Structure 3.2.3 – Homologous Series

A homologous series is a family of organic compounds with the same functional group and similar chemical properties. Members of a homologous series differ from each other by a constant structural unit, typically a methylene group \( (\text{CH}_2) \).

 Characteristics of a Homologous Series

• All members contain the same functional group.
• Each successive compound differs by an additional \( \text{CH}_2 \) group (14 amu).
• They share the same general formula.
• They have similar chemical properties due to the common functional group.
• They exhibit gradual variation in physical properties (e.g., boiling point, solubility).

Why Homologous Series Are Important

• They help in systematically classifying organic compounds.
• They simplify the study of chemical properties by grouping similar compounds together.
• They assist in understanding trends in physical and chemical behavior across a group.

Major Homologous Series in Organic Chemistry

Below are the main homologous series required for IBDP Chemistry. Each series contains compounds with the same functional group and follows a general formula. Successive members differ by a –\( \text{CH}_2 \)– unit.

1. Alkanes

Alkanes are saturated hydrocarbons containing only single bonds between carbon atoms. They are relatively unreactive except in combustion or substitution reactions.

• Homologous series: Alkanes (saturated hydrocarbons)
• General formula: \( \text{C}_n\text{H}_{2n+2} \)
• Functional group: None (single C–C bonds only)
• First compound: Methane – \( \text{CH}_4 \)
• Next members: Ethane –\( \text{CH}_3\text{CH}_3 \), Propane – \( \text{CH}_3\text{CH}_2\text{CH}_3 \), Butane –\( \text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_3 \)
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2. Alkenes

Alkenes are unsaturated hydrocarbons that contain at least one carbon–carbon double bond. They are more reactive than alkanes and undergo addition reactions.

• Homologous series: Alkenes (unsaturated hydrocarbons with C=C)
• General formula: \( \text{C}_n\text{H}_{2n} \)
• Functional group: C=C double bond
• First compound: Ethene – \( \text{CH}_2\text{CH}_2 \)
• Next members: Propene – \( \text{CH}_2\text{CHCH}_3 \), Butene – \( \text{CH}_2\text{CHCH}_2\text{CH}_3 \), Pentene – \( \text{CH}_2\text{CHCH}_2\text{CH}_2\text{CH}_3 \)
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3. Alkynes

Alkynes are unsaturated hydrocarbons that contain at least one carbon–carbon triple bond. They undergo similar addition reactions to alkenes.

• Homologous series: Alkynes (unsaturated hydrocarbons with C≡C)
• General formula: \( \text{C}_n\text{H}_{2n-2} \)
• Functional group: C≡C triple bond
• First compound: Ethyne – \( \text{CH}\equiv\text{CH} \)
• Next members: Propyne – \( \text{CH}\equiv\text{CCH}_3 \), Butyne – \( \text{CH}_3\text{C}\equiv\text{CCH}_3 \), Pentyne – \( \text{CH}_3\text{CH}_2\text{C}\equiv\text{CH} \)
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4. Halogenoalkanes

These are compounds in which one or more hydrogen atoms in an alkane have been replaced by halogens (Cl, Br, or I). They undergo nucleophilic substitution reactions.

• Homologous series: Halogenoalkanes (alkyl halides)
• Functional group: –X (X = Cl, Br, I)
• First compound: Chloromethane – \( \text{CH}_3\text{Cl} \)
• Next members: Chloroethane – \( \text{CH}_3\text{CH}_2\text{Cl} \), 1-Chloropropane – \( \text{CH}_3\text{CH}_2\text{CH}_2\text{Cl} \), 1-Chlorobutane – \( \text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{Cl} \)
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5. Alcohols

Alcohols contain the hydroxyl (–OH) functional group. They are polar and can hydrogen bond, making them soluble in water. They undergo combustion and oxidation.

• Homologous series: Alcohols
• General formula: \( \text{C}_n\text{H}_{2n+1}\text{OH} \)
• Functional group: –OH (hydroxyl group)
• First compound: Methanol – \( \text{CH}_3\text{OH} \)
• Next members: Ethanol – \( \text{CH}_3\text{CH}_2\text{OH} \), Propanol – \( \text{CH}_3\text{CH}_2\text{CH}_2\text{OH} \), Butanol – \( \text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{OH} \)
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6. Aldehydes

Aldehydes have a carbonyl group (C=O) at the end of the carbon chain. They are produced by the partial oxidation of primary alcohols.

• Homologous series: Aldehydes
• Functional group: –CHO (carbonyl at the end)
• First compound: Methanal – \( \text{HCHO} \)
• Next members: Ethanal – \( \text{CH}_3\text{CHO} \), Propanal – \( \text{CH}_3\text{CH}_2\text{CHO} \), Butanal – \( \text{CH}_3\text{CH}_2\text{CH}_2\text{CHO} \)
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7. Ketones

Ketones contain a carbonyl group located within the carbon chain (not at the end). They are formed by oxidation of secondary alcohols.

• Homologous series: Ketones
• Functional group: –CO– (carbonyl in the middle)
• First compound: Propanone – \( \text{CH}_3\text{COCH}_3 \)
• Next members: Butanone – \( \text{CH}_3\text{COCH}_2\text{CH}_3 \), Pentanone – \( \text{CH}_3\text{COCH}_2\text{CH}_2\text{CH}_3 \), Hexanone – \( \text{CH}_3\text{COCH}_2\text{CH}_2\text{CH}_2\text{CH}_3 \)
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8. Carboxylic Acids

These are organic acids containing the carboxyl (–COOH) group. They are weak acids and react with metals, carbonates, and bases.

• Homologous series: Carboxylic acids
• General formula: \( \text{C}_n\text{H}_{2n+1}\text{COOH} \)
• Functional group: –COOH (carboxyl group)
• First compound: Methanoic acid – \( \text{HCOOH} \)
• Next members: Ethanoic acid – \( \text{CH}_3\text{COOH} \), Propanoic acid – \( \text{CH}_3\text{CH}_2\text{COOH} \), Butanoic acid – \( \text{CH}_3\text{CH}_2\text{CH}_2\text{COOH} \)
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9. Ethers

Ethers contain an oxygen atom connected to two alkyl groups (R–O–R′). They are generally unreactive and used as solvents.

• Homologous series: Ethers
• Functional group: –O– (alkoxy group)
• First compound: Methoxymethane – \( \text{CH}_3\text{OCH}_3 \)
• Next members: Methoxyethane – \( \text{CH}_3\text{OCH}_2\text{CH}_3 \), Ethoxyethane – \( \text{CH}_3\text{CH}_2\text{OCH}_2\text{CH}_3 \), Propoxyethane – \( \text{CH}_3\text{CH}_2\text{CH}_2\text{OCH}_2\text{CH}_3 \)
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10. Amines

Amines contain a nitrogen atom bonded to carbon and/or hydrogen. They are basic and derived from ammonia.

• Homologous series: Primary amines
• Functional group: –NH₂ (amino group)
• First compound: Methanamine – \( \text{CH}_3\text{NH}_2 \)
• Next members: Ethanamine – \( \text{CH}_3\text{CH}_2\text{NH}_2 \), Propanamine – \( \text{CH}_3\text{CH}_2\text{CH}_2\text{NH}_2 \), Butanamine – \( \text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{NH}_2 \)
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11. Amides

Amides contain both a carbonyl group and an amine group. They are formed from reactions between carboxylic acids and amines or ammonia.

• Homologous series: Amides
• Functional group: –CONH₂ (carboxamide group)
• First compound: Methanamide – \( \text{HCONH}_2 \)
• Next members: Ethanamide – \( \text{CH}_3\text{CONH}_2 \), Propanamide – \( \text{CH}_3\text{CH}_2\text{CONH}_2 \), Butanamide – \( \text{CH}_3\text{CH}_2\text{CH}_2\text{CONH}_2 \)
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12. Esters

Esters are formed from the reaction between a carboxylic acid and an alcohol. They are known for their sweet/fruity smells and are used in perfumes and food flavoring.

• Homologous series: Esters
• Functional group: –COO– (ester linkage)
• First compound: Methyl methanoate – \( \text{HCOOCH}_3 \)
• Next members: Methyl ethanoate – \( \text{CH}_3\text{COOCH}_3 \), Ethyl ethanoate – \( \text{CH}_3\text{COOCH}_2\text{CH}_3 \), Propyl ethanoate – \( \text{CH}_3\text{COOCH}_2\text{CH}_2\text{CH}_3 \)
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