CBSE Class 12 Chemistry HOTs Haloalkanes And Haloarenes

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Unit 10 Haloalkanes and Haloarenes Class 12 Chemistry HOTS

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HOTS Questions Unit 10 Haloalkanes and Haloarenes Class 12 Chemistry with Answers

 

CHAPTER:10
HALOALKANES AND HALOARENES
 

Case Based Questions :
 

Free radical chlorination or bromination of alkanes usually gives a complex mixture of isometric mono and poly- haloalkane which is difficult to separate into pure compounds . For example , chlorination of propane yield 1-Chloropropane and 2 -Chloropropane in addition to polychlorinated products .Therefore ,haloalkanes are generally not prepared in laboratory by halogenation of alkanes .However, in certain cases Where formation of isometric products is not possible or where there is large difference in the reactivity of halogen attached to different carbon atoms ,this method can be used for the preparation of haloalkanes

Question. How many isometric monochloinated products are possible for iso-pentane?  
(a) 3.
(b) 4.
(c) 5.
(d) 6

Answer: B

Question. Which of the following halide cannot be prepared in good yield by free radical halogenatom of corresponding hydrocarbon?  
(a) Benzyl chloride.
(b) Allyl chloride.
(c) neo-Pentyl chloride.
(d) iso- Pentyl chloride

Answer: D

Question. Chlorination of methane in the presence of light yield a mixture of CH3Cl ,CH2Cl2,CHCl3 and CClThe most polar compound among these is . 
(a) CH3Cl.
(b) CH2Cl2.
(c) CCl4
(d) CHCl3

Answer: B

Question. Which of the following lighter than water?  
(a) CCl4.
(b) CH2Cl2.
(c) CH3Cl.
(d) CHCl3

Answer: C

 

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) SN1 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 .

class_12_chemistry_hot_3

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 SN2 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.

CBSE_Class_12_Chemistry_Haloalkanes_Set_B_1

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

9 the structural formulas of the organic compounds. A, B, C, D in the following sequence of reactions.
CBSE_Class_12_Chemistry_Haloalkanes_Set_B_2 
10 Rearrange the following in order of increasing ease of dehydrohalogenation:
CH3CH2CH2Cl, CH3CHClCH3, CH3CCl(CH3)2. Give reasons.
 
11 Write formulae for structural and geometrical isomers of C3H4Cl2
[HINT: total 7 structures
 
12 When toluene is chlorinated:
a. in presence of sunlight
b. in dark, in the presence of lewis acid, two separate compounds are obtained. Explain with suitable mechanism.
 
13 Predict the order of reactivity of the following compounds in SN1 and SN2 reactions, giving reasonsa.
C6H5CH2Br, C6H5CH(C6H5)Br, C6H5CH(CH3)Br, C6H5C(CH3)(C6H5)Br The 4 isomeric bromobutanes
 
14 Arrange the following isomeric substituted haloarenes in ascending order of their reactivity towards NaOH to form corresponding substituted phenols.
CBSE_Class_12_Chemistry_Haloalkanes_Set_B_3
15 Arrange the following halocompounds in decreasing order of reactivity towards SN1 nucleophilic substitution reaction, Vinyl chloride, Benzyl chlorides, iso propyl bromide.
 
 
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
 
CBSE_Class_12_Chemistry_Haloalkanes_1
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.

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