Previous year Questions (2016-20) - Haloalkanes and Haloarenes Notes | EduRev

Q.1. The dipole moments of CCl₄, CHCl₃ and CH₄ are in the order    (2020)
(1) CHCl₃ < CH₄ = CCl₄
(2) CCl₄ < CH₄ < CHCl₃
(3) CH₄ < CCl₄ < CHCl₃
(4) CH₄ = CCl₄ < CHCl₃
Ans. (4)
Molecules having symmetrical geometry and identical side atoms have zero dipole moment, that is, the resultant dipole moment is zero. Whereas unsymmetrical molecules shows net dipole moment.


Q.2. For the following reactions

Where

k_s and k_e, are the rate constants for substitution and elimination respectively, and µ = k_s/k_e, the correct option is _______.    (2020)
(1)
(2)
(3)
(4)
Ans. (2)
The reaction involved is


Since, the nucleophile (A) is small so it will favor substitution rather than elimination product. Nucleophile (B) is bulkier, so it will favor elimination product. Thus,  and

Q.3. The decreasing order of reactivity towards dehydrohalogenation (E₁) reaction of the following compounds is    (2020)
(I)
(II)
(III)
(IV)
(1) (IV) > (II) > (III) > (I)
(2) (II) > (IV) > (I) > (III)
(3) (II) > (IV) > (III) > (I)
(4) (II) > (I) > (IV) > (III)
Ans. (1) 

The strength of dehydrohalogenation depends upon the stability of alkene formed. More the stable alkene form, more the strength of dehydrohalogenation of the haloalkane.
(I)
(II)
(III)
(IV)
Since, the transition state of (IV) is more stable than (II), so, the alkene formed in (IV) is more stable than in (II). Thus, the correct decreasing order of dehydrohalogenation of the following compound is: (IV) > (II) > (III) > (I).

Q.4. Arrange the following bonds according to their average bond energies in descending order    (2020)
C – Cl, C – Br, C – F, C – I
(1) C – F > C – Cl > C – Br > C – I
(2) C – Br > C – I > C – Cl > C – F
(3) C – I > C – Br > C – Cl > C – F
(4) C – CI > C – Br > C – I > C – F
Ans. (1)
The bond enthalpy of C–X bond depends upon the size of the halogen X, as the size of the halogen increases, the bond energy decreases. Thus, the correct decreasing order of C–X, bond enthalpy is: C – F > C – Cl > C – Br > C – I.

Q.5. Which of these will produce the highest yield in Friedel Crafts reaction?   (2020)
(1)
(2)
(3)
(4)
Ans. (3)
The highest yield in Friedel Crafts reaction ins given by chlorobenzene. Since, phenol and aniline react with Lewis acid so reaction could not occur. –CONH₂ group is a strong deactivating groups towards electrophilic aromatic subsitution reactions.

Q.6. X melts at low temperature and is a bad conductor of electricity in both liquid and solid state. X is    (2020)
(1) zinc sulphide
(2) mercury
(3) silicon carbide
(4) carbon tetrachloride
Ans. (4)
Since, CCl₄ is a non-polar compound, so, it has low melting point and it is covalent compound, thus, it is bad conductor of electricity in both solid and liquid state.

Q.7. Which of the following reactions will not produce a racemic product?    (2020)
(1)
(2)
(3)
(4)
Ans. (4)
The reaction involved is


Q.8. The major product of the following reaction is:    (2019)

(1)
(2)
(3)
(4)
Ans. (1)
Mechanism involved for the given reaction is:


Q.9. Which amongst the following is the strongest acid?    (2019)
(1) CHBr₃
(2) CHI₃
(3) CH(CN)₃
(4) CHCl₃
Ans. (3)
Due to the resonance stabilisation of the conjugate base, CH(CN)₃ is the strongest acid amongst the given compounds.


The conjugate bases of CHBr₃ and CHl₃ are stabilised by inductive effect of halogens. This is why, they are less stable. Also, the conjugate base of CHCl₃ involves back-bonding between 2p and 3p orbitals

Q.10. The major product of the following reaction is:    (2019)

(1)
(2)
(3)
(4)
Ans. (1)
Dehydrohalogenation (β-elimination) occurs as:


Q.11. Which hydrogen in compound (E) is easily replaceable during bromination reaction in presence of light?    (2019)

(1) α - hydrogen
(2) γ - hydrogen
(3) δ - hydrogen
(4) β - hydrogen
Ans. (2)
Allylic H is replaced due to the greater stability of allylic free radical.



Q.12. The major product of the following reaction is:    (2019)

(1)
(2)
(3)
(4)
Ans. (2)


Q.13. The major product of the following reaction is:    (2019)

(1)
 
(2)
 
(3)
 
(4) 

Ans. (2)
Heating of the given compound in presence of strong base is favoured for elimination reaction resulting in more stable alkene.


Q.14. The major product of the following reaction is:    (2019)

(1)
(2)
(3)
(4)
Ans. (1)


Q.15. The major product of the following reaction is:    (2019)

(1)
(2)
(3)
(4)
Ans. (3)


Q.16. The major product of the following reaction is:    (2019)

(1)
(2)
(3)
(4)
Ans. (3)


Q.17. Increasing order of reactivity of the following compounds for S_N1 substitution is:    (2019)

 
(1) (B) < (C) < (D) < (A)
(2) (B) < (C) < (A) < (D)
(3) (B) < (A) < (D) < (C)
(4) (A) < (B) < (D) < (C)
Ans. (3)
In S_N1 reaction carbocation acts as an intermediate.

Carbocation produced by (C) is more stable than carbocation produced by (D) due to + I effect of -OCH₃ group.
Further in (A) there is formation of tertiary carbocation after rearrangement while (B) is primary carbocation.

So, the correct order is (C) > (D) > (A) > (B).

Q.18. Which of the following potential energy (PE) diagrams represents the S_N1 reaction?    (2019)
(1)
(2)
(3)
(4)
Ans. (1)
The S_N1 reaction energy diagram illustrates the dominant part of the substrate with respect to the reaction rate.
The rate determining step is the formation of the intermediate carbocation.


Q.19. Increasing rate of S_N1 reaction in the following compounds is:    (2019)

 
(1) (A) < (B) < (C) < (4)
(2) (B) < (A) < (C) < (D)
(3) (B) < (A) < (D) < (3) 
(4) (A) < (B) < (D) < (C)
Ans. (2)
The rate of S_N1 is decided by the stability of carbocation formed in the rate determining step.

Carbocation(D) is most stable due to +R effect of CH₃O group, (C) is stabilised by +I and +H effects of the CH₃ group; (B) is least stable due to -I effect of MeO group. So increasing order of rate of S_N1 is (B) < (A) < (C) < (D)

Q.20. The major product of the following reaction is    (2019)

(1)
(2)
(3)
(4)
Ans. (3)



Q.21. The major product ‘Y’ in the following reaction is:    (2019)

(1)
(2)
(3)
(4)
Ans. (3)


Q.22. Which one of the following is likely to give a precipitate with AgNO₃ solution?    (2019)
(1) CH₂ = CH —Cl
(2) CCI₄
(3) CHCl₃
(4) (CH₃)₃CCl
Ans. (4)
Carbocation is formed on reaction with Ag⁺
 
is most stable carbocation compared to other carbocations i.e.


Q.23. Heating of 2-chloro-1-phenylbutane with EtOK/EtOH gives X as the major product. Reaction of X with Hg (OAc)₂/H₂O followed by NaBH₄ gives Y as the major product. Y is:    (2019)
(1)
(2)
(3)
(4)
Ans. (3)


Q.24. An ‘Assertion’ and a ‘Reason’ are given below. Choose the correct answer from the following options :
Assertion (A): Vinyl halides do not undergo nucleophilic substitution easily.
Reason (R): Even though the intermediate carbocation is stabilized by loosely held π-electrons, the cleavage is difficult because of strong bonding.    (2019)
(1) Both (A) and (R) are wrong statements.
(2) Both (A) and (R) are correct statements and (R) is the correct explanation of (A).
(3) Both (A) and (R) are correct statements but (R) is not the correct explanation of (A).
(4) (A) is a correct statement but (R) is a wrong statement.
Ans. (4)

Due to partial double bond character of C-halogen bond, halogen leaves with great difficulty, if at all it does. Hence, vinyl halides do not undergo nucleophilic substitution easily. So, assertion is correct.

Intermediate carbocation is not stabilised by loosely held-π electrons because empty orbital, being at 90°, cannot overlap with p-orbitals of π bond. So, reason is wrong.

Q.25. The major product of the following reaction is:    (2018)

(1)
(2)
(3)
(4)
Ans. (2)


Q.26. Which of the following will most readily give the dehydrohalogenation product?    (2018)
(1)
(2)
(3)
(4)
Ans. (2)
Carbanion formed after removal of hydrogen is resonance stabilised as it is adjacent to phenyl ring.

Q.27. The major product of the following reaction is:    (2018)

(1)
(2)
(3)
(4)
Ans. (3)

Inversion takes place at the carbon containing bromine atom. 

Q.28. The increasing order of the reactivity of the following halides for the S_N1 reaction is    (2017) 

(1) (III) < (II) < (I) 
(2) (II) < (I) < (III) 
(3) (I) < (III) < (II) 
(4) (II) < (III) < (I) 
Ans. (2)
For any S_N1 reaction, reactivity is decided by ease of dissociation of alkyl halide
R - X ⇌ R⁺ + X^-
Higher the stability of R⁺ (carbocation) higher would be reactivity of S_N1 reaction. Since the stability of cation follows the order.
 

Q.29. The major product obtained in the following reaction is    (2017)

(1) (±)C₆H₅CH(OtBu)CH₂C₆H₅ 
(2) C₆H₅CH = CHC₆H₅ 
(3) (+)C₆H₅CH(OtBu)CH₂C₆H₅ 
(4) (-)C₆H₅CH(OtBu)CH₂C₆H₅ 
Ans. (2)


Q.30. The major product of the following reaction is:    (2017)

(1)
(2)
(3)
(4)
Ans. (1)


Q.31. The major product of the following reaction is:    (2017)

(1) CH₃CH = CH – CH = CHCH₃
(2) CH₂ = CHCH = CHCH₂CH₃
(3) CH₃CH = C = CHCH₂CH₃
(4) CH₂ = CHCH₂CH = CHCH₃
Ans. (1)


Q.32. 2-chloro-2-methylpentane on reaction with sodium methoxide in methanol yields:    (2016)
 

(1) All of these 
(2) (a) and (c)
(3) (c) only
(4) (a) and (b) 
Ans. (1)
Strong nucleophile (O^- Me) polar solvent (MeOH) gives elimination products over substitution products but all products are possible in different yields.

Q.33. Which one of the following reagents is not suitable for the elimination reaction?    (2016)

(1) NaI
(2) NaOH/H₂O-EtOH
(3) NaOH/H₂O
(4) NaOEt/EtOH
Ans. (1)
NaI gives Nucleophilic substitution reaction.