NCERT Exemplar (Part - 3) - Haloalkanes and Haloarenes Class 12 Notes | EduRev

Chemistry Class 12

Created by: Mohit Rajpoot

Class 12 : NCERT Exemplar (Part - 3) - Haloalkanes and Haloarenes Class 12 Notes | EduRev

The document NCERT Exemplar (Part - 3) - Haloalkanes and Haloarenes Class 12 Notes | EduRev is a part of the Class 12 Course Chemistry Class 12.
All you need of Class 12 at this link: Class 12

Q.71. Elimination reactions (especially β-elimination) are as common as the nucleophilic substitution reaction in case of alkyl halides. Specify the reagents used in both cases.
Ans. Elimination reactions are as common as the nucleophilic substitution reaction in case of alkyl halides as two reactions occur simultaneously. Generally, at lower temperature and by using weaker base, nucleophilic substitution reaction occurs while at higher temperature and by using a stronger base, elimination reactions (especially  b-elimination) take place. e.g., if ethyl bromide is treated with aq. KOH, at low temperature it gives ethanol while if it is treated with alc. KOH at high temperature then it gives ethene.
NCERT Exemplar (Part - 3) - Haloalkanes and Haloarenes Class 12 Notes | EduRev(Nucleophilic substitution reaction)
NCERT Exemplar (Part - 3) - Haloalkanes and Haloarenes Class 12 Notes | EduRev(Elimination reaction)

Q.72. How will you obtain monobromobenzene from aniline?
Ans. When aniline, dissolved or suspended in cold aqueous mineral acid, is treated with sodium nitrite, a diazonium salt is formed. This diazo salt on treatment with cuprous bromide gives monobromobenzene.
NCERT Exemplar (Part - 3) - Haloalkanes and Haloarenes Class 12 Notes | EduRevThis reaction is named as Sandmeyer's reaction. If benzene diazonium chloride is treated with copper in HBr then the product formed is bromobenzene. This reaction is known as Gattermann reaction.

Q.73. Aryl halides are extremely less reactive towards nucleophilic substitution. Predict and explain the order of reactivity of the following compounds towards nucleophilic substitution:
(I) NCERT Exemplar (Part - 3) - Haloalkanes and Haloarenes Class 12 Notes | EduRev
(II)NCERT Exemplar (Part - 3) - Haloalkanes and Haloarenes Class 12 Notes | EduRev
(III)NCERT Exemplar (Part - 3) - Haloalkanes and Haloarenes Class 12 Notes | EduRev
Ans. Aryl halides are less reactive towards nucleophilic substitution reaction. Presence of electron withdrawing group at ortho and para position increases the stability of intermediates and hence increases the reactivity of aryl halides towards nucleophilic substitution reaction.
Now, more the number of electron withdrawing groups (EWG) at ortho and para position, higher will be the reactivity of aryl halide. Compound (III) has three EWG so, it is most reactive and compound (I) has only one EWG so, it is least reactive, So, the order of reactivity is (I) < (II) < (III)

Q.74. tert-Butylbromide reacts with aq. NaOH by SN1 mechanism while n-butylbromide reacts by SN2 mechanism. Why?
Ans. Tert. butyl bromide reacts with aq. NaOH as follows
NCERT Exemplar (Part - 3) - Haloalkanes and Haloarenes Class 12 Notes | EduRev
tert. butyl bromide when treated with aq. NaOH, it forms tert. corbocation which is more stable intermediate. This intermediate is further attacked by OH ion. As tert. carbocation is highly stable so tert butylbromide follow SN1 mechanism.
In case of n-butylbromide, primary carbocation is formed which is least stable so, it does not follow SN1 mechanism. Here, stearic hindrance is very less so, it follows SN2 mechanism. In SN2 mechanism, −OH will attack from backside and a transition state is formed. The leaving group is then pushed off the opposite side and the product is formed.
NCERT Exemplar (Part - 3) - Haloalkanes and Haloarenes Class 12 Notes | EduRev

Q.75. Predict the major product formed when HCl is added to isobutylene. Explain the mechanism involved.
Ans. Reaction between the isobutylene added to HCl
NCERT Exemplar (Part - 3) - Haloalkanes and Haloarenes Class 12 Notes | EduRev
Electrophilic addition reaction takes place in accordance with Markownikoff s rule,
NCERT Exemplar (Part - 3) - Haloalkanes and Haloarenes Class 12 Notes | EduRev
We know that 3° carbocation is more stable than 1° carbocation, so in further step 3° carbocation is further attacked by Cl− ion
NCERT Exemplar (Part - 3) - Haloalkanes and Haloarenes Class 12 Notes | EduRev

Q.76. Discuss the nature of C–X bond in the haloarenes.
Ans. In haloarenes, carbon of benzene is bonded to halogen. Electronegativity of halogen is more than that of sp2 hybridised carbon of benzene ring. So, C–X bond is a polar bond. Apart from this, lone pair of electrons of halogen atom is involved in resonance with benzene ring. So, this C–X bond has acquired partial bond character.
This C–X bond of haloarenes is less polar than C–X bond of haloalkanes. This is supported by the fact that dipole moment of chlorobenzene (μ = 1.69D) is little lower than that of CH3Cl (μ = 1.83D).


Q.77. How can you obtain iodoethane from ethanol when no other iodine containing reagent except NaI is available in the laboratory?
Ans. Ethanol is treated with red phosphorous and bromine mixture and the product formed will be bromoethane. Bromoethane so formed is then treated with NaI to give lodoethane.
NCERT Exemplar (Part - 3) - Haloalkanes and Haloarenes Class 12 Notes | EduRev
NCERT Exemplar (Part - 3) - Haloalkanes and Haloarenes Class 12 Notes | EduRev
Thus reaction is known as Finkelstein reaction

Q.78. Cyanide ion acts as an ambident nucleophile. From which end it acts as a stronger nucleophile in aqueous medium? Give reason for your answer.
Ans. Cyanide ion (NCERT Exemplar (Part - 3) - Haloalkanes and Haloarenes Class 12 Notes | EduRev C ≡ N) is an ambident nucleophile because it can react either through carbon or through nitrogen. Since, C — C bond is stronger than C — N bond so, cyanide ion will mainly attack through carbon to form alkyl cyanide.
NCERT Exemplar (Part - 3) - Haloalkanes and Haloarenes Class 12 Notes | EduRev


MATCHING TYPE

Note : Match the items given in Column I and Column II in the following questions.
Q.79. Match the the compounds given in Column I with the effects given in Column II.

 Column I Column II
 (i) Chloramphenicol (a) Malaria
 (ii) Thyroxine (b) Anaesthetic
 (iii) Chloroquine (c) Typhoid fever
 (iv) Chloroform (d) Goiter

 (e) Blood substituent

Ans. i → (c), ii → (d), iii → (a), iv → (b)
Solution.
(i) Chloramphenicol is a broadspectrum antibiotic. It is used in the treatment of typhoid fever.
(ii) Thyroxine is a hormone secreated by thyroid gland. Execessive secretion of thyroxine in the body is known as hyperthyroidism. Most patient with hyper thyroidism have an enlarged thyroid gland i.e., goitre.
(iii) Chloroquine prevents the development of malaria parasite Plasmodium vivax in the blood.
(iv) IUPAC name of chloroform is trichloromethane with formula CHCl3. It is a colourless, volatile, sweet-smelling liquid. Its vapours depresses the central nervous system and used as an anaesthetic.

Q.80. Match the items of Column I and Column II.

 Column I Column II
 (i) SN1 reaction (a) vic-dibromides
 (ii) Chemicals in fire extinguisher (b) gem-dihalides
 (iii) Bromination of alkenes (c) Racemisation
 (iv) Alkylidene halides (d) Saytzeff rule
 (v) Elimination of HX from alkylhalide (e) Chlorobromocarbons

Ans. i → (c), ii → (e), iii → (a), iv → (b), v → (d)
Solution.
(i) A mixture containing two enantiomers in equal proportions will have zero optical rotation, such a mixture is known as racemic mixture. The process of conversion of enantiomer into a racemic mixture is known as racemisation. If an alkyl halide follows
SN1 mechanism then racemisation takes place while if it follows SN2 mechanism than inversion takes places.
(ii) Chlorobromocarbons are used in fire extinguishers,
(iii). In vicinal dihalides, halogen atoms are present on the adjacent carbon atom. Bromination of alkenes will give vicinal dihalides.
(iv) Alkylidene haiides are named as gem-dihalides. In gem-dihalides halogen atoms are present on same carbon atom.
(v) Elimination of HX from alkylhalide follows Saytzeff rule. This rule states that "in dehydrohalogenation reactions, the preferred product is that alkene which has the greater number of alkyl groups attached to the doubly bonded carbon atoms".

Q.81. Match the structures of compounds given in Column I with the classes of compounds given in Column II.

  Column I   Column II
 (i) NCERT Exemplar (Part - 3) - Haloalkanes and Haloarenes Class 12 Notes | EduRev (a) Aryl halide
 (ii) CH= CH—CH2—X (b) Alkyl halide
 (iii) NCERT Exemplar (Part - 3) - Haloalkanes and Haloarenes Class 12 Notes | EduRev (c) Vinyl halide
 (iv) CH= CH—X (d) Allyl halide

Ans. i → (b), ii → (d), iii → (a), iv → (c)
Solution.
(i) In alkyl halide, halogen atom is bonded to sp3 hybridised carbon atom, which may be further bonded to one, two or three alkyl groups, i.e., CH3−CH(X) −CH3
(ii) Allyl halides are the compounds in which the halogen atom is bonded to sp3 hybridised carbon atom next to carbon-carbon double bond, i.e., CH2− =CH −CH2−X
(iii) Aryl halides are the compounds in which the halogen atom is bonded to sp2 hybridised carbon atom of an aromatic ring, i.e., C6H5X
(iv) Vinyl halides are the compounds in which the halogen atom is bonded to an sp2 hybridised carbon atom of a carbon-carbon double bond, i.e., CH=CH−X

Q.82. Match the reactions given in Column I with the types of reactions given in Column II.

 Column I Column II
 (i) NCERT Exemplar (Part - 3) - Haloalkanes and Haloarenes Class 12 Notes | EduRev (a) Nucleophilic aromatic substitution
 (ii) NCERT Exemplar (Part - 3) - Haloalkanes and Haloarenes Class 12 Notes | EduRev (b) Electrophilic aromatic substitution
 (iii) NCERT Exemplar (Part - 3) - Haloalkanes and Haloarenes Class 12 Notes | EduRev (c) Saytzeff elimination
 (iv) NCERT Exemplar (Part - 3) - Haloalkanes and Haloarenes Class 12 Notes | EduRev (d) Electrophilic addition
 (v) NCERT Exemplar (Part - 3) - Haloalkanes and Haloarenes Class 12 Notes | EduRev (e) Nucleophilic substitution (SN1) 

Ans. i → (b), ii → (d), iii → (e), iv → (a), v → (c)
Solution.
(i) In this reaction, an electrophile attacks on the benzene ring and substitution takes place.
(ii) In this reaction, addition of HBr takes place on to the doubly bonded carbons of propene in accordance with Markownikoff's rule and electrophilic addition takes place.
(iii) In this reaction, the reactant is secondary halide. Here, halogen atom is substituted by hydroxy ion. As it is secondary halide so it follows SN1 mechanism.
(iv) In this reaction, halogen atom is directly bonded to aromatic ring. So, It is nucieophilic aromatic substitution as –OH group has substituted halogen of given compound.
(v) It is an elimination reaction. It follows Saytzeff elimination rule.

Q.83. Match the structures given in Column I with the names in Column II.

  Column I Column II
 (i)  NCERT Exemplar (Part - 3) - Haloalkanes and Haloarenes Class 12 Notes | EduRev (a) 4-Bromopent-2-ene
 (ii) NCERT Exemplar (Part - 3) - Haloalkanes and Haloarenes Class 12 Notes | EduRev (b) 4-Bromo-3-methylpent-2-ene
 (iii) NCERT Exemplar (Part - 3) - Haloalkanes and Haloarenes Class 12 Notes | EduRev (c) 1-Bromo-2-methylbut-2-ene
 (iv) NCERT Exemplar (Part - 3) - Haloalkanes and Haloarenes Class 12 Notes | EduRev (d) 1-Bromo-2-methylpent-2-ene

Ans. i → (a), ii → (b), iii → (c), iv → (d)
Solution.
(i) The IUPAC name of compound (A) is 4-bromopent-2-ene.
(ii) The IUPAC name of compound (B) is 4-bromo-3-methyl pent-2-ene.
(iii) The IUPAC name of compound (C) is 1-bromo-2- methyl but-2-ene.
(iv) The IUPAC name of compound (D) is 1-bromo-2-methylpent-2-ene.

Q.84. Match the reactions given in Column I with the names given in Column II.

 Column I Column II
 (i) NCERT Exemplar (Part - 3) - Haloalkanes and Haloarenes Class 12 Notes | EduRev (a) Fittig reaction
 (ii) NCERT Exemplar (Part - 3) - Haloalkanes and Haloarenes Class 12 Notes | EduRev (b) Wurtz Fittig reaction
 (iii)NCERT Exemplar (Part - 3) - Haloalkanes and Haloarenes Class 12 Notes | EduRev (c) Finkelstein reaction
 (iv) NCERT Exemplar (Part - 3) - Haloalkanes and Haloarenes Class 12 Notes | EduRev (d) Sandmeyer reaction

Ans. i → (b), ii → (a), iii → (d), iv → (c)
Solution.
(i) A mixture of an alkyl hailde and aryl halide gives an alkylarene when treated with sodium in dry ether and this is called Wurtz-Fittig reaction
(ii) Aryl halides give analogous compounds when treated with sodium in dry ether, in which two aryl groups are joined together. It is called Fittig reaction.
(iii) Diazonium salt when treated with cuprous chloride or cuprous bromide gives chlorobenzene or bromobenzene.
The reaction is known as Sandmeyer's reaction
(iv) Alkyl iodides are prepared by the reaction of alkyl chlorides with sodium iodide in dry acetone. The reaction is known as Finkelstein reaction.


ASSERTION AND REASON TYPE

Note : In the following questions a statement of assertion followed by a statement of reason is given. Choose the correct answer out of the following choices.
(i) Assertion and reason both are correct and reason is correct explanation of assertion.
(ii) Assertion and reason both are wrong statements.
(iii) Assertion is correct but reason is wrong statement.
(iv) Assertion is wrong but reason is correct statement.
(v) Assertion and reason both are correct statements but reason is not correct explanation of assertion.

Q.85. 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.
(i) Assertion and reason both are correct and reason is correct explanation of assertion.
(ii) Assertion and reason both are wrong statements.
(iii) Assertion is correct but reason is wrong statement.
(iv) Assertion is wrong but reason is correct statement.
(v) Assertion and reason both are correct statements but reason is not correct explanation of assertion.

Ans. (ii)
Solution.
Correct Assertion Thionyl chloride is preferred over PCl3 and PCl5 for the preparation of alkyl chlorides from alcohols.
Correct Reason Thionyl chloride gives pure alkyl halide as other two products (SO2 + HCl) are escapable gases.

Q.86. Assertion: The boiling points of alkyl halides decrease in the order : RI > RBr > RCl > RF
Reason: The boiling points of alkyl chlorides, bromides and iodides are considerably higher than that of the hydrocarbon of comparable molecular mass.
(i) Assertion and reason both are correct and reason is correct explanation of assertion.
(ii) Assertion and reason both are wrong statements.
(iii) Assertion is correct but reason is wrong statement.
(iv) Assertion is wrong but reason is correct statement.
(v) Assertion and reason both are correct statements but reason is not correct explanation of assertion.

Ans. (v)
Solution.

For the same hydrocarbon part boiling point depends upon the atomic mass of halogen atom. Higher the mass of the halogen atom, higher will be the boiling point.
So, we can say that boiling point decreases with decrease in atomic mass of halogen atom.

Q.87. Assertion: KCN reacts with methyl chloride to give methyl isocyanide
Reason: CN – is an ambident nucleophile.
(i) Assertion and reason both are correct and reason is correct explanation of assertion.
(ii) Assertion and reason both are wrong statements.
(iii) Assertion is correct but reason is wrong statement.
(iv) Assertion is wrong but reason is correct statement.
(v) Assertion and reason both are correct statements but reason is not correct explanation of assertion.

Ans. (iv)
Solution.

Correct Assertion KCN reacts with methyl chloride to give the mixture of methyl cyanide and methyl isocyanide in which methyl cyanide predominates because of stable C—C bond in methyl cyanide.

Q.88. Assertion: tert-Butyl bromide undergoes Wurtz r eaction to give 2, 2, 3, 3-tetramethylbutane.
Reason: 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.
(i) Assertion and reason both are correct and reason is correct explanation of assertion.
(ii) Assertion and reason both are wrong statements.
(iii) Assertion is correct but reason is wrong statement.
(iv) Assertion is wrong but reason is correct statement.
(v) Assertion and reason both are correct statements but reason is not correct explanation of assertion.

Ans. (iv) 
Solution.

Correct Assertion- sec-butyl bromide (2° alkyl halide) undergoes Wurtz reaction to give 2, 5-dimethylhexane.

Q.89. Assertion: Presence of a nitro group at ortho or para position increases the reactivity of haloarenes towards nucleophilic substitution.
Reason: Nitro group, being an electron withdrawing group decreases the electron density over the benzene ring.
(i) Assertion and reason both are correct and reason is correct explanation of assertion.
(ii) Assertion and reason both are wrong statements.
(iii) Assertion is correct but reason is wrong statement.
(iv) Assertion is wrong but reason is correct statement.
(v) Assertion and reason both are correct statements but reason is not correct explanation of assertion.

Ans. (iv)
Solution.

Presence of nitro group at ortho or para position increases the reactivity of haloarenes towards nucleophilic substitution because —NO2 group, being an electron withdrawing group decreases the electron density over the benzene ring.

Q.90. Assertion: In monohaloarenes, further electrophilic substitution occurs at ortho and para positions.
Reason : Halogen atom is a ring deactivator.
(i) Assertion and reason both are correct and reason is correct explanation of assertion.
(ii) Assertion and reason both are wrong statements.
(iii) Assertion is correct but reason is wrong statement.
(iv) Assertion is wrong but reason is correct statement.
(v) Assertion and reason both are correct statements but reason is not correct explanation of assertion.

Ans. (v)
Solution.

Correct explanation in monohaloarenes, halogen atom increases the electron density at ortho and para position. So, further electrophilic substitution occurs at ortho and para positions.

Q.91. Assertion: Aryl iodides can be prepared by reaction of arenes with iodine in the presence of an oxidising agent.
Reason: Oxidising agent oxidises I2 into HI.
(i) Assertion and reason both are correct and reason is correct explanation of assertion.
(ii) Assertion and reason both are wrong statements.
(iii) Assertion is correct but reason is wrong statement.
(iv) Assertion is wrong but reason is correct statement.
(v) Assertion and reason both are correct statements but reason is not correct explanation of assertion.

Ans. (iii)
Solution.

Correct Reason Oxidising agent oxidises HI into l2 to prevent the possibility of backward reaction.

Q.92. Assertion: It is difficult to replace chlorine by –OH in chlorobenzene in comparison to that in chloroethane.
Reason: Chlorine-carbon (C—Cl) bond in chlorobenzene has a partial double bond character due to resonance.
(i) Assertion and reason both are correct and reason is correct explanation of assertion.
(ii) Assertion and reason both are wrong statements.
(iii) Assertion is correct but reason is wrong statement.
(iv) Assertion is wrong but reason is correct statement.
(v) Assertion and reason both are correct statements but reason is not correct explanation of assertion.

Ans. (i)
Solution.

It is difficult to replace chlorine by —OH in chlorobenzene in comparison to that in chloromethane because C—Cl bond in chlorobenzene has a partial double bond character due to resonance.

Q.93. Assertion: Hydrolysis of (–)-2-bromooctane proceeds with inversion of configuration.
Reason: This reaction proceeds through the formation of a carbocation.
(i) Assertion and reason both are correct and reason is correct explanation of assertion.
(ii) Assertion and reason both are wrong statements.
(iii) Assertion is correct but reason is wrong statement.
(iv) Assertion is wrong but reason is correct statement.
(v) Assertion and reason both are correct statements but reason is not correct explanation of assertion.

Ans. (iii)
Solution.
Correct Reason This reaction proceeds through SN2 mechanism, in which OH ion attacks at 180° to the halogen atom of 2-bromooctane which leads to the inversion of configuration.

Q.94. Assertion: Nitration of chlorobenzene leads to the formation of m-nitrochlorobenzene
Reason: —NO2 group is a m -directing group.
(i) Assertion and reason both are correct and reason is correct explanation of assertion.
(ii) Assertion and reason both are wrong statements.
(iii) Assertion is correct but reason is wrong statement.
(iv) Assertion is wrong but reason is correct statement.
(v) Assertion and reason both are correct statements but reason is not correct explanation of assertion.

Ans. (iv)
Solution.
Correct Assertion Chlorination of nitrobenzene leads to the formation of mnitrochlorobenzene because —NO2 group deactivates the ring as it is meta directing.


LONG ANSWER TYPE QUESTIONS

Q.95. Some alkylhalides undergo substitution whereas some undergo elimination reaction on treatment with bases. Discuss the structural features of alkyl halides with the help of examples which are responsible for this difference.
Ans. Primary alkyl halides follow SN2 mechanism in which a nucleophile attacks at 180° to the halogen atom. A transition state is formed in which carbon is bonded to two nucleophiles and finally halogen atom is pushed out. In SN2 mechanism, substitution of nucleophile takes place as follows
NCERT Exemplar (Part - 3) - Haloalkanes and Haloarenes Class 12 Notes | EduRevThus, in SN2 mechanism, substitution takes place. Tertiary alkyl halides follow SN1 mechanism. In this case, tert alkyl halides form 3° carbocations. Now, if the reagent used is a weak base then substitution occurs while if it is a strong base than instead of substitution elimination occurs.
NCERT Exemplar (Part - 3) - Haloalkanes and Haloarenes Class 12 Notes | EduRevHere, the reagent used is aq. KOH. It is a weak base so, substitution takes place.
NCERT Exemplar (Part - 3) - Haloalkanes and Haloarenes Class 12 Notes | EduRevAs alc. KOH is a strong base, so elimination competes over substitution and alkene is formed.

Q.96. Some halogen containing compounds are useful in daily life. Some compounds of this class are responsible for exposure of flora and fauna to more and more of UV light which causes destruction to a great extent. Name the class of these halocompounds. In your opinion, what should be done to minimise harmful effects of these compounds.
Ans. Some halogen containing compounds are useful in daily life are as follows
Dichloromethane: It is used as a solvent, as a paint remover, as a propellant in aerosols, and as a process solvent in the manufacture of drugs. It is also used as a metal cleaning and finishing solvent.
Trichloromethane: It is employed as a solvent for fats, alkaloids, iodine and other substances.
Triodomethane: It is used as an antiseptic. Now, it has been replaced by some other compounds because of its objectionable smell.
But some compounds of this class are responsible for exposure of flora and fauna to more and more of UV light which causes destruction to great extent.
These are as follows.
(i) Tetrachloromethane: When carbon tetrachloride is released into the air, it rises to the atmosphere and depletes the ozone layer. Depletion of the ozone layer is believed to increase human exposure to UV rays leading to increased skin cancer, eye diseases and disorders, and possible disruption of the immune system. These UV rays cause damage to plants, and reduction of plankton populations in the ocean's photic zone.
(ii) Freons: Freon-113 is likely to remain in the air long enough to reach the upper atomsphere. Here, it provides chlorine atoms which damage the ozone layer. Because of this depletion UV rays enters in our atmosphere and become responsible for damage to great extent.
(iii) p-p'-Dichlorodiphenyl trichloroethane (DDT)
DDT is not completely biodegradable. Instead, it gets deposited in fatty tissues. If ingestion continues for a long time, DDT builds up within the animal and effect the reproductive system.
To minimise the harmful impacts of these compounds i.e., freons, hydrofluorocarbons, fluorocarbons and hydrocarbons can be straight used to make refrigerants and air-conditioning equipments. They are stable in the stratosphere and secure for flora and fauna.

Q.97. Why are aryl halides less reactive towards nucleophilic substitution reactions than alkyl halides? How can we enhance the reactivity of aryl halides?
Ans. Aryl halides are less reactive towards nucleophilic substitution reaction due to the following reasons (i) In haloarenes, the lone pair of electron on halogen are in resonance with benzene ring So, C—Cl bond acquires partial double bond character which strengthens C— Cl bond.
Therefore, they are less reactive towords nucleophilic substitution reaction.
NCERT Exemplar (Part - 3) - Haloalkanes and Haloarenes Class 12 Notes | EduRev
(ii) In haloarenes, the carbon atom attached to halogen is sp2 hybridised. The sphybridised carbon is more electronegative than sp3 hybridised carbon. This sp2 hybridised carbon in haloarenes can hold the electron pair of C—X bond more tightly and make this C — Cl bond shorter than C — Cl bond of haloalkanes.
Since, it is difficult to break a shorter bond than a longer bond, therefore, haloarenes are less reactive than haloarenes.
In haloarenes, the phenyl cation will not be stabilised by resonance therefore SN1 mechanism ruled out.
(iv) Because of the repulsion between the nucleophile and electron rich arenes, aryl halides are less reactive than alkyl halides.
The reactivity of aryl halides can be increased by the presence of an electron withdrawing group (-(NO2) at ortho and para positions. However, no effect on reactivity of haloarenes is observed by the presence of electron withdrawing group at meta-position. Mechanism of the reaction is as depicted with OH ion.
NCERT Exemplar (Part - 3) - Haloalkanes and Haloarenes Class 12 Notes | EduRevNCERT Exemplar (Part - 3) - Haloalkanes and Haloarenes Class 12 Notes | EduRevNCERT Exemplar (Part - 3) - Haloalkanes and Haloarenes Class 12 Notes | EduRevNCERT Exemplar (Part - 3) - Haloalkanes and Haloarenes Class 12 Notes | EduRevNCERT Exemplar (Part - 3) - Haloalkanes and Haloarenes Class 12 Notes | EduRevFrom the above resonance, it is very clear that electron density is rich at ortho and para positions. So, presence of EWG will facilitate nucleophilic at ortho and para postitions not on meta position.

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