IB MYP 4-5 Chemistry -Homologous Series and Functional Groups - Study Notes - New Syllabus
IB MYP 4-5 Chemistry -Homologous Series and Functional Groups – Study Notes
Key Concepts
- Homologous Series and Functional Groups
Homologous Series and Functional Groups
Homologous Series
A homologous series is a group of organic compounds with the same functional group and similar chemical properties, where each successive member differs by a –CH₂– (methylene) unit in their molecular formula.
Characteristics of a Homologous Series
- All members have the same general formula.
- Each member differs from the next by one –CH₂– group (molecular mass difference of 14 units).
- Show a gradual change in physical properties (e.g., boiling point, solubility) with increasing molecular mass.
- Have similar chemical properties because they contain the same functional group.
- Can be represented by a general formula that applies to all members of the series.
Examples of Homologous Series
| Homologous Series | General Formula | Functional Group | Examples |
|---|---|---|---|
| Alkanes | \( \mathrm{C_{n}H_{2n+2}} \) | None (single bonds only) | Methane, Ethane, Propane |
| Alkenes | \( \mathrm{C_{n}H_{2n}} \) | C=C double bond | Ethene, Propene, Butene |
| Alkynes | \( \mathrm{C_{n}H_{2n-2}} \) | C≡C triple bond | Ethyne, Propyne, Butyne |
| Alcohols | \( \mathrm{C_{n}H_{2n+1}OH} \) | –OH (Hydroxyl) | Methanol, Ethanol, Propanol |
| Carboxylic Acids | \( \mathrm{C_{n}H_{2n+1}COOH} \) | –COOH (Carboxyl) | Methanoic acid, Ethanoic acid |
Functional Groups
A functional group is an atom or group of atoms within a molecule that determines the compound’s characteristic chemical reactions.
| Functional Group | Structure | Compound Type | Example |
|---|---|---|---|
Hydroxyl | –OH | Alcohols | Ethanol (\( \mathrm{C_2H_5OH} \)) |
Carboxyl | –COOH | Carboxylic acids | Ethanoic acid (\( \mathrm{CH_3COOH} \)) |
Carbonyl | C=O | Aldehydes / Ketones | Propanone (\( \mathrm{CH_3COCH_3} \)) |
Amino | –NH₂ | Amines | Ethylamine (\( \mathrm{C_2H_5NH_2} \)) |
General Trends in a Homologous Series
- Chemical properties: Similar due to the same functional group.
- Physical properties: Change gradually (e.g., boiling point increases with molecular mass).
- Solubility: Decreases with increasing chain length in polar solvents like water.
- Flammability: Decreases as molecular size increases.
Importance of Functional Groups
- Determine the compound’s reactivity and behavior.
- Help in classifying and naming organic compounds.
- Enable prediction of chemical reactions.
- Provide the basis for identifying new substances in organic chemistry.
Example
Identify the homologous series for the compound \( \mathrm{C_3H_8} \) and name it.
▶️ Answer / Explanation
Step 1: The formula fits the general form \( \mathrm{C_{n}H_{2n+2}} \).
Step 2: This corresponds to the alkane series.
Final Answer: The compound is propane, an alkane.
Example
Two compounds, \( \mathrm{CH_3OH} \) and \( \mathrm{C_2H_5OH} \), belong to the same homologous series. What is the molecular difference between them, and how does this affect their properties?
▶️ Answer / Explanation
Step 1: They differ by one –CH₂– group.
Step 2: This increases molecular mass by 14 units.
Step 3: Higher molecular mass → higher boiling point and lower solubility in water.
Final Answer: \( \mathrm{C_2H_5OH} \) (ethanol) has a higher boiling point and lower solubility than \( \mathrm{CH_3OH} \) (methanol).
Example
Compare the chemical properties of \( \mathrm{C_2H_5OH} \) (ethanol) and \( \mathrm{CH_3COOH} \) (ethanoic acid). Explain how their functional groups cause different behaviors.
▶️ Answer / Explanation
Step 1: Ethanol contains the hydroxyl (–OH) group; ethanoic acid contains the carboxyl (–COOH) group.
Step 2: The hydroxyl group gives ethanol neutral properties and allows it to form hydrogen bonds.
Step 3: The carboxyl group makes ethanoic acid acidic and able to donate protons (H⁺).
Final Answer: The functional group determines behavior — alcohols are neutral, while carboxylic acids are acidic due to the –COOH group.