313_Chemistry_Eng_Lesson25.pdf). Content covers sections 25.1–25.4.When H in a hydrocarbon is replaced by F, Cl, Br or I, the products are haloalkanes (alkyl) or haloarenes (aryl). They are not common in nature but are major industrial solvents, pharmaceuticals, pesticides, refrigerants, anaesthetics and fire extinguishers. This lesson covers IUPAC names, preparation, C–X bonding, physical and chemical properties (especially SN1/SN2), elimination, organometallic reactions, and polyhalogen compounds (CHCl₃, CHI₃, DDT).
Select the longest chain that includes the halogen; number so X gets the lowest number; prefix fluoro/chloro/bromo/iodo. With several X atoms, choose the chain with maximum halogen atoms and use di/tri/tetra. Haloarenes: chlorobenzene, bromobenzene; substituted as 1-chloro-2-nitrobenzene, etc.
Haloalkanes:
Haloarenes:
C–X is polar (Cδ⁺–Xδ⁻) via sp³–p overlap. Bond strength falls F→I; reactivity of alkyl halides: RI > RBr > RCl > RF. Lower alkyls are gases; many liquids with higher b.p. than parent hydrocarbons (dipole–dipole + higher mass). Polar but often immiscible in water (poor H-bonding). p-Dichlorobenzene has higher m.p. than o-isomer (symmetric packing).
Haloarenes are less reactive toward nucleophiles than haloalkanes because resonance gives C–X partial double-bond character.
Cδ⁺ is attacked by nucleophiles: OH⁻ → alcohol; CN⁻ → nitrile; NH₃ → amine; OR⁻ → ether; SH⁻ → thiol; RC≡C⁻ → higher alkyne.
SN2: one step; Nu attacks from backside as X leaves; inversion of configuration; preferred for primary RX; rate depends on both [RX] and [Nu].
SN1: two steps; slow ionisation to carbocation, then fast Nu capture; preferred for tertiary RX; rate depends only on [RX]. Carbocation stability: 3° > 2° > 1° (+I and hyperconjugation).
Haloarenes: Nu substitution only under drastic conditions (e.g. NaOH, 623 K, 300 atm → phenol). o/p–NO₂ groups activate Ar–X. Electrophilic substitution on the ring is easier — Cl is o/p director (nitration → o- and p-nitrochlorobenzene).
aq KOH: substitution → alcohol. alc KOH: β-elimination → alkene (Saytzeff: more substituted alkene major, e.g. 2-bromobutane → mainly but-2-ene).
Metals: Grignard RMgX (dry ether); Wurtz (2RX + 2Na → R–R); Wurtz–Fittig (ArX + RX + Na → Ar–R); Fittig (2ArX + 2Na → Ar–Ar); tetraethyl lead from EtBr + Pb (antiknock, historical).
Reduction: R–X → R–H (H₂/Ni or HI/red P).
Distinction R–X vs Ar–X: after NaOH, only alkyl chlorides give AgCl with AgNO₃ under ordinary conditions.
Chloroform (CHCl₃): from ethanol or acetone + Cl₂ + alkali (via chloral). Sweet-smelling liquid. Oxidised by light/air to toxic phosgene (COCl₂) — store in dark, full bottles with a little ethanol. Used in carbylamine test for primary amines (RNH₂ + CHCl₃ + alc KOH → RNC, foul smell).
Iodoform (CHI₃): yellow solid with characteristic odour; from ethanol or acetone + I₂ + NaOH. Iodoform test for CH₃CO– and CH₃CH(OH)–. Antiseptic use historically.
DDT: dichlorodiphenyltrichloroethane — mosquito control; accumulates in environment, toxic to wildlife; banned in many countries.
Also important industrially: CCl₄ (solvent, fire extinguisher — ozone concerns), BHC (hexachlorocyclohexane).
High-yield: naming; ROH→RX reagents; Sandmeyer; C–X polarity and RI>…>RF; SN1 vs SN2 with mechanisms; aq vs alc KOH; Saytzeff elimination; Grignard/Wurtz; Ar–X inertness and o/p–NO₂ activation; AgNO₃ distinction; CHCl₃ storage; iodoform test; DDT issues.
Builds on L23 (mechanisms, E⁺/Nu) and L24 (alkanes/alkenes). Next: alcohols, phenols, ethers (L26) use many of these substitutions.
Most exam-important points from this chapter:
ROH→RX (HX, PCl₅, SOCl₂). ArH+X₂/Fe; Sandmeyer for ArCl/Br. IUPAC: lowest number for X.
Polar Cδ⁺–Xδ⁻. RI most reactive. Ar–X less Nu-reactive (resonance).
S_N2: 1°, inversion, one step. S_N1: 3°, C⁺, rate = k[RX]. C⁺: 3°>2°>1°.
aq KOH alcohol; alc KOH alkene (Saytzeff). Grignard, Wurtz, Fittig, Wurtz–Fittig.
CHCl₃: dark + EtOH (phosgene). CHI₃: iodoform test. DDT: persistent toxin.
Extracted from NIOS Chemistry (313) board exam papers in your PDF. Chapter L25 — Compounds of Carbon Containing Halogens only. Use Model Answer for marking points; Explanation for concept clarity.
6 question(s) · Sources: 313/MAY/205A, 313/MAY/205B, 313/MAY/205C, 313/TUS/105A
PYQ1. Read the passage given below and answer the following questions : In alkyl halides, the carbon-halogen bond is formed by the overlap of the sp3 hybrid orbital of carbon atom with the p-orbital of the halogen atom. As the size of the halogen atom increases, the overlap decreases. Also due to the high electronegativity of the halogen atom, the electron density along the C—X bond is displaced in the direction of the halogen atom. Thus, C—X bond becomes polar in nature. Though polar in nature, haloalkanes are immiscible in water. Why? Give one example of substitution reaction of haloalkanes.
Model Answer
Give the chemical reason linked to structure/bonding/equilibrium. Start with the principle, then apply to the species named in the question.
Explanation
Reasoning marks require principle + application. Cite electron effects, stability, or Le Chatelier as relevant.
How to write for NIOS: Use 30–50 words (VSA) or short objective. Open with definition/equation, then reason, end with conclusion. Paper 313/MAY/205A · Q26 · 2 mark(s) · L25.
PYQ2. Read the passage given below and answer the following questions : In alkyl halides, the carbon-halogen bond is formed by the overlap of the sp3 hybrid orbital of carbon atom with the p-orbital of the halogen atom. As the size of the halogen atom increases, the overlap decreases. Also due to the high electronegativity of the halogen atom, the electron density along the C—X bond is displaced in the direction of the halogen atom. Thus, C—X bond becomes polar in nature. Though polar in nature, haloalkanes are immiscible in water. Why? Give one example of substitution reaction of haloalkanes.
Model Answer
Give the chemical reason linked to structure/bonding/equilibrium. Start with the principle, then apply to the species named in the question.
Explanation
Reasoning marks require principle + application. Cite electron effects, stability, or Le Chatelier as relevant.
How to write for NIOS: Use 30–50 words (VSA) or short objective. Open with definition/equation, then reason, end with conclusion. Paper 313/MAY/205B · Q21 · 2 mark(s) · L25.
PYQ3. Read the passage given below and answer the following questions : In alkyl halides, the carbon-halogen bond is formed by the overlap of the sp3 hybrid orbital of carbon atom with the p-orbital of the halogen atom. As the size of the halogen atom increases, the overlap decreases. Also due to the high electronegativity of the halogen atom, the electron density along the C—X bond is displaced in the direction of the halogen atom. Thus, C—X bond becomes polar in nature. Though polar in nature, haloalkanes are immiscible in water. Why? Give one example of substitution reaction of haloalkanes.
Model Answer
Give the chemical reason linked to structure/bonding/equilibrium. Start with the principle, then apply to the species named in the question.
Explanation
Reasoning marks require principle + application. Cite electron effects, stability, or Le Chatelier as relevant.
How to write for NIOS: Use 30–50 words (VSA) or short objective. Open with definition/equation, then reason, end with conclusion. Paper 313/MAY/205C · Q21 · 2 mark(s) · L25.
PYQ4. The melting and boiling points of haloalkanes and haloarenes are higher than those of their parent hydrocarbons due to — (A) lower molecular masses (B) lower magnitude of van der Waals forces of attraction (C) the existence of intermolecular dipole-dipole interaction (D) their capability to form intermolecular hydrogen bonds h¡bmoEoë
Model Answer
Model approach (select the best option):
Eliminate options that contradict definitions/equations from the chapter notes. NIOS awards full mark for the single correct choice.
Explanation
This MCQ belongs to L25. Recall the core definition or formula from notes, then match it to one option. Paper: 313/TUS/105A · Q10.
Tip: For numerical MCQs, write the formula first, substitute values, then pick the option.
PYQ5. Represent the mechanism of SN2 reaction taking place between a nucleophile (–OH) and a haloalkane molecule. Zm{^H$amJr (–OH) Am¡a h¡bmoEëHo$Z AUw Ho$ ‘ܶ hmoZo dmbr SN2 A{^{H«$¶m
Model Answer
Answer using key concepts from L25 (definitions, equations, and one example where useful). Stay within the suggested word range for a 2-mark NIOS question.
Explanation
Cross-check with L25 notes. Structure: definition/law → working → conclusion. Partial marks for correct equations even if explanation is short.
How to write for NIOS: Use 30–50 words (VSA) or short objective. Open with definition/equation, then reason, end with conclusion. Paper 313/MAY/205B · Q36 · 2 mark(s) · L25.
PYQ6. State Saytzeff’s rule. Give a suitable example
Model Answer
Saytzeff’s rule: in elimination of HX from alkyl halides, the preferred alkene is the more substituted one (hydrogen is removed preferentially from the β-carbon bearing fewer hydrogens). Example: 2-bromobutane → but-2-ene major.
Explanation
More substituted alkenes are more stable (hyperconjugation). Contrast Hofmann product when bulky base is used (higher level).
How to write for NIOS: Use 30–50 words (VSA) or short objective. Open with definition/equation, then reason, end with conclusion. Paper 313/TUS/105A · Q37 · 2 mark(s) · L25.
Six problems spanning this chapter’s NIOS syllabus. Every question is built from the notes and formula sheet: solve with equations first, then read the formal textbook-style write-up, the easy explanation, and the topic in depth (formulas, meaning, exam tips). If the question says draw, a labelled pencil sketch is provided. Explanations open by default.
Describe stereochemistry of SN2 at a chiral carbon.
Final answer: Inversion (Walden inversion)
Concerted bimolecular nucleophilic substitution.
Working formulas: (see solution steps). State the definition or law first (NIOS style), use SI units, and box the final numerical answer with unit.
Nucleophile attacks opposite the leaving group—like an umbrella flipping.
Read once for the idea, once for the numbers. Write the formula, substitute, then simplify. Check whether you used moles, grams, or litres correctly.
Favoured by primary substrates.
Linked to chapter notes (L25). Remember: (see solution steps). Most exam errors are unit mix-ups (g vs mol, mL vs L) or wrong mole ratios from the equation.
Write (see solution steps) before substituting. Keep three significant figures until the end when data allow.
Why do tertiary alkyl halides prefer SN1?
Final answer: Stable 3° carbocation
Rate depends only on substrate concentration in RDS.
Working formulas: (see solution steps). State the definition or law first (NIOS style), use SI units, and box the final numerical answer with unit.
Tertiary can lose X⁻ and sit as a stable ion then grab nucleophile.
Read once for the idea, once for the numbers. Write the formula, substitute, then simplify. Check whether you used moles, grams, or litres correctly.
Racemisation often observed.
Linked to chapter notes (L25). Remember: (see solution steps). Most exam errors are unit mix-ups (g vs mol, mL vs L) or wrong mole ratios from the equation.
Write (see solution steps) before substituting. Keep three significant figures until the end when data allow.
Convert ROH to RCl (one common reagent).
Final answer: e.g. SOCl₂ or HCl/ZnCl₂
Hydroxyl replaced by halogen.
Working formulas: (see solution steps). State the definition or law first (NIOS style), use SI units, and box the final numerical answer with unit.
Swap OH for Cl using chlorinating agents.
Read once for the idea, once for the numbers. Write the formula, substitute, then simplify. Check whether you used moles, grams, or litres correctly.
Lucas test distinguishes alcohol classes.
Linked to chapter notes (L25). Remember: (see solution steps). Most exam errors are unit mix-ups (g vs mol, mL vs L) or wrong mole ratios from the equation.
Write (see solution steps) before substituting. Keep three significant figures until the end when data allow.
Why are aryl halides less reactive in nucleophilic substitution than alkyl halides?
Final answer: Resonance stabilised C–X; sp² C
Electron-rich ring and geometry hinder SN pathways.
Working formulas: (see solution steps). State the definition or law first (NIOS style), use SI units, and box the final numerical answer with unit.
Cl stuck harder on benzene than on alkyl chain.
Read once for the idea, once for the numbers. Write the formula, substitute, then simplify. Check whether you used moles, grams, or litres correctly.
Electron-withdrawing groups ortho/para can activate.
Linked to chapter notes (L25). Remember: (see solution steps). Most exam errors are unit mix-ups (g vs mol, mL vs L) or wrong mole ratios from the equation.
Write (see solution steps) before substituting. Keep three significant figures until the end when data allow.
State one environmental concern of freons (CFCs).
Final answer: Ozone depletion
CFCs release Cl radicals that catalyse O₃ destruction.
Working formulas: (see solution steps). State the definition or law first (NIOS style), use SI units, and box the final numerical answer with unit.
Old fridge gases climb up and damage ozone shield.
Read once for the idea, once for the numbers. Write the formula, substitute, then simplify. Check whether you used moles, grams, or litres correctly.
Montreal Protocol phased them out.
Linked to chapter notes (L25). Remember: (see solution steps). Most exam errors are unit mix-ups (g vs mol, mL vs L) or wrong mole ratios from the equation.
Write (see solution steps) before substituting. Keep three significant figures until the end when data allow.
One use of chloroform (historical/modern caution).
Final answer: Solvent / former anaesthetic (toxic)
Polyhalogen compounds have specialised uses and hazards.
Working formulas: (see solution steps). State the definition or law first (NIOS style), use SI units, and box the final numerical answer with unit.
Useful but handle carefully—liver toxicity etc.
Read once for the idea, once for the numbers. Write the formula, substitute, then simplify. Check whether you used moles, grams, or litres correctly.
CCl₄ former fire extinguisher, now restricted.
Linked to chapter notes (L25). Remember: (see solution steps). Most exam errors are unit mix-ups (g vs mol, mL vs L) or wrong mole ratios from the equation.
Write (see solution steps) before substituting. Keep three significant figures until the end when data allow.