Chapter 9: Hydrocarbons
9.1 Introduction
Hydrocarbons are organic compounds containing only carbon and hydrogen.
| Type | General Formula | C–C Bond | Example |
|---|---|---|---|
| Alkanes | \(C_nH_{2n+2}\) | Single | CH₄, C₂H₆ |
| Alkenes | \(C_nH_{2n}\) | Double | C₂H₄, C₃H₆ |
| Alkynes | \(C_nH_{2n-2}\) | Triple | C₂H₂, C₃H₄ |
| Aromatic | Variable | Delocalised π | C₆H₆ (benzene) |
PART A: ALIPHATIC HYDROCARBONS
9.2 Alkanes (Paraffins)
Nomenclature (IUPAC)
| No. of C | Root | Alkane | Formula |
|---|---|---|---|
| 1 | Meth- | Methane | CH₄ |
| 2 | Eth- | Ethane | C₂H₆ |
| 3 | Prop- | Propane | C₃H₈ |
| 4 | But- | Butane | C₄H₁₀ |
| 5 | Pent- | Pentane | C₅H₁₂ |
| 6 | Hex- | Hexane | C₆H₁₄ |
Isomerism in Alkanes (Chain Isomerism)
C₄H₁₀ has 2 isomers: n-butane, isobutane (2-methylpropane)
C₅H₁₂ has 3 isomers: pentane, 2-methylbutane, 2,2-dimethylpropane (neopentane)
Conformation of Ethane
Conformations are different spatial arrangements obtained by rotation about a C–C single bond.
Energy difference: Eclipsed form is ~12 kJ/mol higher in energy than staggered form due to torsional strain.
Physical Properties of Alkanes
- First four members (C₁–C₄) are gases; C₅–C₁₇ are liquids; C₁₈+ are solids
- Boiling point increases with molecular mass
- Branching decreases boiling point
- Insoluble in water (non-polar), soluble in organic solvents
Chemical Reactions of Alkanes
1. Halogenation (Free Radical Substitution)
\[ \text{CH}_4 + \text{Cl}_2 \xrightarrow{h\nu} \text{CH}_3\text{Cl} + \text{HCl} \]
Mechanism (Free Radical Chain Reaction):
Step 1: Initiation
\[ \text{Cl}_2 \xrightarrow{h\nu} 2\text{Cl}· \]
Step 2: Propagation
\[ \text{CH}_4 + \text{Cl}· \rightarrow \text{CH}_3· + \text{HCl} \]
\[ \text{CH}_3· + \text{Cl}_2 \rightarrow \text{CH}_3\text{Cl} + \text{Cl}· \]
Step 3: Termination
\[ \text{CH}_3· + \text{Cl}· \rightarrow \text{CH}_3\text{Cl} \]
Reactivity of H atoms: 3° > 2° > 1°
2. Combustion
\[ \text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O} \quad \Delta_c H = -890;\text{kJ/mol} \]
General: \(\text{C}n\text{H}{2n+2} + \frac{3n+1}{2}\text{O}_2 \rightarrow n\text{CO}_2 + (n+1)\text{H}_2\text{O}\)
3. Pyrolysis (Cracking)
\[ \text{C}{10}\text{H}{22} \xrightarrow{500°\text{C}} \text{C}5\text{H}{12} + \text{C}5\text{H}{10} \]
Higher alkanes → smaller alkanes + alkenes (thermal decomposition)
9.3 Alkenes (Olefins)
Structure of Double Bond (Ethene)
- Each carbon is sp² hybridised
- Bond angle ≈ 120°, planar geometry
- One σ bond (head-on overlap of sp² orbitals) + One π bond (lateral overlap of unhybridised p orbitals)
Geometrical (cis-trans) Isomerism
Alkenes with two different groups on each doubly-bonded carbon show geometrical isomerism due to restricted rotation about the C=C bond.
Methods of Preparation of Alkenes
- Dehydrohalogenation of alkyl halides (Saytzeff’s rule):
\[ \text{CH}_3\text{CHBrCH}_3 \xrightarrow{\text{alc. KOH}} \text{CH}_3\text{CH=CH}_2 + \text{HBr} \]
- Dehydration of alcohols:
\[ \text{CH}_3\text{CH}_2\text{OH} \xrightarrow{\text{H}_2\text{SO}_4, 443\text{K}} \text{CH}_2=\text{CH}_2 + \text{H}_2\text{O} \]
Chemical Reactions of Alkenes
Addition Reactions
(a) Addition of hydrogen (hydrogenation):
\[ \text{CH}_2=\text{CH}_2 + \text{H}_2 \xrightarrow{\text{Ni/Pt/Pd}} \text{CH}_3\text{CH}_3 \]
(b) Addition of halogens (halogenation):
\[ \text{CH}_2=\text{CH}_2 + \text{Br}_2 \rightarrow \text{CH}_2\text{BrCH}_2\text{Br} \]
This is used as a test for unsaturation — decolorization of brown Br₂ water.
(c) Addition of HX (Markovnikov’s rule):
“The negative part of the addendum (X) adds to the carbon bearing fewer hydrogen atoms.”
\[ \text{CH}_3\text{CH=CH}_2 + \text{HBr} \rightarrow \text{CH}_3\text{CHBrCH}_3 \quad \text{(Markovnikov product)} \]
(d) Anti-Markovnikov addition (Peroxide effect / Kharash effect):
In the presence of organic peroxides (e.g., benzoyl peroxide), HBr adds in anti-Markovnikov fashion:
\[ \text{CH}_3\text{CH=CH}_2 + \text{HBr} \xrightarrow{\text{peroxide}} \text{CH}_3\text{CH}_2\text{CH}_2\text{Br} \]
This works only with HBr, not with HCl or HI.
(e) Addition of water (hydration):
\[ \text{CH}_2=\text{CH}_2 + \text{H}_2\text{O} \xrightarrow{\text{H}^+} \text{CH}_3\text{CH}_2\text{OH} \]
Oxidation
- With cold, dilute KMnO₄ (Baeyer’s test): Forms glycol
\[ \text{CH}_2=\text{CH}_2 + \text{H}_2\text{O} + [\text{O}] \xrightarrow{\text{KMnO}_4} \text{HOCH}_2\text{CH}_2\text{OH} \]
(Decolorization of pink KMnO₄ → test for unsaturation)
- Ozonolysis: Alkene + O₃ → ozonide → cleaved by Zn/H₂O → carbonyl compounds
\[ \text{R}_1\text{R}_2\text{C=CR}_3\text{R}_4 \xrightarrow{1.,\text{O}_3,\ 2.,\text{Zn/H}_2\text{O}} \text{R}_1\text{R}_2\text{C=O} + \text{R}_3\text{R}_4\text{C=O} \]
9.4 Alkynes
Structure of Triple Bond (Ethyne)
- Each carbon is sp hybridised
- Linear geometry, bond angle = 180°
- One σ bond + Two π bonds
Acidic Character of Alkynes
Terminal alkynes (with C≡C-H) show acidic character because the sp-hybridised carbon has more s-character (50%) → holds electrons more tightly → makes H more acidic.
\[ \text{CH≡CH} + \text{Na} \rightarrow \text{CH≡CNa} + \frac{1}{2}\text{H}_2 \]
Acidity order: HC≡CH > H₂C=CH₂ > H₃C−CH₃ (sp > sp² > sp³)
Chemical Reactions of Alkynes
(a) Addition of H₂:
\[ \text{CH≡CH} \xrightarrow{\text{H}_2/\text{Pd-BaSO}_4} \text{CH}_2=\text{CH}_2 \xrightarrow{\text{H}_2/\text{Ni}} \text{CH}_3\text{CH}_3 \]
Lindlar’s catalyst (Pd/BaSO₄ + quinoline) → gives cis-alkene (partial reduction)
(b) Addition of HX: Two molecules add (Markovnikov’s rule applies)
\[ \text{CH≡CH} \xrightarrow{\text{HCl}} \text{CH}_2=\text{CHCl} \xrightarrow{\text{HCl}} \text{CH}_3\text{CHCl}_2 \]
(c) Addition of water (hydration):
\[ \text{CH≡CH} + \text{H}_2\text{O} \xrightarrow{\text{H}_2\text{SO}_4/\text{HgSO}_4} \text{CH}_3\text{CHO} \quad \text{(acetaldehyde)} \]
9.5 Aromatic Hydrocarbons
Benzene: Structure and Aromaticity
Benzene (C₆H₆) was proposed by Kekulé (1865) as a cyclic structure with alternating single and double bonds.
Actual structure: All C–C bonds in benzene are equal (139 pm, intermediate between C–C 154 pm and C=C 134 pm) due to resonance / delocalised π system.
Hückel’s Rule for Aromaticity
A cyclic planar compound is aromatic if it has \((4n + 2)\) π electrons, where \(n = 0, 1, 2, 3, \ldots\)
| Compound | π electrons | \(n\) | Aromatic? |
|---|---|---|---|
| Benzene | 6 | 1 | ✓ |
| Cyclooctatetraene | 8 | — | ✗ (anti-aromatic) |
| Naphthalene | 10 | 2 | ✓ |
| [14]-Annulene | 14 | 3 | ✓ |
Electrophilic Aromatic Substitution (EAS)
The characteristic reaction of benzene is electrophilic substitution (not addition), because substitution preserves aromaticity.
General Mechanism:
\[ \text{ArH} + \text{E}^+ \rightarrow [\text{Arenium ion}] \rightarrow \text{ArE} + \text{H}^+ \]
| Reaction | Electrophile (E⁺) | Conditions | Product |
|---|---|---|---|
| Nitration | NO₂⁺ | conc. HNO₃ + H₂SO₄ | Nitrobenzene |
| Sulphonation | SO₃ / SO₃H⁺ | Fuming H₂SO₄ | Benzenesulphonic acid |
| Halogenation | Cl⁺ (or Br⁺) | Cl₂/AlCl₃ or Br₂/FeBr₃ | Chlorobenzene |
| Friedel-Crafts Alkylation | R⁺ | RCl/AlCl₃ | Alkylbenzene |
| Friedel-Crafts Acylation | RCO⁺ | RCOCl/AlCl₃ | Acylbenzene |
Directive Influence of Substituents
Existing substituents on benzene direct incoming groups to specific positions:
| Type | Position | Effect on rate | Examples |
|---|---|---|---|
| Activating, ortho/para-directing | o- and p- | Faster than benzene | −OH, −NH₂, −OCH₃, −CH₃, −NHCOCH₃ |
| Deactivating, meta-directing | m- | Slower than benzene | −NO₂, −CN, −COOH, −CHO, −COR, −SO₃H |
| Deactivating, ortho/para-directing | o- and p- | Slower than benzene | −F, −Cl, −Br, −I |
Carcinogenicity and Toxicity
- Benzene is a known carcinogen (causes leukaemia)
- Polynuclear aromatic hydrocarbons (PAHs) like benzo[a]pyrene (found in coal tar and tobacco smoke) are potent carcinogens
- Toluene poisoning can cause damage to the central nervous system
Practice Questions
Multiple Choice Questions (MCQs)
1. The product formed when propene reacts with HBr in the presence of peroxide is:
(a) 2-Bromopropane
(b) 1-Bromopropane
(c) 2-Bromobutane
(d) Propan-2-ol
2. Which of the following is NOT aromatic?
(a) Benzene
(b) Naphthalene
(c) Cyclooctatetraene
(d) Pyridine
3. The electrophile in Friedel-Crafts acylation is:
(a) R⁺
(b) RCO⁺
(c) AlCl₃
(d) Cl⁺
4. Lindlar’s catalyst converts alkynes to:
(a) Alkanes
(b) trans-Alkenes
(c) cis-Alkenes
(d) Alkynes remain unchanged
5. The correct order of decreasing acidity is:
(a) \(\text{HC≡CH} > \text{H}_2\text{C=CH}_2 > \text{H}_3\text{C-CH}_3\)
(b) \(\text{H}_3\text{C-CH}_3 > \text{H}_2\text{C=CH}_2 > \text{HC≡CH}\)
(c) \(\text{H}_2\text{C=CH}_2 > \text{HC≡CH} > \text{H}_3\text{C-CH}_3\)
(d) \(\text{HC≡CH} > \text{H}_3\text{C-CH}_3 > \text{H}_2\text{C=CH}_2\)
6. Which substituent is meta-directing?
(a) −OH
(b) −CH₃
(c) −NO₂
(d) −NH₂
Short Answer Questions (2–3 Marks)
7. State Markovnikov’s rule. What is the exception (anti-Markovnikov addition)?
8. Give the mechanism of free radical halogenation of methane.
9. What are conformational isomers? Draw the eclipsed and staggered conformations of ethane.
10. Write the reactions of ethyne with: (i) HCl (ii) water
11. Explain why benzene undergoes electrophilic substitution rather than addition.
Long Answer Questions (5 Marks)
12. (a) Explain Hückel’s rule for aromaticity. Using this rule, predict whether the following are aromatic: (i) Cyclopentadienyl anion (ii) Cycloheptatrienyl cation (iii) Cyclooctatetraene.
(b) What are polynuclear aromatic hydrocarbons? Why are they considered carcinogenic?
13. (a) Compare the reactivity of alkanes, alkenes, and alkynes towards addition reactions.
(b) Write the products formed by ozonolysis of: (i) But-1-ene (ii) But-2-ene
(c) An alkene on ozonolysis gives methanal and ethanal. Identify the alkene.
14. (a) What is Friedel-Crafts reaction? Explain both alkylation and acylation with mechanisms.
(b) Classify the following substituents as ortho/para-directing or meta-directing: −Cl, −OH, −NO₂, −CH₃, −COOH.
Assertion-Reason Questions
15. Assertion (A): Terminal alkynes are weakly acidic.
Reason (R): sp-hybridised carbon has more s-character, making the C–H bond more polar.
16. Assertion (A): Toluene is ortho/para-directing in electrophilic substitution.
Reason (R): The methyl group has +I effect and activates the ring.
Answer Key
| Q | Answer |
|---|---|
| 1 | (b) — Peroxide effect → anti-Markovnikov addition → 1-bromopropane |
| 2 | (c) — Cyclooctatetraene has 8π electrons (not 4n+2), non-planar |
| 3 | (b) — RCO⁺ (acylium ion) is the electrophile |
| 4 | (c) — Lindlar’s catalyst gives cis-alkene (syn addition) |
| 5 | (a) — sp > sp² > sp³ in terms of acidity |
| 6 | (c) — −NO₂ is a deactivating meta-directing group |
| 13 (c) | Propene (CH₃CH=CH₂) — ozonolysis gives CH₃CHO + HCHO |
| 15 | (a) — Both true, R correctly explains A |
| 16 | (a) — Both true, R correctly explains A |