CBSE Class 12 Chemistry HOTs Haloalkanes And Haloarenes

Case Based Questions :

Short type Question And Answers

Question. Why haloalkanes are more reactive than haloarenes?
Answer: In haloarenes, there is partial double bond character b/w carbon and halogen due to resonance effect which makes him less reactive.
(ii) In benzene, carbon being sp² hybridised which is smaller in size than sp³ present in haloalkanes. So C–Cl bond in aryl halides is shorter and stronger.

Question. Why do haloalkenes under go nucleophillic substitution whereas haloarenes under go electophillic substitution?
Answer: Due to more electro negative nature of halide atom in haloalkanes carbon atom
becomes slightly positive and is easily attacked by nucleophillic reagents. While in haloarenes due to resonance, carbon atom becomes slightly negative and attacked by electrophillic reagents.

Question. The treatment of alkyl chlorides with aqueous KOH lead to the formation of alcohols but in presence of alcoholic KOH alkenes are major products. Explain?
Answer: In aqueous KOH,OH^– is nucleophile which replaces another nucleophile.
R-X + KOH → R-OH + KX
Where as in alcoholic KOH, C₂H₅O– ion is produced which is a strong base hence b-elimination took place to form alkane
C₂H₅OH + KOH → C₂H₅O- + K+
CH₃CH₂-Cl + alcoholic KOH → CH₂ = CH₂ + C₂H₅OH

Question. Explain why vinyl chloride is unreactive in nucleophillic substitution reaction?
Answer: Vinyl chloride is unreactive in nucleophillic substitution reaction because of double bond character between C–Cl bond which is difficult to break.

Question. Arrange the following compounds according to reactivity towards nucleophillic substitution reaction with reagents mentioned :-
(i) 4-nitrochlorobenzene> 2,4 dinitrochlorobemzene > 2,4,6, trinitrochlorobenzene with CH₃ONa
Answer: ,4,6, trinitrochlorobenzene > 2,4 dinitrochlorobemzene > 4- nitrochlorobenzene

Question. Why Grignard reagent should be prepared under an hydrous conditions?
Answer: Grignard reagent react with H₂O to form alkanes, therefore they are prepared under anhydrous condition.

Question. Why is Sulphuric acid not used during the reaction of alcohols wiht KI?
Answer: It is because HI formed will get oxidized to I₂ by concentrated Sulphuric acid which is an oxidizing agent.

Question. p-dichlorobenzene has highest m.p. than those of ortho and m-isomers?
Answer: p-dichlorobenzene is symmetrical, fits into crystal lattice more readily and has higher melting point.

Question. Give reasons:
(i) C–Cl bond length in chlorobenzene is shorter than C–Cl bond in CH₃Cl.
(ii) The dipole moment of chlorobenzene is lower than that of cyclohexyl chloride.
(iii) S_N1 reactions are accompained by racemization in optically active alkyl halides.
Answer: (i) In chlorobenzene, each carbon atom is sp² hybridised/ resonating structures / partial donable bond character.
(ii) Due to + R effect in chlorobenzene / difference in hybridization i.e., sp² and sp³ respectively.
(iii) Due to formation of planer carbocation.

More Questions..

1 Iodoform gives the precipitate with AgNO₃ on heating while chloroform does not. Give reasons

2 The following reaction give 2 products. Write their structures .

3 Predict the products of the following reactions:
a. HCl with CH₃CCl=CH₂
b. HBr with CH₃CH=C(CH₃)₂

4 Monochlorination of ethane to ethyl chloride is more practical than chlorination of npentane. Give reasons

5 An optically active compound having molecular formula C₇H₁₅Br reacts with aq. KOH to give racemic mixture of products. Write the mechanism involved in the reaction. [hint: a carbocation being planar, allows attack of nucleophile from either direction.]

6 a. Which of the following 2 compounds would react faster by S_N2 pathway:- 1- bromobutane or 2-bromobutane and why?

b. Allyl chloride is more reactive than n-propyl chloride towards nucleophilic substitution reaction. Explain why?

c. Haloalkanes react with KCN to give alkyl cyanides as main product while with AgCN, they form isocyanide as the main product. Give reasons.

7 In each of the following pairs of organic compounds, identify the compound which will undergo SN1reaction faster. Also give reason with related structures.

8 p-nitrobenzene undergoes nucleophilic substitution faster than chlorobenzene. Explain giving the resonating structures as well.

1 - MARK QUESTIONS

 

1. Identify the most reactive among the given compounds.

a. H₂C = CH-Cl b. CH₃CH₂Cl c. H₂C = CHCH₂Cl d. C₆H₅Cl 1

 

2. What is the main product formed when 2-bromopentane is heated with potassium ethoxide and ethanol ? 1

 

3. In presence of peroxide HCl and HI do not give antimarkovnikov’s addition to alkenes. Why ? 1

 

4. Arrange the following in the order of increasing ease of dehydrohalogenation.

i. CH₃CH₂Cl ii. CH₃CHClCH₃ iii. CH₃CCl(CH₃)₂ 1

 

5. Which is the better nucleophile. Bromide ion or Iodide ion. Why ? 1

 

6. A hydrocarbon C₅H₁₂ gives only one monochlorination product. Identify it. 1

 

7. Which isomer of C₄H₉Br have lowest boiling point ? 1

 

 

2 - MARKS QUESTIONS

 

8. How to convert i. 2-methyl propane to isobutyl bromide ii. 2-iodopropane to 1-iodopropane 2

 

9. Grignard reagent cannot be prepared from BrCH2C≡CH. Explain why ? 2

 

10. Wurtz reaction fails incase of tertiary butyl halide. Why ? 2

 

11. Write down the structures of A and B

 

12. How can you distinguish between the following pair

 

a. CH₃-CH=CHBr & H₂C=CH-CH₂Br

 

b. 1,1-dichloroethane & 1,2 – dichloroethane 2

 

13. RCl is hydrolysed to ROH slowly, but the reaction is rapid if a catalytic amount of KI is added to the reaction mixture. Explain. 2

 

 

3 - MARKS QUESTIONS

 

14. Predict the order of reactivity of the following compounds in SN¹ and SN² reactions

 

a. Four isomeric bromobutanes

 

b. Benzyliodide, benzylchloride, benzylbromide.

 

c. CH₃Cl, CH₃Br, CH₃CH₂Cl, (CH₃)₂CHCl ₃

 

15. Arrange the following in the increasing order of boiling point and state the reason for the order.

 

a. CH₃CH₂CH₂CH₂Br, (CH₃)₃CBr, (CH₃)₂CHCH₂Br

 

b. 1,2-dichlorobenzene, 1,3-dichlorobenzene, 1,4-dichlorobenzene

 

c. CH₃CH₂CH₂Br, CH₃CH₂CH₂Cl, CH₃CH2CH₂I 3

 

16. State the reasons for the following

 

a. Iodoform is obtained by the reaction of acetone with Hypoiodite but not with iodide ion

 

b. Trichloromethane is stored in dark coloured bottles.

 

c. Haloarenes are insoluble in water but soluble in benzene. 3

 

17. An alkyl halide X of formula C₆H₁₃Cl on treatment with potassium tertiary butoxide gives two isomeric alkenes Y and Z (C₆H₁₂). Both alkenes on hydrogenation give 2,3-dimethylbutane. Predict the structures of X, Y and Z. Write chemical equations involved. 3

 

18. A Grignard reagent A and a haloalkane B react together to give C. Compound C on heating with KOH gives a mixture of two geometrical isomers D and E of which D predominates. C and E have same molecular formula and C gives 1-bromo-3- phenylpropane on reaction with HBr in presence of peroxide. Give the structures of A, B and C and configurations of D and E with reasons.