CBSE Class 12 Chemistry Haloalkanes And Haloarenes Board Exam Notes

(B) IMPORTANT MECHANISM
i) Nucleophilic Substitution ii) Elimination reaction iii) Electrophilic substitution
i) Nucleophilic Substitution :‐ it involves the replacement of an atom or group of atoms by a nucleophile (electron rich species)
A‐B + Nu →A‐ Nu +B

It is of two types:‐
a) Unimolecular nucleophi lic substitution reaction (SN1)
b) Bimolecular nucleophilic substitution reaction (SN22)
a) Unimolecular substitution reaction (SN1)
(i) it involves two steps
(ii) in this retention/Racemization of configuration take place.
(iii) unimolecular and all are first order.
Mechanism (step 1) formation of carbocation.
Step 2 Attack of nucleophile on carbocation

Order of reactivity: 30> 20>10 ( Because 30 carbocation is more stable)
(b) Bimolecular Nucleophilic Substitution SN2(i) it involves one step. (ii) in this inversion of configuration takes place.(iii) bimolecular mechanism: . in this nucleophile attacks from the back side Where halogen atom is attached.
It facilitates the departure of leaving halide group.

R‐X + AgCN → RNC + AgX
R‐X + Na‐C ≡C‐R → R‐C≡ C‐R + Na‐X
R-X + KNO2 → R-O-N=O + KX
ii) Elimination reaction : Haloalkanes having hydrogen atom, when heated with alcoholic KOH, there is elimination of hydrogen atom from β‐carbon and a halogen atom from the α‐ carbon atom & alkene is formed.
Saytzeff rule “in dehydrohalogenation reactions, the preferred product is that alkene which has the

Q. 1 What are ambident nucleophiles? (1 Mark)
Ans. Nucleophiles which can attack through two different sites are called ambident nucleophiles.
Example: ‐ Cyanide ion :CN–: & C=N:–

Q. 2 Which is a better nucleophile, a bromide ion or iodide ion? (1 Mark)
Ans. Iodide ion because it has lower electronegativity and larger size.

Q. 3 Arrange the compounds of each set in order of reactivity towards SN2 displacement:
2‐ Bromo‐2‐methylbutane, 1‐Bromopentane, 2‐Bromopentane. (1 Mark)
Ans. The reactivity in SN2 reactions depend upon steric hindrance; more the steric hindrance slower the reaction.The order of reactivity in SN2 reactions follows the order : 10> 20> 30. 1‐ Bromopentane>
2‐Bromopentane> 2‐Bromo‐2‐methylbutane .

Q. 4 Arrange the following in increasing order of boiling point.
CH₃CH₂CH₂CH₂Br, (CH₃)₃CBr, (CH₃)2CHCH₂Br. (1Mark)
Ans. The boiling point increases as the branching decreases, so the increasing order of boiling pointis:‐(CH3)3CBr < (CH3)2CHCH2Br < CH3CH2CH2CH2Br

Q. 5 Define optical activity? (1Mark)
Ans. The compounds which rotate the plane of polarized light when it is passed through their solutions are
called optically active compounds and this property is known as optical activity.

Q. 6 What is racemic mixture or racemic modification? (1 Mark)
Ans. A mixture containing two enantiomers in equal proportions will have zero optical rotation, As the rotation due to one isomer will be cancelled by the rotation due to other isomer. Such a mixture is known as racemic mixture or racemic modification.

Q. 7 What are enantiomers? (1 Mark)
Ans. The stereo isomers related to each other as non-superimposable mirror images are called enantiomers.

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