CBSE Class 12 Chemistry Haloarenes Worksheet

Very Short Answer Questions

Question. Which halogen compound in each of the following pairs will react faster in S_N2 reaction :
(i) CH₃Br or CH₃I
(ii) (CH₃)₃CCl or CH₃Cl
Answer. (i) CH₃I will give faster S_N2 reaction.
(ii) CH₃Cl will give faster S_N2 reaction.

Question. How may methyl bromide be preferentially converted to methyl isocyanide? 
Answer. KCN is predominantly ionic and provides cyanide ions in solution
CH₃Br + KCN              →         CH₃C ≡ N + KBr
Methyl Methyl                             cyanide
bromide
AgCN is mainly covalent in nature and nitrogen is free to donate electron pair forming isocyanide as main product.
CH₃Br + AgCN          →          CH₃N ≡ C + AgBr
Methyl Methyl                            isocyanide
bromide

Question. What happens when ethyl chloride is treated with aqueous KOH?
Answer. When ethyl chloride is treated with aqueous KOH, ethanol is formed,
CH₃CH₂Cl + KOH_(aq) → CH₃CH₂OH + KCl

Question. Give reason :
n-Butyl bromide has higher boiling point than t-butyl bromide.
Answer. n-Butyl bromide, being a straight chain molecule have strong intermolecular forces whereas t-butyl bromide being a branched chain molecule have weaker intermolecular forces due to smaller surface area. Hence, boiling point of n-butyl bromide is higher than that of t-butyl bromide.

Question. Why are alkyl halides insoluble in water?
Answer. Alkyl halides are polar but are insoluble in water because energy required to break the intermolecular H – bond among water molecules is much higher than energy released by water halide interaction.

Question. Which would undergo S_N2 reaction faster in the following pair and why?
CH₃ CH₂ — Br and CH₃ — CH₂ — I
Answer. Since I^– is a better leaving group than Br–, thus, CH₃CH₂I undergoes S_N2 reaction faster than CH₃CH₂Br.

Short Answer Questions

Question. Suggest a possible reason for the following observations :
(i) The order of reactivity of haloalkanes is RI > RCl > RBr.
(ii) Neopentyl chloride (CH₃)₃CCH₂Cl does not follow S_N2 mechanism.
Answer. (i) Among the various halides with same alkyl group the order of reactivity is RI > RBr > RCl. Due to increasing bond strength of C I, C Br and C Cl the reactivity decreases.
(ii) Neopentyl chloride being a primary halide reacts slowly through S_N1 and the carbon carrying halogen is sterically more hindered. Hence it does not follow S_N2 mechanism.

Question. Which compound in the following couple will react faster in S_N2 displacement and why?
(i) 1-Bromopentane or 2-bromopentane
(ii) 1-Bromo-2-methylbutane or 2-bromo-2-methylbutane.
Answer. (i) 1-Bromopentane, as it is a primary alkyl halide.
(ii) 1-Bromo-2-methylbutane, as it is a primary alkyl halide.

1.Give IUPAC names of the following:- 

(a) Iso-butylchloride

(b) tert- butylchloride

(c) neo-pentylbromide

(d) ethylidene chloride

(e) Ethylene dichloride

(f) ethylene dichloride

(g) bromoform

(h) vinylchloride

(i) allyl bromide

(j) methylene chloride

(k) benzyl chloride

2.Give reasons for the following:-

(a) Flourine is more electronegative than chlorine still dipolemoment of C-F bond is less than that of CCl bond in methylflouride & methylchloride.

(b) For the conversion of alcohols to alkyl halides, thionyl chloride is preferred over phosphorous halide or halogen acid.

(c) Conversion of alcohols to alkyl iodide is done in absence of catalyst zinc chloride.

(d) Phenol cannot be converted to halobenzene by reaction with thionyl chloride or phosphorous halide or halogen acid.

(e) Free radical halogenation of alkanes is not a very good method for preparation of holoalkanes.

(f) Electrophilic substitution of benzene with iodine requires presence of oxidizing agents like HIO₃.

(g) Electrophilic substitution of benzene with fluorine is not done for preparation of flourobenzene.

(h) Finkelstein reaction is carried out in dry acetone medium.

(i) Rate of reaction between t-butyl bromide & hydroxide ion depends only on concentration of hydroxide ions.

(j) Allylic & benzylic halides show high reactivity towards SN1 reaction.

(k) Dehydrohalogenation of 2-bromopentane gives 2-pentene as the major product.

(l) Optically active 2-bromobutane on treatment with aq KOH results in formation of optically inactive product.

(m) Aryl halides are extremely less reactive towards nucleophilic substitution reactions.

(n) Rate of electrophilic substitution reactions in haloarenes is less than that of benzene.

(o) Choloform is stored in dark bottles.

(p) P-Nitrochlorobenzene can be more easily hydrolysed with aq. NaOH thatn chlorobenzene.

(q) Reaction of (-) 2-Bromo octane with aq NaOH results in formation of (+)octane-2-ol.

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