Environmental Chemistry Class 11 Notes

• Clean air: roughly 78% nitrogen, 21% oxygen, 0.93% argon, about 0.03–0.04% carbon dioxide, plus water vapour and traces of other gases.
• A pollutant is a substance present in harmful excess; a contaminant becomes a pollutant when the amount crossed a danger threshold.
• Primary pollutants are released directly (CO, SO₂, NO, particulates); secondary pollutants form in the air (O₃, PAN, NO₂).
• Sources: stationary combustion, motor vehicles, industry and natural events like volcanic eruptions.

• Carbon monoxide binds haemoglobin ~300 times stronger than oxygen, causing oxygen starvation.
• SO₂ and NOx irritate lungs, form acid rain and corrode buildings and stone monuments.
• Particulates (PM) cause respiratory and lung disease; lead from old fuels damages the nervous system.
• Fly ash, hydrocarbon vapours, and chlorofluorocarbons (CFCs) complete the list of major air pollutants.
• Catalytic converters convert CO, NOx and hydrocarbons to CO₂, N₂ and H₂O; electrostatic precipitators trap particulates.

• Photochemical smog needs sunlight, NO₂, hydrocarbons and a warm inversion layer.
• Ozone near the ground is a harmful pollutant in photochemical smog — unlike the protective layer in the stratosphere.
• Control: reduce hydrocarbon emissions, use cleaner fuels and catalytic converters.

• CO₂ dominates: burning of fossil fuels and deforestation are the main drivers.
• Methane (livestock, rice, leaks) and nitrous oxide (fertilisers) are potent per-molecule.
• F-Gases (CFCs, HFCs) also contribute; CFCs additionally destroy ozone.
• Consequences: rising sea levels, glacier retreat, extreme weather and habitat shifts.

• Catalytic cycle: CL + O₃ → ClO + O₂; ClO + O → Cl + O₂, regenerating the Cl that destroys more ozone.
• NOVA compounds (NO) natural cycle plus volcanic particles also deplete ozone.
• The Montreal Protocol phased out CFCs; ozone levels are now slowly recovering.
• Ground-level ozone is a pollutant (smog), stratospheric ozone is protective — altitude decides.

• Water pollution: domestic sewage raises biological oxygen demand (BOD), starving aquatic life; industrial effluents add heavy metals (Hg, Pb, Cd).
• Eutrophication: excess phosphates and nitrates from fertilisers make algal blooms that deplete oxygen.
• Biochemical Oxygen Demand measures the oxygen microorganisms need to decompose organic waste.
• Green chemistry: design products/processes that minimize hazardous substances — the Twelve Principles (prevention, atom economy, safer solvents, ...).

Example: Distinguish photochemical smog from classical smog.

Solution: Classical smog is reducing, forms in cool humid mornings, and is dominated by SO₂ and soot from coal. Photochemical smog is oxidising, forms in sunny warm conditions from NOx and hydrocarbons, and contains O₃ and PAN.

Example: Why is ground-level ozone harmful while stratospheric ozone is essential?

Solution: In the stratosphere, ozone absorbs harmful UV-B. At ground level (smog), ozone is a powerful oxidant that damages lungs, plants and rubber, so the same molecule is a protectant up high and a pollutant near the surface.

Exercises (NCERT Q9.1–Q9.25)

• 9.1) How do you account for the formation of ethane during the chlorination of methane?
• 9.2) Write the IUPAC names of the following compounds: (a) CH₃CH=C(CH₃)₂. (b) CH₂=CH–C≡C–CH₃. (c) CH₂=CH–CH=CH₂. (d) C₆H₅–CH₂–CH₂–CH=CH₂. (e) o-CH₃–C₆H₄–OH, with the methyl and hydroxyl groups adjacent. (f) CH₃(CH₂)₄CH(CH₂)₃CH₃, with a –CH₂–CH(CH₃)₂ group attached to the central CH. (g) CH₃–CH=CH–CH₂–CH=CH–CH(C₂H₅)–CH₂–CH=CH₂.
• 9.3) Write structural formulas and IUPAC names for all possible isomers having the indicated number of double or triple bonds: (a) C₄H₈, one double bond. (b) C₅H₈, one triple bond.
• 9.4) Write the IUPAC names of the products obtained by ozonolysis of: (i) pent-2-ene; (ii) 3,4-dimethylhept-3-ene; (iii) 2-ethylbut-1-ene; and (iv) 1-phenylbut-1-ene.
• 9.5) An alkene A on ozonolysis gives a mixture of ethanal and pentan-3-one. Write the structure and IUPAC name of A.
• 9.6) An alkene A contains three C–C σ bonds, eight C–H σ bonds and one C–C π bond. Ozonolysis of A gives two moles of an aldehyde of molar mass 44 u. Write the IUPAC name of A.
• 9.7) Propanal and pentan-3-one are the ozonolysis products of an alkene. What is the structural formula of the alkene?
• 9.8) Write balanced chemical equations for complete combustion of: (i) butane; (ii) pentene; (iii) hexyne; and (iv) toluene.
• 9.9) Draw the cis and trans structures of hex-2-ene, using condensed formulae and stating the relative positions of the two larger groups. Which isomer has the higher boiling point and why?
• 9.10) Why is benzene extraordinarily stable although it contains three carbon–carbon double bonds?
• 9.11) What are the necessary conditions for a system to be aromatic?
• 9.12) Explain why the following systems are not aromatic: (i) the displayed system with six π-electrons and an sp³-hybridised carbon; (ii) the displayed system with four π-electrons and an sp³-hybridised carbon; and (iii) cyclooctatetraene.
• 9.13) How will you convert benzene into: (i) p-nitrobromobenzene; (ii) m-nitrochlorobenzene; (iii) p-nitrotoluene; and (iv) acetophenone?
• 9.14) In CH₃–CH₂–C(CH₃)₂–CH₂–CH(CH₃)₂, identify the primary, secondary and tertiary carbon atoms and give the number of hydrogens bonded to each of them.
• 9.15) What effect does branching of an alkane chain have on its boiling point?
• 9.16) Addition of HBr to propene yields 2-bromopropane, while in the presence of benzoyl peroxide the same reaction yields 1-bromopropane. Explain the difference and give the mechanisms.
• 9.17) Write down the products of ozonolysis of 1,2-dimethylbenzene (o-xylene). How does the result support the Kekulé structure for benzene?
• 9.18) Arrange benzene, n-hexane and ethyne in decreasing order of acidic character and explain the order.
• 9.19) Why does benzene undergo electrophilic substitution easily but nucleophilic substitution with difficulty?
• 9.20) How would you convert: (i) ethyne into benzene; (ii) ethene into benzene; and (iii) hexane into benzene?
• 9.21) Write the structures of all alkenes which on hydrogenation give 2-methylbutane.
• 9.22) Arrange each set in decreasing order of relative reactivity with an electrophile E⁺: (a) chlorobenzene, p-nitrochlorobenzene and 2,4-dinitrochlorobenzene; (b) toluene, p-nitrotoluene and p-dinitrobenzene.
• 9.23) Out of benzene, m-dinitrobenzene and toluene, which undergoes nitration most easily and why?
• 9.24) Suggest a Lewis acid other than anhydrous aluminium chloride that can be used during ethylation of benzene.
• 9.25) Why is the Wurtz reaction not preferred for preparing alkanes containing an odd number of carbon atoms? Illustrate the answer with one example.

Answers at a glance

1. 1
   Q9.1 → Ethane is formed in a termination step when two methyl free radicals combine: CH₃• + CH₃• → CH₃CH₃.
2. 2
   Q9.2 → (a) 2-methylbut-2-ene; (b) pent-1-en-3-yne (pent-1-ene-3-yne); (c) buta-1,3-diene; (d) 4-phenylbut-1-ene; (e) 2-methylphenol; (f) 5-(2-methylpropyl)decane; (g) 4-ethyldeca-1,5,8-triene.
3. 3
   Q9.3 → (a) CH₂=CH–CH₂–CH₃, but-1-ene; CH₃–CH=CH–CH₃, but-2-ene; CH₂=C(CH₃)₂, 2-methylprop-1-ene. (b) HC≡C–CH₂–CH₂–CH₃, pent-1-yne; CH₃–C≡C–CH₂–CH₃, pent-2-yne; HC≡C–CH(CH₃)–CH₃, 3-methylbut-1-yne.
4. 4
   Q9.4 → (i) Ethanal and propanal; (ii) butan-2-one and pentan-2-one; (iii) methanal and pentan-3-one; (iv) benzaldehyde and propanal.
5. 5
   Q9.5 → A is CH₃–CH=C(CH₂CH₃)₂, and its IUPAC name is 3-ethylpent-2-ene.
6. 6
   Q9.6 → A is but-2-ene.
7. 7
   Q9.7 → The alkene is CH₃CH₂–CH=C(CH₂CH₃)₂, named 4-ethylhex-3-ene.
8. 8
   Q9.8 → The balanced equations are 2C₄H₁₀ + 13O₂ → 8CO₂ + 10H₂O; C₅H₁₀ + 15/2 O₂ → 5CO₂ + 5H₂O; 2C₆H₁₀ + 17O₂ → 12CO₂ + 10H₂O; and C₇H₈ + 9O₂ → 7CO₂ + 4H₂O.
9. 9
   Q9.9 → Cis-hex-2-ene has the higher boiling point because its polar bond dipoles add to give a larger molecular dipole moment and therefore stronger intermolecular attraction.
10. 10
    Q9.10 → Benzene is unusually stable because its six π electrons are delocalised over a planar, fully conjugated ring, giving resonance stabilisation and six equivalent C–C bonds.
11. 11
    Q9.11 → An aromatic system is cyclic, planar, fully conjugated through overlapping p orbitals at every ring atom, and contains 4n+2 delocalised π electrons.
12. 12
    Q9.12 → All three are non-aromatic: (i) has incomplete conjugation caused by an sp³ carbon despite six π-electrons; (ii) has an sp³ carbon and only four π-electrons; and (iii) is non-planar with eight π-electrons.
13. 13
    Q9.13 → Use bromination then nitration for p-nitrobromobenzene; nitration then chlorination for m-nitrochlorobenzene; methylation then nitration for p-nitrotoluene; and Friedel–Crafts acetylation for acetophenone.
14. 14
    Q9.14 → There are five primary carbons with 15 H atoms, two secondary carbons with 4 H atoms, one tertiary carbon with 1 H atom, and one quaternary carbon with no H.
15. 15
    Q9.15 → Greater branching lowers the boiling point because the compact molecules have less surface contact and weaker London dispersion forces.
16. 16
    Q9.16 → The ionic mechanism follows Markovnikov's rule through a secondary carbocation and gives 2-bromopropane; the peroxide-initiated radical mechanism gives the more stable secondary radical orientation and ultimately 1-bromopropane, the anti-Markovnikov product.
17. 17
    Q9.17 → The products are glyoxal, methylglyoxal and butane-2,3-dione (diacetyl). Their formation supports benzene as a resonance hybrid of equivalent Kekulé structures.
18. 18
    Q9.18 → Decreasing acidic character is ethyne > benzene > n-hexane, because the order of carbon hybridisation is sp > sp² > sp³ and acidity increases with s-character.
19. 19
    Q9.19 → Benzene's electron-rich π cloud readily attracts electrophiles, while nucleophiles are repelled and substitution by a nucleophile would require loss of aromaticity.
20. 20
    Q9.20 → Ethyne and ethene each cyclotrimerise in a red-hot iron tube, while hexane is aromatised over Cr₂O₃/Al₂O₃ at high temperature and pressure.
21. 21
    Q9.21 → The three alkenes are 2-methylbut-1-ene, 2-methylbut-2-ene and 3-methylbut-1-ene, with the structures shown above.
22. 22
    Q9.22 → (a) Chlorobenzene > p-nitrochlorobenzene > 2,4-dinitrochlorobenzene. (b) Toluene > p-nitrotoluene > p-dinitrobenzene.
23. 23
    Q9.23 → Toluene nitrates most easily. The order of reactivity is toluene > benzene > m-dinitrobenzene because CH₃ activates the ring whereas NO₂ groups deactivate it.
24. 24
    Q9.24 → Anhydrous ferric chloride, FeCl₃, can be used; boron trifluoride, BF₃, is another acceptable Lewis acid.
25. 25
    Q9.25 → Odd-carbon alkanes require two different alkyl halides in Wurtz coupling, so self-coupling gives a mixture. Ethyl bromide plus propyl bromide gives pentane together with butane and hexane.

MCQ Questions

1 Mark Questions

• Define pollutant.
• What is smog?
• Define acid rain.
• What are biodegradable pollutants?

2 Mark Questions

• Differentiate between primary and secondary pollutants.
• Write any two harmful effects of acid rain.
• How do CFCs cause ozone depletion? (short)

3 Mark Questions

• Explain greenhouse effect and its consequences.
• Describe photochemical smog formation and effects.
• Explain water pollution control measures (brief).

5 Mark Questions

• What is ozone depletion? Explain causes, effects and prevention measures.
• Explain various strategies to control environmental pollution.

Assignments walk the chapter's topics in order; the tests follow the current CBSE blueprint (MCQ/VSA, 2M, 3M and 5M questions) with answer keys. They will appear here as direct download links.