Haloalkanes & Haloarenes — Advanced Notes & 50 MCQs

1. In SN2 reactions the rate depends on:

1. Only substrate concentration
2. Only nucleophile strength
3. Both substrate and nucleophile concentrations (bimolecular) ✅
4. Only temperature

2. Which substrate favors SN1 mechanism?

1. Primary alkyl halide
2. Tertiary alkyl halide ✅
3. Methyl halide
4. All equally

3. Which halogenation method gives aryl halides from aniline derivatives?

1. Free radical halogenation directly
2. Sandmeyer reaction via diazonium salt ✅
3. SN2 on benzene
4. Wurtz coupling

4. Aryl halides are less reactive toward nucleophilic substitution because:

1. They lack leaving groups
2. C atom bearing halogen is sp2 hybridized and resonance stabilizes C–X bond ✅
3. They are too soluble in water
4. They undergo SN2 easily

5. Which reagent converts an alcohol to an alkyl halide via SN1 (tertiary alcohol)?

1. HCl/ZnCl2 (Lucas reagent) ✅
2. Cold NaOH
3. KMnO4 only
4. None

6. In SN2 reaction, stereochemistry at the carbon changes by:

1. No change
2. Complete inversion (Walden inversion) ✅
3. Racemization
4. Optical activity lost only

7. Benzylic halides are unusually reactive because:

1. They are sp hybridized
2. Benzylic carbocations and radicals are resonance stabilized ✅
3. They have no resonance stabilization
4. They are ionic compounds

8. Which mechanism is concerted (single step) for alkyl halide substitution?

1. SN1
2. SN2 ✅
3. Free radical substitution only
4. Elimination only

9. Which condition favors E2 elimination over SN2?

1. Weak base and low temperature
2. Strong, bulky base and secondary/tertiary substrate ✅
3. Polar protic solvent only
4. Methyl substrate

10. Which reagent is used in the Sandmeyer reaction to replace diazonium group with Cl?

1. CuCN
2. CuCl ✅
3. NaCl only
4. HCl alone

11. Which halide undergoes SN2 fastest (all else equal)?

1. Tertiary bromide
2. Methyl bromide ✅
3. Primary chloride
4. Tertiary chloride

12. Which is a common method to prepare haloarenes from aromatic amines?

1. Nucleophilic aromatic substitution directly
2. Diazotization followed by Sandmeyer reaction ✅
3. Friedel–Crafts alkylation
4. Wurtz reaction

13. Which of these is true about aryl chlorides reacting with nucleophiles?

1. They undergo SN2 easily
2. They are generally unreactive unless activated by electron-withdrawing groups (Ar–NO2) ✅
3. They react only at benzylic position
4. They react via radical substitution only

14. What is the major product of dehydrohalogenation of 2-bromobutane with t-BuOK?

1. 1-Butene
2. 2-Butene (more substituted alkene, Zaitsev product) ✅
3. Butane
4. 2-butanol

15. Which reagent converts alkyl halide to alkene via elimination under mild conditions?

1. NaBH4 only
2. KOH in alcohol (E2 elimination) ✅
3. H2/Pd
4. KMnO4

16. Which halogen is most reactive in free radical halogenation of alkanes?

1. Chlorine
2. Fluorine (most reactive, unselective) ✅
3. Bromine
4. Iodine

17. The Wurtz reaction couples two alkyl halides using:

1. Silver metal
2. Sodium metal in dry ether ✅
3. Potassium permanganate
4. Hydrogen gas only

18. Which alkyl halide gives substitution product with cyanide ion faster in SN2?

1. Tertiary chloride
2. Primary iodide ✅
3. Aryl bromide
4. Methyl chloride

19. Which factor does NOT favor SN2?

1. Strong nucleophile
2. Tertiary substrate (steric hindrance inhibits SN2) ✅
3. Polar aprotic solvent
4. Good leaving group

20. Which reagent converts aryl diazonium salt to aryl fluoride?

1. CuCl
2. HBF4 (via Balz–Schiemann reaction) ✅
3. NaF alone
4. HCl

21. Which halide is least likely to undergo radical substitution under typical conditions?

1. Chlorides (moderately reactive)
2. Aryl halides (C–X bond strong and resonance stabilized) ✅
3. Allylic halides
4. Benzylic halides

22. Which reagent is used to prepare alkyl bromides from alcohols?

1. PCC only
2. PBr3 or HBr (PBr3 for primary/secondary alcohols) ✅
3. KMnO4
4. NaBH4

23. Which condition favors SN1 racemization?

1. Strong nucleophile and primary substrate
2. Tertiary substrate in polar protic solvent ✅
3. Polar aprotic with methyl substrate
4. None of the above

24. Which halide is commonly used as a solvent for SN2 reactions (polar aprotic)?

1. Water (polar protic)
2. DMSO or DMF (polar aprotic) — not halide, but favorable solvents for SN2 ✅
3. Benzene only
4. None

25. Which product forms when benzyl chloride reacts with aqueous KOH?

1. Benzyl alcohol ✅
2. Benzene only
3. Toluene
4. Benzyl ether only

26. Which halide is most reactive toward nucleophilic attack (leaving group ability)?

1. Chloride
2. Iodide (best leaving group among halides) ✅
3. Fluoride
4. Bromide

27. Which reagent replaces halogen by CN (nucleophilic substitution)?

1. NaOH only
2. NaCN (cyanide ion as nucleophile) ✅
3. HCl only
4. KMnO4

28. Which statement is TRUE about allylic halides?

1. They are less reactive than primary alkyl halides
2. They undergo both SN1/SN2 and radical reactions readily due to resonance stabilization ✅
3. They cannot form carbocations
4. They are inert

29. Which reaction converts aryl halide to phenol?

1. Direct SN2 with OH-
2. Reaction of diazonium salt with water or nucleophilic aromatic substitution under forcing conditions (e.g., NaOH, high temp) ✅
3. Wurtz reaction
4. Hydrogenation

30. Which product results from radical bromination of isobutane?

1. Primary bromide only
2. Tertiary bromide predominantly (more stable radical) ✅
3. No reaction
4. Alkene formation only

31. Which halogenation of benzene requires a Lewis acid catalyst (FeCl3)?

1. Nucleophilic substitution
2. Electrophilic aromatic substitution (chlorination/bromination) ✅
3. SN2 on benzene ring
4. Free radical substitution only

32. Which reagent converts alkyl halide to alcohol via substitution under mild conditions?

1. H2 and Pd
2. Aqueous NaOH (nucleophilic substitution) ✅
3. Br2 only
4. Cl2 only

33. Which of the following halides is carcinogenic and tightly regulated?

1. Methyl chloride (CH3Cl)
2. Certain haloarenes (e.g., chlorinated benzenes, some polychlorinated biphenyls) ✅
3. All halides are safe
4. None of the above

34. Which statement about nucleophilicity in protic solvents is TRUE?

1. Smaller anions are stronger nucleophiles
2. Larger, less solvated anions (I-) are stronger nucleophiles in protic solvents ✅
3. Nucleophilicity equals basicity always
4. Solvent has no effect

35. Which reagent can convert alkyl halide to alkane (reduction)?

1. NaOH only
2. H2 with Pd (hydrogenolysis) or LiAlH4 in some cases ✅
3. KMnO4
4. None

36. Which halide is most stable in aqueous solution (least reactive) toward nucleophiles?

1. Allylic iodide
2. Aryl fluoride (strong C–F bond and resonance) ✅
3. Primary bromide
4. Methyl iodide

37. Which mechanism involves formation of a carbocation intermediate?

1. SN2
2. SN1 ✅
3. Free radical substitution only
4. E2 only

38. Which reagent transforms aryl halide into organometallic reagent (Grignard)?

1. NaCN
2. Mg in dry ether to give ArMgX (aryl magnesium halide) ✅
3. H2O
4. HCl

39. Which statement about benzylic oxidation is TRUE?

1. Benzylic C–H cannot be oxidized
2. Benzylic hydrogens are easily oxidized to carboxylic acids (e.g., KMnO4) ✅
3. Only allylic positions oxidize
4. Benzylic position is inert under all conditions

40. Which halogenated compound is used as a refrigerant historically (but phased out)?

1. CH3Cl only
2. Chlorofluorocarbons (CFCs) like CCl2F2 (Freon) ✅
3. NaCl only
4. HCl only

41. Which reagent converts aryl halide into phenylboronic acid (useful in Suzuki coupling)?

1. B2H6 only
2. Mixture with Pd catalyst in borylation reactions (e.g., Pd-catalyzed borylation to give Ar–B(OH)2) ✅
3. NaOH only
4. KMnO4 only

42. Which of the following is true about nucleophilic aromatic substitution via addition-elimination?

1. Requires electron-donating groups on ring
2. Requires strong electron-withdrawing groups ortho/para to leaving group to stabilize intermediate ✅
3. Occurs readily for unsubstituted chlorobenzene
4. Is identical to SN2

43. Which reagent converts aryl halide into phenol via nucleophilic substitution under harsh conditions?

1. Cold NaOH
2. Molten NaOH at high temperature/pressure or diazonium hydrolysis routes ✅
3. H2 and Pd
4. LiAlH4

44. Which of the following is most nucleophilic in polar protic solvent?

1. F-
2. I- (less solvated, more nucleophilic in protic solvents) ✅
3. Cl-
4. Br-

45. Which reagent is used to prepare alkyl iodides from alcohols?

1. PCl5 only
2. PI3 (or red phosphorus + I2) or HI ✅
3. NaBr only
4. KMnO4

46. Which catalyst is commonly used for hydrochlorination of alkenes to form alkyl halides industrially?

1. Pd only
2. HCl with peroxides for anti-Markovnikov additions in presence of radical initiators; general addition uses HCl without catalyst ✅
3. NaOH only
4. None

47. Which of the following best explains leaving group ability?

1. Basicity: stronger base = better leaving group
2. Weaker bases (more stable as anions) are better leaving groups (I- > Br- > Cl- > F-) ✅
3. All leaving groups are equally good
4. Leaving group ability is unrelated to basicity

48. Which reagent can convert benzyl chloride to benzyl cyanide?

1. NaOH only
2. NaCN (nucleophilic substitution at benzylic position) ✅
3. H2O only
4. HCl only

49. Which statement about dehalogenation is TRUE?

1. Dehalogenation always forms alkenes only
2. Catalytic hydrogenation (H2/Pd) can remove halogens to give hydrocarbons ✅
3. Dehalogenation is impossible
4. Only radicals can remove halogens