Class 12 Chemistry — Q&A

Haloalkanes & Haloarenes (हेलोएल्केन और हेलोएरीन) • Alcohols, Phenols & Ethers (अल्कोहल, फिनॉल और ईथर) — Bihar Board format

Chapter: Haloalkanes & Haloarenes — हैलोएल्केन और हैंलोएरीन

Contents: 15 MCQs • 10 Reactions (equations) • Name Reactions (notes) • IUPAC Practice (with answers) • 20 Short Qs & Answers

15 MCQs (with answers) — 15 बहुविकल्पी प्रश्न (उत्तर दिए हुए)

1. Which reagent is used for converting alcohol to alkyl chloride?
   a) NaCl   b) PCl₅ ✅   c) NaOH   d) HCl
2. General formula of haloalkanes:
   a) C_nH_2n+2   b) C_nH_2n+1X ✅   c) C_nH_2n   d) C_nH_2n-2
3. Highest boiling point:
   a) CH₃Cl   b) CH₃Br   c) CH₃I ✅   d) CH₄
4. IUPAC name of C₂H₅Cl:
   a) Ethyl chloride   b) Chloroethane ✅   c) Methyl chloride   d) Chloroethene
5. Most reactive in SN1:
   a) CH₃Cl   b) (CH₃)₃CCl ✅   c) C₂H₅Cl   d) C₆H₅Cl
6. Does not undergo nucleophilic substitution easily:
   a) CH₃Cl   b) C₂H₅Br   c) C₆H₅Cl ✅   d) C₃H₇I
7. Chloroform is:
   a) CH₃Cl   b) CHCl₃ ✅   c) CCl₄   d) CH₂Cl₂
8. Main product of chlorination of methane (sunlight):
   a) CH₃Cl ✅   b) CH₂Cl₂   c) CHCl₃   d) CCl₄
9. Freons are used as:
   a) Insecticides   b) Refrigerants ✅   c) Fuels   d) Solvents
10. Which halogen derivative used in anaesthesia?
    a) CHCl₃ ✅   b) CCl₄   c) CH₂Cl₂   d) CH₃I
11. Wurtz reaction is used for:
    a) Formation of alkanes ✅   b) alkenes   c) alkynes   d) alcohols
12. Bond angle in CCl₄ approx:
    a) 90°   b) 109.5° ✅   c) 120°   d) 180°
13. In SN2, rate depends on:
    a) not nucleophile   b) both substrate & nucleophile ✅   c) only substrate   d) none
14. Compound used to prepare Grignard reagent:
    a) Alkyl halide ✅   b) Alcohol   c) Aldehyde   d) Carboxylic acid
15. Which is not an aryl halide?
    a) Chlorobenzene   b) Bromobenzene   c) Benzyl chloride ✅   d) Iodobenzene

10 Important Reactions (Equations) — 10 महत्वपूर्ण अभिक्रियाएँ (समिकरण)

1. Alcohol → Alkyl chloride (PCl₅)
   

   CH3CH2OH + PCl5 → CH3CH2Cl + POCl3 + HCl

2. Alcohol → Alkyl bromide (PBr₃)
   

   3CH3CH2OH + PBr3 → 3CH3CH2Br + H3PO3

3. Alcohol → Alkyl iodide (HI, heat)
   

   CH3CH2OH + HI → CH3CH2I + H2O

4. Wurtz Reaction (alkyl halide + Na → higher alkane)
   

   2CH3CH2Br + 2Na → CH3CH2-CH2CH3 + 2NaBr

5. Sandmeyer Reaction (diazonium → aryl halide)
   

   C6H5-N2+ Cl- + CuCl → C6H5Cl + N2↑

6. Finkelstein Reaction (halogen exchange)
   

   R-Br + NaI (acetone) → R-I + NaBr (precipitate)

7. Swarts Reaction (to fluorides)
   

   R-Cl + AgF → R-F + AgCl

8. Electrophilic substitution: Benzene → Chlorobenzene
   

   C6H6 + Cl2 —(FeCl3)→ C6H5Cl + HCl

9. Haloform Reaction (methyl ketone → haloform)
   

   CH3COCH3 + 3Cl2 + 4NaOH → CHCl3 + CH3COONa + 3NaCl + 3H2O

10. Alkyl halide → Alcohol (aqueous KOH)
    

    R-Cl + KOH(aq) → R-OH + KCl

Name Reactions — नामांकित अभिक्रियाएँ (संक्षेप में)

• Wurtz Reaction: Coupling of alkyl halides with sodium to give higher alkanes. (2R–X + 2Na → R–R + 2NaX)
• Sandmeyer Reaction: Aryl diazonium salts react with CuX to form aryl halides. Useful to introduce Cl, Br, CN, etc.
• Finkelstein Reaction: Halogen exchange (usually Br → I) in acetone with NaI.
• Swarts Reaction: Replacement of Cl/Br by F using AgF or SbF₅ reagents.
• Rosenmund Reduction: Acid chloride → aldehyde using H₂/Pd-BaSO₄.
• Reimer–Tiemann Reaction: Phenol → ortho-formyl phenol using chloroform + base (used later with phenols).
• Gattermann Reaction: Similar to Reimer–Tiemann but uses HCN/AlCl₃ or HCN + CuCl for formylation of activated aromatics.
• Haloform Reaction: Methyl ketones (or ethanol in some conditions) give haloform (CHI₃/CHCl₃).
• Benzyne mechanism: For nucleophilic substitution on aryl halides under strong base conditions (e.g., NaNH₂).
• Raschig–Hell Process / Diazotization related methods: Industrial/analytical transformations involving halogen introduction/removal.

IUPAC Naming Practice — IUPAC नामकरण अभ्यास (उत्तर सहित)

1. CH₃CH₂Br — Answer: Bromoethane (ब्रोमोएथेन)

2. CH₃CH₂CH₂Cl — Answer: 1-Chloropropane (1-क्लोरोप्रोपेन)

3. CH₃CHClCH₃ — Answer: 2-Chloropropane (2-क्लोरोप्रोपेन)

4. ClCH₂CH₂Cl — Answer: 1,2-Dichloroethane (1,2-डाइक्लोरोएथेन)

5. C₆H₅Cl — Answer: Chlorobenzene (क्लोरोबेंजीन)

20 Short Questions & Answers — 20 छोटे प्रश्न व उत्तर

1. Define haloalkanes.
   Haloalkanes are alkanes in which one or more hydrogen atoms are replaced by halogen atoms (F, Cl, Br, I).
2. Give two uses of chloroform.
   Used as a solvent and formerly as an anaesthetic (now replaced due to toxicity).
3. Why is C–I bond weaker than C–F?
   Because iodine is larger with poorer orbital overlap → weaker bond; fluorine is small and forms a strong bond.
4. Explain SN1 mechanism (brief).
   Two-step: (1) leaving group leaves forming carbocation (rate-determining), (2) nucleophile attacks carbocation. Rate depends on substrate only.
5. Explain SN2 mechanism (brief).
   One-step bimolecular backside attack by nucleophile with simultaneous leaving group departure. Inversion of configuration; rate depends on both nucleophile and substrate.
6. What are freons?
   Chlorofluorocarbons (CFCs) used as refrigerants; harmful to ozone layer.
7. Why are aryl halides less reactive towards nucleophilic substitution?
   Because sp2 carbon attached to halogen has stronger C–X bond and resonance delocalization reduces positive character at carbon; also steric/electronic factors.
8. Example of Sandmeyer reaction.
   Aniline → diazonium salt → Chlorobenzene using CuCl:

   C6H5NH2 → C6H5N2+Cl- —(CuCl)→ C6H5Cl + N2

9. Why chlorobenzene less reactive than benzyl chloride?
   Chlorine on benzene is attached to sp2 carbon (resonance stabilized), benzyl chloride has benzylic sp3 carbon (stabilized carbocation), easier to react.
10. What happens when CH3CH2Br reacts with Na in dry ether?
    Wurtz coupling: gives C4H10 (butane) and NaBr precipitate.
11. What is haloform test?
    Test for methyl ketones (or ethanol) that gives haloform (CHI3/CHCl3) as yellow precipitate (iodoform with I).
12. Give industrial use of chlorobenzene.
    Used as solvent and intermediate for making phenol and herbicides.
13. Reactivity order of alkyl halides in SN2?
    Primary > secondary > tertiary (tertiary hardly undergo SN2).
14. Which solvent favours SN1 reactions?
    Polar protic solvents (e.g., water, alcohols) stabilize carbocations and anions—favour SN1.
15. Which solvent favours SN2?
    Polar aprotic solvents (e.g., acetone, DMSO) favour SN2 by not solvating nucleophile strongly.
16. How is benzyl chloride prepared from toluene?
    Free radical chlorination of toluene at benzylic position in light: C6H5CH3 → C6H5CH2Cl.
17. Why does tertiary alkyl halide prefer SN1?
    Because tertiary carbocation is stabilized by inductive & hyperconjugation effects.
18. Explain Finkelstein reaction use.
    To exchange halogens (commonly Br → I) using NaI in acetone where NaBr precipitates, shifting eqn to product.
19. Give one environmental problem caused by halogen compounds.
    CFCs deplete ozone layer; many halogenated organics are persistent pollutants.
20. How to convert chlorobenzene to phenol (general route)?
    Via diazotization of aniline (after nitration & reduction) or using nucleophilic aromatic substitution under harsh conditions (alternative routes exist industrially).

Chapter: Alcohols, Phenols & Ethers — अल्कोहल, फिनॉल और ईथर

Contents: 15 MCQs • 10 Reactions • Name Reactions • IUPAC Practice • 20 Short Qs & Answers

15 MCQs (with answers) — 15 बहुविकल्पी प्रश्न (उत्तर दिए हुए)

1. General formula of alcohols:
   a) R-OH ✅   b) R-X   c) R-COOH   d) R-CHO
2. Which reagent converts alcohol to alkene (dehydration)?
   a) H₂/Pd   b) Conc. H₂SO₄ (heat) ✅   c) NaBH₄   d) LiAlH₄
3. Which is most acidic?
   a) Ethanol   b) Phenol ✅   c) Methanol   d) Tert-butanol
4. IUPAC name of CH₃CH(OH)CH₃:
   a) 1-Propanol   b) 2-Propanol ✅   c) Propan-2-ol   d) Isopropyl alcohol
5. Which reagent converts phenol to 2,4,6-tribromophenol?
   a) Br₂/FeCl₃   b) Br₂ (in water) ✅   c) NBS   d) HBr
6. Williamson Ether Synthesis requires:
   a) Alkyl halide + alkoxide ✅   b) Alcohol + alcohol   c) Acid chloride + alcohol   d) Aldehyde + Grignard
7. Which gives iodoform test?
   a) Ethanol ✅ (gives iodoform)   b) Methanol   c) Tert-butanol   d) Benzyl alcohol
8. Boiling point order (highest):
   a) Alcohols > Ethers > Alkanes ✅
9. Phenol + NaOH → ?
   a) Phenoxide salt ✅   b) Ether   c) Ester   d) No reaction
10. Which is a primary alcohol?
    a) CH₃CH₂OH ✅   b) CH₃CH(OH)CH₃   c) (CH₃)₃COH
11. Test to distinguish phenol from alcohol?
    a) NaOH (both react)   b) Neutral FeCl₃ (phenol gives violet color) ✅
12. Ether cleavage by HI gives:
    a) Alcohols   b) Alkyl iodides + alcohols (depending on structure) ✅
13. Preparation of alcohol from alkene uses:
    a) HBr   b) H₂/Pt   c) H₂O / H₂SO₄ (acidic hydration) ✅
14. Which reagent oxidizes primary alcohol to carboxylic acid?
    a) PCC   b) KMnO₄/acid or K₂Cr₂O₇/H₂SO₄ ✅
15. Phenol is more acidic than ethanol because:
    a) Resonance stabilization of phenoxide ion ✅

10 Important Reactions (Equations) — 10 महत्वपूर्ण समिकरण

1. Dehydration of alcohol (to alkene):
   

   CH3CH2CH2OH —(conc. H2SO4, Δ)→ CH3CH=CH2 + H2O

2. Oxidation of primary alcohol → carboxylic acid (KMnO4 / K2Cr2O7):
   

   CH3CH2OH —(O)→ CH3COOH

3. Oxidation of secondary alcohol → ketone:
   

   CH3CH(OH)CH3 —(O)→ CH3COCH3

4. Williamson Ether Synthesis:
   

   R-O^- Na+ + R'-X → R-O-R' + NaX

5. Ether cleavage (HI):
   

   R-O-R' + HI → R-I + R'-OH (or 2R-I if symmetrical + excess HI)

6. Phenol nitration (electrophilic substitution):
   

   C6H5OH + HNO3 → 2-nitrophenol / 4-nitrophenol (major/minor depending on conditions)

7. Reimer–Tiemann (Phenol → ortho-formylphenol):
   

   C6H5OH + CHCl3 + NaOH → o-HOC6H4-CHO + other products

8. Bromination of phenol (in water):
   

   C6H5OH + 3Br2 → 2,4,6-tribromophenol + 3HBr

9. Iodoform test (ethanol / methyl ketones):
   

   CH3CH2OH + I2 + NaOH → CHI3 (yellow ppt) + other products

10. Preparation of alcohol from alkyl halide (aqueous KOH):
    

    R-X + KOH(aq) → R-OH + KX

Name Reactions — नामांकित अभिक्रियाएँ (संक्षेप)

• Williamson Ether Synthesis: Alkoxide + alkyl halide → ether (SN2). Good for making asymmetrical ethers when R' is primary.
• Reimer–Tiemann Reaction: Phenol + CHCl₃/NaOH → o-formyl phenol (salicylaldehyde) predominantly.
• Iodoform Reaction: Methyl ketones or ethanol give yellow precipitate CHI₃; used as test.
• Buchner or other named oxidations: Oxidation of alcohols to carbonyl compounds (different reagents give different results).
• Benzyl oxidation: Side-chain oxidation of alkyl side-chains on benzene rings using KMnO₄ to give carboxylic acids.

IUPAC Naming Practice — IUPAC नामकरण अभ्यास (उत्तर)

1. CH₃CH₂OH — Answer: Ethanol (एथेनॉल)

2. CH₃CH(OH)CH₃ — Answer: Propan-2-ol (or 2-Propanol) (प्रोपैन-2-ओल)

3. Ph-OH (C₆H₅OH) — Answer: Phenol (फिनॉल)

4. CH₃OCH₂CH₃ — Answer: 1-Methoxyethane (Ethyl methyl ether) (मेथॉक्सीएथेन)

5. (CH₃)₃COH — Answer: 2-Methylpropan-2-ol (tert-butanol) (टर्ट-ब्यूटेनॉल)

20 Short Questions & Answers — 20 छोटे प्रश्न व उत्तर

1. Define alcohol.
   Organic compound containing hydroxyl (–OH) group attached to saturated carbon (R–OH).
2. Differentiate primary, secondary, tertiary alcohol (one line).
   Primary: –OH on primary carbon (RCH2OH), Secondary: on secondary (R2CHOH), Tertiary: on tertiary (R3COH).
3. Why phenol is more acidic than ethanol?
   Phenoxide ion is resonance stabilized; ethanol's conjugate base (ethoxide) is not stabilized.
4. What is Williamson synthesis used for?
   To prepare ethers from alkoxide + alkyl halide (SN2 mechanism).
5. How to distinguish between alcohol and phenol?
   FeCl3 test: phenol gives violet color; alcohols do not.
6. How to prepare alcohol from alkene?
   Acidic hydration: R-CH=CH2 + H2O —(H2SO4)→ R-CH(OH)-CH3 or oxymercuration-demercuration for markovnikov addition without rearrangement.
7. What is the ether functional group?
   R–O–R', oxygen between two alkyl/aryl groups.
8. Why ethers have lower boiling points than alcohols?
   Ethers cannot hydrogen-bond with themselves (no –OH), only dipole interactions; alcohols H-bond → higher b.p.
9. Give reaction: Phenol + Br2 (in water).
   

   C6H5OH + 3Br2 → 2,4,6-tribromophenol + 3HBr

10. Write reaction: Ethanol + KMnO4 (strong oxidant).
    

    CH3CH2OH —(KMnO4, H+)→ CH3COOH

11. What is iodoform test?
    Formation of yellow CHI3 precipitate used to detect methyl ketones and ethanol.
12. Cleavage of ether by HI — general outcome?
    Produces alkyl iodide and alcohol (or two alkyl halides if excess HI and tertiary structures favor SN1).
13. Action of PCC on primary alcohol?
    Oxidizes to aldehyde (stops at aldehyde in mild conditions).
14. Action of H2/Pd on alcohol?
    Hydrogenation typically reduces double/triple bonds; H2/Pd does not reduce alcohol to hydrocarbon under normal conditions.
15. Why tert-butanol does not give iodoform test?
    Because it lacks CH3CH(OH)- structure or methyl ketone structure required for iodoform.
16. Phenol + NaOH → ?
    Sodium phenoxide (C6H5O^- Na^+), water.
17. Preparation of phenol from chlorobenzene (brief)?
    Harsh nucleophilic aromatic substitution or multistep via diazonium route from aniline → diazonium → hydrolysis to phenol.
18. One application of ethers.
    Diethyl ether used as a solvent and historically as an anesthetic; ethers are good aprotic solvents for reactions.
19. Why phenol gives electrophilic substitution easily?
    –OH is activating and ortho/para directing due to electron donation via lone pair and resonance.
20. Name one reagent to convert secondary alcohol to ketone.
    K2Cr2O7/H2SO4 or PCC (milder gives ketone without over-oxidation).

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