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CHAPTER, , HALOALKANES, AND HALOARENES, , , , mechanism of substitution reactions., , , , Haloalkanes: Nomenclature, nature of C-X bond, physical and chemical properties, optical rotation, , > Haloarenes: Nature of C-X bond, substitution reactions (Directive influence of halogenin monosubstituted, , compounds only), , STAND ALONE MCQs, , , , Q.1. The order of reactivity of following alcohols with, halogen acids is:, (i) CH\CH,-CH,-OH, , (ii) CH,CH,—CH—OH,, bu,, e, , (iii) CH,CH, —C—OH, by,, , (A) @) > Gi) > Gi), , (C) Gi) > @ > Gi), Ans. Option (B) is correct., , (B) Git) > Gi) > @, (D) (i) > (ii) > @), , Explanation: The reactivity order of alcohols, towards halogen acids is 3°>2°>1” as the stability, of carbocations is of the order 3°>2°>1°., , Q.2. Which of the following alcohols will yield the, corresponding alkyl chloride on reaction with, concentrated HCl at room temperature?, , (A) CH,CH) - CH)- OH, (B) CH,CH,—CH OH, , CH,, (©) Ch Llp ere 1,0H, , CH,, , PO enetoil), , , , fm, (D) CH,CH, —C—OH, H,, Ans. Option (D) is correct., , Explanation: As tertiary carbocation is more, stable, so tertiary alcohols will yield the, corresponding alkyl chloride on reaction with, concentrated HCI at room temperature. While, primary and secondary alcohols require the, presence of a catalyst ZnCl., , Q.3. Arrange the following compounds in increasing, ardevor hae builingpeintt, CH,, i CII—CI1,Br, w oe? a), (ii) CH,CH,CH,CH,Br, CH;, , |, (ii) Cy —C— CH, , Br, , (A) Gi) <@ < Gi), (©) Gii)<@ <i), Ans. Option (C) is correct., , (B) @) < (i) < (iii), (D) (ii) < di) < @ Ul

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Explanation: Boiling points of isomeric haloalkanes, decrease with increase in branching as with, increase in branching surface area decreases which, leads to decrease in intermolecular forces., , Q. 4, The conversion of an alkyl halide into an alcohol by, , aqueous NaOH is classified as, , (A) a dehydrohalogenation reaction, , (B) asubstitution reaction, , (C) an addition reaction, , (D) a dehydration reaction, , [R| [CBSE Delhi Set-I 2020], , Ans. Option (B) is correct., , | Explanation: R —X+NaOH+R-OH + nax|, (A) Q. 5. Which of the following alkyl halides will undergo, , Sq! reaction most readily?, (A) (CH3)3C—F (B) (CHa);C—Cl, (C) (CH3);C—Br (D) (CH3)3C—1, Ans. Option (D) is correct., , Explanation: (CH3)sC—I will undergo Sy!, reaction most readily as C—I bond is weakest,, due to the large difference in the size of carbon, and iodine., , (Al) Q. 6. Which reagent will you use for the following, reaction?, CH3CH,CH,CH;—> CH3CH,CH,CH,CI, , + CH;CH,CHCICH;, (A) CL/UV light, (B) NaCl + H,S0,, (C) Cl, gas in dark, (D) Cl, gas in the presence of iron in dark, Ans. Option (A) is correct., , Explanation: The given reaction is a free radical, substitution reaction. It occurs in presence, of ultraviolet ight or at high temperature or, peroxides which are free radical generators. Free, radical substitution cannot take place in dark., , Q.7. In which of the following molecules carbon atom, , marked with asterisk (“) is asymmetric?, (i) (ii), , a, ra Sa j, Br Br, , (iii) H (iv) H, , | |, E, on” Nan Wen, CiHs, (A) ©, (ii), (ii), Gv), , (C) Gi), Git), (iv), Ans. Option (B) is correct., , A;, (B) @, Gi), ©, D) (0), (iit), Gv), , Explanation: Asymmetric/Chiral carbon atom is, that in which all of its four valencies lie with four, different groups or atoms., , In molecules (i), (ii) and (iii), all have asymmetric, carbon as each carbon has satisfied all four, valencies with four different groups or atoms., In molecule (iv) carbon satisfies two of its, valencies with two hydrogen aloms ie. similar, atom. So, it is not an asymmetric carbon atom., , Q. 8. Which of the following structures is enantiomeric, with the molecule (a) given below?, , | ve, a, No:, , (a), (A) H B), , | Gls, A /, HC \ Br Br, a) Lad @), , lv”, , H,C CH;, Ans. Option (A) is correct., , , , , , , , iv, ow, , , , Br, H, lv, a Neu,, , , , Explanation: Compound (a) is enantiomer of, compound (A) because the configuration of two, groups, that is, CH, and C,Hs in themis reversed, at chiral carbon., , Q.9. Identify the compound Y in the following reaction., , NH, na, , or NaNO, +HCL CuCl,, , 273-278 K Y +N,, qk, (A) Cy ) CO, cl, , Cl, ©) O () C, ad cL, , Ans. Option (A) is correct., , Al, , Explanation: When a primary aromatic amine, is dissolved or suspended ‘in cold aqueous, mineral acid and treated with sodium nitrite,, a diazonium salt is formed. When this freshly, prepared diazonium salt is mixed with cuprous, chloride, diazonium group is replaced by CL. Then,, chlorobenzene is formed which is Y in this reaction., , AE a, 1 Cock, Co OO, *, , ™ reattivn), , “Nanogen,, TK

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AI] Q. 10. What is ‘A in the following reaction?, (Al) 8, , CH,-CH=CH,, + HClL—+A, ‘H,-CH=CH, CH3-CH)=CHy-Cl, Cl., SS, (A) |, LE, a, : |, CH, oa =CHy CH-CH,-CH,, a., © ), , a, Ans, Option (C) is correct., Explanation: In this reaction, addition of HCl takes, place on doubly bonded carbons in accordance, with Markovnikov’s rule, that is, addition of, negative addendum will take place on that carbon, , which has lesser number of hydrogen., H, - CH =CH, CH, - CH- CH,, , cl, + HCl ——»>, , Q. 11. The IUPAC name of the compound shown below is:, cl, , A, , SS, , Br, , (A) 2-bromo-6-chlorocyclohex-l-ene, , (B) 6-bromo-2-chlorocyclohexene, , (©) 3-bromo-1-chloracyclohexene, , (D) 1-bromo-3-chlorocyclohexene [CBSE SQP 2020], Ans. Option (C) is correct., , Explanation:, , x Br, TUPAC name: 3-bromo-1-chlorocyclohexene, , Q.12. Toluene reacts with a halogen in the presence, of iron (Ill) chloride giving ortho and para halo, compounds. The reaction is., , (A) Electrophilic elimination reaction, (B) Electrophilic substitution reaction, (C) Free radical addition reaction, (D) Nucleophilic substitution reaction, Ans. Option (B) is correct., Explanation:, CH;, , CH, CHs, x, FeCl,, + X,—— +, x, , Q.13. Which of the following is halogen exchange, , reaction?, (A) RX + Nal > RI + Nax, @B) \c—C%+ HX —> Sc=c%, Es vy |S, zc, H X, , (C) R-OH + HX ————_® R-X+H,0, , Hy H; CH,, Fe, (D) + ya CJ, xX xX, , Ans. Option (A) is correct., , Explanation: Halogen exchange reactions are, those in which one halide replaces another. This, reaction is known as Finkelstein reaction., , (AD Q. 14. Chlorobenzene is formed by reaction of, chlorine with benzene in the presence of AICI., Which of the following species attacks the, benzene ring in this reaction?, , (A) Cl (B) cit, (©) AICI (D) [AICL]—, Ans. Option (B) is correct., Explanation: Cis an electrophile formed by the, following reaction:, AICL, +Cl, > [AICL, | +l", , Cl’ attacks the, , chlorobenzene., , Cl, ., Ov-O, Cc, , hlorobenzene, , benzene ring to give, , Q.15. Reaction of CyH5CH,Br with aqueous sodium, hydroxide follows:, (A) Sx! mechanism, (B) Sx? mechanism, (©) Any of the above two depending upon the, , temperature of reaction, (D) Saytzeff rule, Ans. Option (A) is correct., , Explanation: CsHsCH,Br will follow Sy!, mechanism on reaction with aqueous sodium, hydroxide since the carbocation formed, CgH5CH, is a resonance stabilized cation.

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Benzylic halides show high reactivity towards, the Sy/' reaction., , The carbocation thus formed gets stabilized, through resonance as shown in the structure., , ce dedguor, , Q. 16. Arrange the following compounds in the, increasing order of their densities., , a, " CO ™ CQ, , , , Directions: In the following questions, A statement, of Assertion (A) is followed by a statement of, Reason (R). Mark the correct choice as., , (A) Both Aand Rare true and Ris the correct explanation, ofA, , (B) Both A and R are true but R is NOT the correct, explanation of A, , (C) Ais true but R is false, , (D) Ais false and Ris True, , Q.1. Assertion (A): Aryl halides undergo nucleophilic, substitution reactions with ease,, , Reason(R): The carbon halogen bond in aryl halides, has partial double bond character., [RJ [CBSE SQP 2020], , Ans. Option (D) is correct., , Explanation: Aryl halides are less reactive, towards nucleophilic substitution reactions, because of the carbon halogen bond in aryl, halides has partial double bond character., , Q. 2. Assertion (A): Hydrolysis of (-)-2-bromooctane, proceeds with inversion of configuration., Reason (R): This reaction proceeds through the, formation of a carbocation., Ans. Option (D) is correct., , Explanation: Hydrolysis of (-)-2-bromooctane, proceeds through the formation of a carbocation, following Sy1 reaction., , Q.3. Assertion (A); tert-Butyl bromide undergoes Wurtz, reaction to give 2, 2, 3, 3-tetramethylbutane., Reason (R): In Wurtz reaction, alkyl halides react, with sodium in dry ether to give hydrocarbon, containing double the number of carbon atoms, present in the halide. u, , Ans. Option (A) is correct., , Explanation: In Wurtz reaction, alkyl halides, react with sodium in dry ether to give, hydrocarbon containing double the number of, carbon atoms present in the halide so tert-Butyl, bromide undergoes Wurtz reaction lo give 2, 2, 3,, 3-tetramethylbutane., , ASSERTION AND REASON BASED MCQs, , a Br, , (iii) (iv), cl ‘el, (A) @® <i) < (iii) < Gv), (B) (i) < (iii) < (iv) < (ii), (©) (iv) < Gil) <i) <@, (D) (ii) < (iv) < (itt) < @), Ans. Option (A) is correct., Lxplanation: Density increases with increase in, molecular mass., , POET a ral, , , , Q. 4. Assertion: Phosphorus chlorides (tri and penta) are, preferred over thionyl chloride for the preparation, of alkyl chlorides from alcohols., , Reason: Phosphorus chlorides give pure alkyl, halides., , Ans. Option (B) is correct., , Explanation: Thionyl chloride is best halogen, carrier to convert alcohol to alkyl halide because, it gives by-products in gaseous state. Thus, we, get pure alkyl halide in this reaction., , Q.5. Assertion (A): Presence of a nitro group at ortho or, para position increases the reactivity of haloarenes, towards nucleophilic substitution., , Reason (R): Nitro group, being an electron, withdrawing group decreases the electron density, over the benzene ring., , Ans. Option (A) is correct., , Explanation: Nitro group being an electron, withdrawing group, decreases the electron, density of benzene ring thus increasing the, reactivity of haloarenes towards nucleophilic, substitution., , Q.6. Assertion (A): In monohaloarenes, further, electrophilic substitution occurs at ortho and para, positions., , Reason (R): Halogen atom is a ring deactivator. [R], , Ans. Option (B) is correct., , Explanation: Halogens are ortho-para directing, due to (+M) or (+R) effect. Moreover, they are, deactivating due to high electronegativity., , (A Q. 7. Assertion (A): It is difficult to replace chlorine, by -OH in chlorobenzene in comparison to that in, chloroethane., , Reason (R): Carbon-chlorine (C—Cl) bond in, , chlorobenzene has a partial double bond character, , due to resonance. ul, Ans. Option (A) is correct.

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Explanation: Chlorobenzene is very less reactive, to nucleophilic substitution réaction by —, OH group as Carbon-chlorine (C—Cl) bond, in chlorobenzene has a partial double bond, character due lo resonance., , Q. 8. Assertion (A): Aryl iodides can be prepared by, reaction of arenes with iodine in the presence of an, oxidising agent., , Reason (R): Oxidising agent oxidises Ip into HI., , CASE-BASED MCQs, , , , I, , . Read the passage given below and answer the, following questions:, , Nucleophilic substitution reaction of haloalkane, can be conducted according to both Sy! and $y”, mechanisms. However, which mechanism it is, based on is related to such factors as the structure, of haloalkane, and properties of leaving group,, nucleophilic reagent and solvent., Influences of halogen: No matter which mechanism, the nucleophilic substitution reaction is based on,, the leaving group always leave the central carbon, atom with electron pair. This is just the opposite of, the situation that nucleophilic reagent aitacks the, central carbon atom with electron pair. Therefore,, the weaker the alkalinity of leaving group is ,, the more stable the anion formed is and it will, be more easier for the leaving group to leave the, central carbon atom; thal is to say, the reactant is, more easier to be substituted. The alkalinity order, of halogen ion is IT < Br~ < Cl” < F and the, order of their leaving tendency should be I” > Br, > Cl > F. Therefore, in four halides with the, same alkyl and different halogens, the order of, substitution reaction rate is RI > RBr > RCI > RE In, addition, if the leaving group is very easy to leave,, many carbocation intermediates are generated, in the reaction and the reaction is based on $y!, mechanism. If the leaving group is not easy to, leave, the reaction is based on Sx’ mechanism., Influences of solvent polarity: In Sy? reaction, the, polarity of the system increases from the reactant, to the transition state, because polar solvent has a, greater stabilizing effect on the transition state than, the reactant, thereby reduce activation energy and, accelerate the reaction. In Sy” reaction, the polarity, of the system generally does not change from the, reactant to the transition state and only charge, dispersion occurs. At this time, polar solvent has, a great stabilizing effect on Nu than the transition, state, thereby increasing, activation energy and, slow down the reaction rate. For example, the, decomposition rate ($y) of tertiary chlorobutane, in 25° water (dielectric constant 79) is 300000 times, faster than in ethanol (dielectric constant 24). The, reaction rate ($y) of 2-bromopropane and NaOH, in ethanol containing 40% water is lwice slower, than in absolute ethanol. In a word, the level of, solvent polarity has influence on both Sx! and, Sy? reactions, but with different results. Generally, , , , , , Ans. Option (C) is correct., , Explanation: Oxidising agent like (HIO3) converts, HI to I, otherwise HI may reduce aryl halide to, arenes., , 5HI+ HIO, -> 3H,0 +31,, , , , speaking, weak polar solvent is favorable for Sx?, reaction, while strong polar solvent is favorable for, Sy! reaction, because only under the action of polar, solvent can halogenated hydrocarbon dissociate, into carbocation and halogen ion and solvents, with a strong polarity is favorable for solvation, of carbocation, increasing, its stability. Generally, speaking, the substitution reaction’ of tertiary, haloalkane is based on Sy! mechanism in solvents, with a strong polarity (for example, ethanol, containing water). [CBSE QB 2021], , The following questions are multiple choice, questions. Choose the most appropriate answer:, , Q. 1. Sy! mechanism is favoured in which of the, following solvents:, (A) benzene, (C) acetic acid, , Ans. Option (C) is correct., , (B) carbon tetrachloride, (D) carbon disulphide, , Q. 2. Nucleophilic substitution will be fastest in case of:, (A) 1-Chloro-2,2-dimethyl propane, (B) 1-Iodo-2,2-dimethyl propane, (C) 1-Bromo-2,2.dimethyl propane, (D) 1-Fluoro-2,2-dimethyl propane, Ans. Option (B) is correct., , Q.3. Sy reaction will be fastest in which of the following, solvents?, (A) Acetone (dielectric constant 21), (B) Ethanol (dielectric constant 24), (D) Methanol (dielectric constant 32), (D) Chloroform (dielectric constant 5), Ans. Option (C) is correct,, , Q.4. Polar solvents make the reaction faster as they:, (A) destabilize transilion slale and decrease the, activation energy, (B) destabilize transition state and increase the, activation energy, (C) stabilize transition state and increase the, activation energy, (D) stabilize transition state and decrease the, activation energy, Ans. Option (C) is correct., , Q. 5. Sy)' reaction will be fastest in case of:, (A) 1-Chloro-2-methyl propane, (B) 1-Iodo-2-methyl propane, (C) 1-Chlorobutane, (D) 1-lodobutane