldehydes ($R–CHO$)

• Structure: Contain a carbonyl group (C=O) at the end of the carbon chain.
• Example: Methanal ($HCHO$), ethanal ($C{H}_{3}CHO$).
• Chemical Properties: Can be oxidized to carboxylic acids.

6.Ketones ($R–CO–R$)

• Structure: Contain a carbonyl group (C=O) within the carbon chain.
• Example: Propanone ($C{H}_{3}COC{H}_3$), butanone ($C{H}_{3}CO{C}_2{H}_5$).
• Chemical Properties: Less reactive than aldehydes but can undergo nucleophilic addition.

7.Carboxylic Acids ($R–COOH$)

• Structure: Contain a carboxyl group (–COOH).
• Example: Methanoic acid ($HCOOH$), ethanoic acid ($C{H}_{3}COOH$).
• Chemical Properties: Acidic, react with bases to form salts and water.

8.Ethers ($R–O–R$)

• Structure: Contain an oxygen atom bonded to two alkyl or aryl groups.
• Example: Methoxymethane ($C{H}_{3}OC{H}_3$), ethoxyethane ($C_2{H}_{5}O{C}_2{H}_5$).
• Chemical Properties: Relatively unreactive but can act as solvents.

9.Esters ($R–COOR$)

• Structure: Derived from carboxylic acids and alcohols.
• Example: Methyl ethanoate ($C{H}_{3}COOC{H}_3$), ethyl propanoate ($C_2{H}_{5}COO{C}_2{H}_5$).
• Chemical Properties: Pleasant-smelling, used in perfumes and flavorings.

10.Amines ($R–N{H}_2$)

• Structure: Contains a nitrogen atom bonded to one or more alkyl groups.
• Example: Methylamine ($C{H}_{3}N{H}_2$), Ethylamine ($C_2{H}_{5}N{H}_2$).
• Chemical Properties: Act as weak bases due to the nitrogen’s lone pair, capable of accepting protons.

11.Amides ($R–CON{H}_2$)

• Structure: Derived from carboxylic acids with a nitrogen atom bonded to the carbonyl group.
• Example: Ethanamide ($C{H}_{3}CON{H}_2$), Propanamide ($C_2{H}_{5}CON{H}_2$).
• Chemical Properties: Polar and capable of hydrogen bonding, leading to high boiling points and water solubility.

12.Halogenoalkanes ($R–X$)

• Structure: Contains a halogen atom (Cl, Br, or I) bonded to a carbon.
• Example: Chloroethane ($C{H}_{3}C{H}_{2}Cl$), Bromomethane ($C{H}_{3}Br$).
• Chemical Properties: Undergo nucleophilic substitution reactions due to the polar carbon-halogen bond.

Reflection and Practice