This MCQ module is based on: Hydrocarbons, Functional Groups and Homologous Series
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Hydrocarbons, Functional Groups and Homologous Series
🎓 Class 10
Science
CBSE
Theory
Ch 4 — Carbon and its Compounds
⏱ ~20 min
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4.2.5 Hydrocarbons — Compounds of Carbon & Hydrogen
A hydrocarbon is a compound containing only carbon and hydrogen. Hydrocarbons are the simplest organic compounds and form the starting material for nearly every organic chemical you use — plastics, fuels, dyes, medicines, solvents.
Types of Carbon Chains
Saturated Hydrocarbons — Alkanes
Alkanes contain only C–C single bonds. They follow the general formula \(C_nH_{2n+2}\). Each successive alkane differs from the previous one by a –CH2– (methylene) unit.
| n | Formula | Name | State at RT |
|---|---|---|---|
| 1 | CH4 | Methane | Gas |
| 2 | C2H6 | Ethane | Gas |
| 3 | C3H8 | Propane | Gas (LPG) |
| 4 | C4H10 | Butane | Gas (LPG) |
| 5 | C5H12 | Pentane | Liquid |
Unsaturated Hydrocarbons — Alkenes & Alkynes
Alkenes contain one C=C double bond. General formula \(C_nH_{2n}\): C2H4 (ethene), C3H6 (propene), C4H8 (butene).
Alkynes contain one C≡C triple bond. General formula \(C_nH_{2n-2}\): C2H2 (ethyne, commonly called acetylene), C3H4 (propyne).
4.2.6 Nomenclature — Naming Carbon Compounds (IUPAC)
Chemists use a systematic naming system developed by the International Union of Pure and Applied Chemistry (IUPAC). A compound's name has three parts:
- Root/Prefix — indicates the number of carbons in the longest chain.
- Suffix — indicates the type of bonds or functional group.
- Position numbers — show where the bond/group is located (for chains of 4+ C).
| Carbons (n) | Prefix | Carbons (n) | Prefix |
|---|---|---|---|
| 1 | Meth- | 6 | Hex- |
| 2 | Eth- | 7 | Hept- |
| 3 | Prop- | 8 | Oct- |
| 4 | But- | 9 | Non- |
| 5 | Pent- | 10 | Dec- |
| Bond/Family | Suffix | Example |
|---|---|---|
| All single bonds (alkane) | -ane | CH3CH3 → ethane |
| One double bond (alkene) | -ene | CH2=CH2 → ethene |
| One triple bond (alkyne) | -yne | HC≡CH → ethyne |
4.2.7 Homologous Series
A homologous series is a family of carbon compounds sharing the same functional group and the same general formula. Key features:
- Successive members differ by a –CH2– group, i.e., by 14 atomic mass units (12 + 2).
- Members show a gradual change in physical properties (MP, BP, density) as molecular mass increases.
- All members show similar chemical properties because the functional group is the same.
- All members can be represented by a common general formula (e.g., \(C_nH_{2n+2}\) for alkanes).
Example — alkane homologous series: CH4 (M=16) → C2H6 (M=30) → C3H8 (M=44) → C4H10 (M=58). Each step adds 14 units.
Functional Groups (Table 4.3)
A functional group is the reactive centre of an organic compound — it decides its chemistry. The carbon chain is the "skeleton"; the functional group is the "hand" that reacts.
| Family | Functional group | Suffix | Example (IUPAC name) |
|---|---|---|---|
| Alcohol | –OH | -ol | CH3OH methanol |
| Aldehyde | –CHO | -al | HCHO methanal |
| Ketone | >C=O | -one | CH3COCH3 propanone |
| Carboxylic acid | –COOH | -oic acid | CH3COOH ethanoic acid |
| Halide | –X | halo- (prefix) | CH3Cl chloromethane |
Naming rule for functional groups: the carbon of the functional group is counted in the longest chain. The suffix of the parent alkane (-ane) changes to -anol, -anal, -anone, or -anoic acid. The "e" is dropped before a vowel.
Activity 4.2 — Build a Homologous SeriesL3 Apply
Predict: If the boiling point of methane is −162 °C and ethane is −89 °C, what trend do you expect for propane and butane?
- List the first five alkanes: CH4, C2H6, C3H8, C4H10, C5H12.
- Calculate the molecular mass of each (C=12, H=1). What difference do you notice?
- Look up (or predict) the boiling points. What trend do you see?
- Each member burns to give CO2 and H2O — similar chemistry, different physical state.
Molecular masses: 16 → 30 → 44 → 58 → 72, differing by exactly 14 (–CH2–).
Boiling points: −162 → −89 → −42 → 0 → 36 °C. BP rises steadily as chain lengthens (more molecular mass, stronger van der Waals forces). All members burn completely in excess O2: \(C_nH_{2n+2} + \tfrac{3n+1}{2}O_2 \rightarrow nCO_2 + (n+1)H_2O\). Same chemistry, graded physical properties — classic homologous behaviour.
Boiling points: −162 → −89 → −42 → 0 → 36 °C. BP rises steadily as chain lengthens (more molecular mass, stronger van der Waals forces). All members burn completely in excess O2: \(C_nH_{2n+2} + \tfrac{3n+1}{2}O_2 \rightarrow nCO_2 + (n+1)H_2O\). Same chemistry, graded physical properties — classic homologous behaviour.
Worked Examples
Example 1. Write the general formula and name the fourth member of the alkene series.
Solution: General formula: \(C_nH_{2n}\). Members: ethene (n=2), propene (n=3), butene (n=4), pentene (n=5). The fourth member is pentene, C5H10.
Example 2. Give the IUPAC name of CH₃–CH₂–CH₂–OH.
Solution: Chain has 3 carbons → prop-. Functional group is –OH → -ol. Dropping "e" of propane: propan-1-ol (or simply propanol).
Example 3. Which of the following is an unsaturated hydrocarbon: C₃H₈ or C₃H₆? Justify.
Solution: Checking \(C_nH_{2n+2}\): for n=3 that gives C3H8 — saturated (propane). C3H6 fits \(C_nH_{2n}\), matching the alkene formula → propene, unsaturated (has a C=C).
Example 4. Write the name of the compound with formula CH₃COOH and identify its functional group.
Solution: Two carbons in the chain (including the C of –COOH) → eth-. Functional group –COOH → -oic acid suffix. Name: ethanoic acid (common: acetic acid, present in vinegar). Functional group: carboxylic acid (–COOH).
Example 5. Write the next homologue of CH₃COCH₃.
Solution: CH3COCH3 is propanone (3 C, ketone). The next homologue adds one –CH2– giving CH3COCH2CH3 — butanone (4 C).
Interactive: Name That Compound L3 Apply
Type the IUPAC name of each compound (lowercase).
Q1. CH3–CH2–CH3
Q2. CH2=CH–CH3
Q3. CH3–CH2–OH
Q4. HC≡CH
Q5. HCOOH
Competency-Based Questions
A science club collects four bottles labelled only with their molecular formulae: C2H6, C2H4, C2H5OH and CH3COOH. The team has to identify each and write their IUPAC names.
Q1. L1 Remember The general formula of alkanes is:
Answer: B. Alkanes are saturated hydrocarbons with general formula \(C_nH_{2n+2}\).
Q2. L2 Understand Identify and name the functional group in CH₃CH₂CHO. (2 marks)
Answer: The –CHO group is the aldehyde functional group. The compound is propanal (3 C + aldehyde suffix -al).
Q3. L3 Apply Write the next two homologues of methanol (CH₃OH). (2 marks)
Answer: Each homologue adds a –CH2– unit. Next: C2H5OH (ethanol); next: C3H7OH (propanol).
Q4. L4 Analyse Two members of a homologous series have similar chemical properties but different physical properties. Explain why. (3 marks)
Answer: The chemical properties of an organic compound are governed by the functional group, which is identical across a homologous series — hence similar chemistry. Physical properties (MP, BP, solubility, density) depend on molecular mass and chain length; these increase steadily across the series, giving graded physical properties.
Q5. L5 Evaluate A student writes the formula of butanone as CH₃CH₂CH₂CHO. Evaluate whether this is correct. (3 marks)
Answer: The student is incorrect. CH3CH2CH2CHO has an aldehyde group (–CHO) — that is butanal, not butanone. Butanone is a ketone with the structure CH3COCH2CH3 — the C=O is on the second carbon, not at the end of the chain.
Assertion-Reason Questions
Assertion (A): All members of a homologous series have the same general formula.
Reason (R): Consecutive members in the series differ by a –CH2– unit.
Answer: A. Both are true; differing by –CH2– consistently is exactly what gives rise to the common general formula.
Assertion (A): C2H4 is an unsaturated hydrocarbon.
Reason (R): It contains a carbon–carbon double bond.
Answer: A. Both true; the C=C bond defines it as unsaturated (alkene).
Assertion (A): Ethanoic acid and methanoic acid belong to the same homologous series.
Reason (R): Both have the same molecular mass.
Answer: C. Both are carboxylic acids (–COOH), so A is true. But their molecular masses differ by 14 (HCOOH = 46, CH3COOH = 60), so R is false.
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