Haloalkanes & Haloarenes

31 NEET-UG practice questions on Haloalkanes & Haloarenes, part of Chemistry. The first 25 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

1. Question 1 · Easy

   Ethylmagnesium bromide reacts at once with water. Name the organic product.

   1. AEthane
   2. BButane
   3. CEthene
   4. DEthanol

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   Answer: A · Ethane

   The carbon bound to magnesium is strongly carbanion-like and takes a proton from water: CH₃CH₂MgBr + H₂O → CH₃CH₃ + Mg(OH)Br.

   • B: This is a coupling product, not formed by reaction with water.
   • C: This would require elimination, which does not occur here.
   • D: This assumes OH is transferred to carbon, but carbon picks up H instead.

   Remember this

   Grignard reagents are destroyed by any source of acidic hydrogen, giving the hydrocarbon, so they must be kept dry.

2. Question 2 · Easy

   When an SN2 reaction takes place at a chiral carbon, what happens to the configuration at that carbon?

   1. AIt is fully racemised
   2. BIt becomes planar in the product
   3. CIt is retained
   4. DIt is inverted

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   Answer: D · It is inverted

   The nucleophile attacks from the side opposite the leaving group, in a single step, so the other three groups flip over like an umbrella.

   • A: Racemisation is the feature of SN1, which has a planar intermediate.
   • B: The product carbon is still sp³; only the transition state has the planar arrangement.
   • C: Retention would need attack from the same side as the leaving group.

   Remember this

   SN2 means backside attack and inversion; SN1 goes through a planar carbocation and leads to racemisation.

3. Question 3 · Hard

   Follow this sequence. 1-Bromopropane + Mg, dry ether → P P + propanone, then H₃O⁺ → Q Q + conc. H₂SO₄, heat → R (major) What is R?

   1. A2-Methylpent-1-ene
   2. B2-Methylpent-2-ene
   3. C4-Methylpent-2-ene
   4. DHex-3-ene

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   Answer: B · 2-Methylpent-2-ene

   P is CH₃CH₂CH₂MgBr. It adds to the C=O of propanone, giving the tertiary alcohol Q, 2-methylpentan-2-ol. Acid dehydration then follows Saytzeff's rule, losing H from the chain CH₂ next to the carbocation centre to give (CH₃)₂C=CHCH₂CH₃.

   • A: This is the less substituted alkene, the minor product.
   • C: This moves the methyl branch and the double bond away from the tertiary carbon.
   • D: This has a straight chain, ignoring the branch set by the propanone carbon.

   Remember this

   A Grignard reagent and a ketone give a tertiary alcohol, which dehydrates easily to the more substituted alkene.

4. Question 4 · Medium

   Arrange these bromides in increasing order of reactivity by the SN1 mechanism. A. (CH₃)₃CBr B. CH₃CH₂Br C. (CH₃)₂CHBr D. CH₃Br

   1. AD, B, C, A
   2. BA, C, B, D
   3. CD, C, B, A
   4. DB, D, C, A

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   Answer: A · D, B, C, A

   The rate-determining step of SN1 forms a carbocation, so the rate follows carbocation stability: methyl < 1° < 2° < 3°.

   • B: This is the SN2 order, which is controlled by crowding.
   • C: This places the secondary bromide below the primary one, against carbocation stability.
   • D: This places ethyl below methyl, though a primary cation is more stable than methyl.

   Remember this

   SN1 reactivity is 3° > 2° > 1° > CH₃X, exactly the reverse of the steric order for SN2.

5. Question 5 · Medium

   On heating 2-bromo-2-methylbutane with alcoholic KOH, which alkene is formed as the major product?

   1. A2-Methylbut-1-ene
   2. B2-Methylbutan-2-ol
   3. C2-Methylbut-2-ene
   4. D3-Methylbut-1-ene

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   Answer: C · 2-Methylbut-2-ene

   Alcoholic KOH causes β-elimination. By Saytzeff's rule the more substituted alkene is preferred, so H is lost from the CH₂ group to give (CH₃)₂C=CHCH₃.

   • A: This is the less substituted alkene, formed in the smaller amount.
   • B: This is the substitution product expected with aqueous alkali, and it is not an alkene.
   • D: This has its double bond away from the carbon that held bromine, which simple elimination cannot give.

   Remember this

   In dehydrohalogenation the major alkene carries the greater number of alkyl groups on the double bond.

6. Question 6 · Medium

   Counting enantiomers separately, how many isomers in all have the molecular formula C₄H₉Br?

   1. A4
   2. B3
   3. C5
   4. D6

   Answer and explanation in LearnFree account

7. Question 7 · Medium

   Assertion A: Chlorobenzene is much less reactive than chloroethane towards nucleophilic substitution. Reason R: The C-Cl bond in chlorobenzene is longer than that in chloroethane. Choose the correct answer.

   1. AA is correct but R is not correct
   2. BBoth A and R are correct but R is not the correct explanation of A
   3. CBoth A and R are correct and R is the correct explanation of A
   4. DA is not correct but R is correct

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8. Question 8 · Hard

   Follow the sequence shown, starting from bromobenzene. What is Y?

   

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   1. APhenylethyne
   2. BEthylbenzene
   3. C1-Phenylethanol
   4. DEthenylbenzene

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9. Question 9 · Medium

   The enantiomerically pure bromide drawn is converted to an alcohol by a purely SN2 pathway. Name the product P.

   

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   1. A(S)-Butan-2-ol
   2. B(±)-Butan-2-ol
   3. C(R)-Butan-2-ol
   4. DButan-1-ol

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10. Question 10 · Easy

    Refer to bromides X and Y in the figure. Assertion A: Towards OH⁻ under SN2 conditions, X reacts faster than Y. Reason R: The carbocation formed from Y is more stable than the one formed from X. Choose the correct answer.

    

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    1. AA is correct but R is not correct
    2. BBoth A and R are correct and R is the correct explanation of A
    3. CA is not correct but R is correct
    4. DBoth A and R are correct but R is not the correct explanation of A

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11. Question 11 · Medium

    Assertion A: Chlorobenzene undergoes nucleophilic substitution more readily than chloromethane. Reason R: Resonance gives the C-Cl bond in chlorobenzene partial double bond character. Choose the correct answer.

    1. AA is correct but R is not correct
    2. BBoth A and R are correct and R is the correct explanation of A
    3. CA is not correct but R is correct
    4. DBoth A and R are correct but R is not the correct explanation of A

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12. Question 12 · Medium

    Chlorobenzene is treated with chloromethane in the presence of anhydrous AlCl₃. What is the major product?

    1. A1-Chloro-3-methylbenzene
    2. BMethylbenzene
    3. C1-Chloro-4-methylbenzene
    4. D1-Chloro-2-methylbenzene

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13. Question 13 · Medium

    Ethylmagnesium bromide is added to methanal in dry ether, and the adduct is then hydrolysed with dilute acid. What is the organic product?

    1. APropan-2-ol
    2. BEthanol
    3. CPropanal
    4. DPropan-1-ol

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14. Question 14 · Easy

    An alcohol is to be converted into a pure alkyl chloride with all the by-products escaping as gases. The best reagent is

    1. Athionyl chloride, SOCl₂
    2. Bphosphorus pentachloride, PCl₅
    3. CHCl with anhydrous ZnCl₂
    4. Dphosphorus trichloride, PCl₃

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15. Question 15 · Hard

    Read the statements: A. SN1 hydrolysis of an optically active tertiary alkyl halide gives a racemic product. B. The rate of an SN2 reaction depends on the concentrations of both the alkyl halide and the nucleophile. C. A nitro group at the meta position activates chlorobenzene towards nucleophilic substitution as strongly as one at the para position. D. With alcoholic KOH, 2-bromobutane gives but-1-ene as the major alkene. Which of them are correct?

    1. AA and B only
    2. BA, B and C only
    3. CB and D only
    4. DA, B and D only

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16. Question 16 · Hard

    Chlorobenzene is treated with a mixture of concentrated HNO₃ and H₂SO₄ to give the major product P. P is heated with aqueous NaOH at 443 K, and the mixture is then acidified to give Q. What is Q?

    1. A4-Chlorophenol
    2. B4-Nitrophenol
    3. C1-Chloro-4-nitrobenzene, unchanged
    4. D3-Nitrophenol

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17. Question 17 · Easy

    Bromoethane is warmed with sodium iodide in dry acetone. What is the organic product?

    1. AButane
    2. BEthene
    3. CEthanol
    4. DIodoethane

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18. Question 18 · Medium

    Arrange the compounds in increasing order of reactivity towards aqueous NaOH (replacement of Cl by OH). A. 1-Chloro-2,4-dinitrobenzene B. Chlorobenzene C. 1-Chloro-2,4,6-trinitrobenzene D. 1-Chloro-4-nitrobenzene

    1. AC, A, D, B
    2. BD, B, A, C
    3. CB, A, D, C
    4. DB, D, A, C

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19. Question 19 · Medium

    Optically active 3-bromo-3-methylhexane is hydrolysed in water by the SN1 mechanism. The product, 3-methylhexan-3-ol, is obtained as

    1. Aa meso compound, because the two faces of the carbocation are equivalent
    2. Ba racemic mixture, because the planar carbocation is attacked from both faces
    3. Can optically inactive compound, because the product has no chiral carbon
    4. Da single enantiomer with inverted configuration, because of backside attack

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20. Question 20 · Medium

    Given below are two statements: Statement I: Haloalkanes react with KCN to give alkyl cyanides as the main product. Statement II: Haloalkanes react with AgCN to give alkyl cyanides as the main product. Choose the correct answer.

    1. AStatement I is correct but Statement II is incorrect
    2. BBoth Statement I and Statement II are correct
    3. CStatement I is incorrect but Statement II is correct
    4. DBoth Statement I and Statement II are incorrect

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21. Question 21 · Easy

    The Swarts reaction, in which an alkyl chloride or bromide is heated with a metallic fluoride such as AgF, Hg₂F₂ or SbF₃, is used to prepare

    1. Aalkyl fluorides
    2. Baryl bromides
    3. Calkyl chlorides
    4. Dalkyl iodides

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22. Question 22 · Medium

    A mixture of bromomethane and bromoethane is treated with sodium in dry ether. How many different alkanes are formed?

    1. A1
    2. B3
    3. C2
    4. D4

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23. Question 23 · Medium

    Chlorobenzene is treated with sodium in dry ether. What is the main organic product?

    1. AToluene
    2. BBiphenyl
    3. CBenzene
    4. DDiphenyl ether

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24. Question 24 · Medium

    Assertion A: (CH₃)₃CBr undergoes SN2 substitution faster than CH₃Br does. Reason R: In an SN2 reaction, bulky groups around the carbon bearing the halogen hinder the approach of the nucleophile. Choose the correct answer.

    1. AA is correct but R is not correct
    2. BA is not correct but R is correct
    3. CBoth A and R are correct but R is not the correct explanation of A
    4. DBoth A and R are correct and R is the correct explanation of A

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25. Question 25 · Easy

    Bromoethane is heated with silver cyanide. What is the main organic product?

    1. AEthyl isocyanide, CH₃CH₂NC
    2. BPropanenitrile, CH₃CH₂CN
    3. CEthene
    4. DEthanol

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