Q41Multiple choice
Which of the following compounds can be classified as aryl halides?
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Correct option: A
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Option (i) and (iv) are the answers.
NCERT Exemplar · Class 12 · Chemistry
Chapter 10: Haloalkanes and Haloarenes
Practice each question, then open the answer or worked solution.
Questions
55 questionsQ42Written answer
Alkyl halides are prepared from alcohols by treating with
(i) HCl + ZnCl2
(ii) Red P + Br2
(iii) H2SO4+ KI
(iv) All the above
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Worked solution
Option (i) and (ii) are the answers
Q43Written answer
Alkyl fluorides are synthesised by heating an alkyl chloride/bromide in
presence of ____________ or ____________.
(i) Ca F2
(ii) CoF2
(iii) Hg2F2
(iv) NaF
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Worked solution
Option (ii) and (iii) are the answers.
III. Short Answer Type
Q44Written answer
Aryl chlorides and bromides can be easily prepared by electrophilic substitution of arenes with chlorine and bromine respectively in the presence of Lewis acid catalysts. But why does the preparation of aryl iodides requires the presence of an oxidising agent?
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Iodination of arenes is reversible due to the formation of HI. To move the reaction forward, an oxidizing agent like HNO3 or HIO4 oxidises HI, thus stabilizing the product.
Q45Written answer
Out of o-and p-bromobenzene which one has a higher melting point and why?
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p-bromobenzene has a higher melting point because the symmetry of p-dibromobenzene makes the molecule fit better in a crystal lattice. So that it requires a higher temperature to break the bonds between the molecules and thus has a higher melting point.
Q46Written answer
Which of the compounds will react faster in SN1 reaction with the –OH ion?
CH3— CH2— Cl or C6H5— CH2— Cl
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Worked solution
C6H5— CH2— Cl will react faster in an SN1 reaction with the OH- ion. This happens due to the stability of the carbocation in the compound. C6H5 group is already stable due to resonance, and the CH2 attached will gain that stability, thus forming a stable C6H5CH2+ carbocation after the cleavage in the first step of the SN1 reaction.
Q47Written answer
Why iodoform has appreciable antiseptic property?
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Due to the liberation of free iodine iodoform has an appreciable antiseptic property.
Q48Written answer
Haloarenes are less reactive than haloalkanes and haloalkenes. Explain.
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Worked solution
This is because of the resonance stabilization of the aryl ring. If we take an example of C6H5-Cl, there will be a conjugation of chlorine electrons with the electrons in the ring. This resonance makes C-Cl bond to get partial double bond character and makes less reactive to nucleophilic substitution.
Q49Written answer
Discuss the role of Lewis acids in the preparation of aryl bromides and chlorides in the dark.
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Worked solution
Aryl bromides and chlorides can be prepared from arenes by electrophilic substitution. This reaction is carried out by treating the arene with chlorine or bromine in the presence of iron (III) chloride in the absence of light. Iron (III) chloride, is a Lewis acid, which generates the electrophile required to take the reaction forward. FeCl3 forms a coordination compound with Cl2, making the complex Cl+[FeCl4-].
Q50Written answer
Which of the following compounds (a) and (b) will not react with a mixture of
NaBr and H2SO4. Explain why?
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Worked solution
A mixture of NaBr and H2SO4 gives Br2 gas as a product. Molecule (b) will not react with Br2 gas because of the stable molecule that is formed due to resonance stabilization.