NCERT Solutions for Class 12 Chemistry Chapter 10 - Haloalkanes and Haloarenes

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Eight isomers are possible.

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(i) 4-Bromo-3-methylpent-2-ene (ii) 3-Bromo-2-methylbut-1-ene (iii) 4-Bromopent-2-ene (iv) 1-Bromo-2-methylbut-2-ene (v) 1-Bromobut-2-ene (vi) 3-Bromo-2-methylpropene

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There are four different types of hydrogen atoms. Replacement of these hydrogen atoms will give the following isomers.

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The cyanide ion is a resonance hybrid of the following structures :

It is clear from the resonating structures that cyanide ion can attack the nucleophilic site through C as well as through N. Such a nucleophile which is capable of attacking through more than one site is called ambident nucleophile. Thus, CN^– behaves as an ambident nucleophile.

KCN is predominantly ionic and therefore, both C and N atoms are free to donate electron pair. Since, C—C bond is relatively more stronger than C—N bond, therefore, the attack occurs mostly through the carbon atom of the cyanide group forming alkyl cyanides as the major product.

On the other hand, AgCN is predominantly covalent. Therefore, in the case only N atom is free to donate electron pair and the attack mostly occurs through the N atom of the cyanide group forming alkyl isocyanides as the major product.

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(a) The four isomeric bromobutanes are :

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1. In structure (i), the central carbon atom is bonded to four different substituents (H, OH, Br and CH₃) and hence (i) is chiral. Structure (ii) has two identical Br atoms attached to central carbon atom and hence it is achiral molecule.
2. (i) is chiral and (ii) is achiral. 
3. (i) is chiral and (ii) is achiral.

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Chlorine is an electron withdrawing group and has –I (inductive) effect. Therefore, it withdraws electrons from the benzene ring and tends to destabilize the intermediate carbocation formed during the electrophilic substitution reaction. The intermediate carbocations for ortho- and para- attacks are shown below :

Since –I effect of Cl is stronger than its +R effect, therefore, causes electron withdrawal and this causes net deactivation. The resonance effect tends to oppose the inductive effect for attack at ortho and para position and hence makes deactivation less for ortho- and para- attack. Thus, we can say that the reactivity is controlled by the stronger inductive effect and orientation is controlled by the resonance effect. Thus, although chlorobenzene is less reactive than benzene but it is ortho, para directing in electrophilic substitution reaction.

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Sulphuric acid is an oxidising agent. It will oxidise HI produced during the reaction to I2 and therefore, will prevent the reaction between an alcohol and HI to form alkyl iodide.

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Four isomeric dihalogen derivatives of propane are formed. For example,

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1. For the same alkyl group, boiling point increases with the increase in size of the halogen atom. Therefore, boiling point of bromomethane is more than that of chloromethane. Further, the boiling point increases with increase in number of halogen atoms. Therefore, boiling point of bromoform is higher than that of dibromomethane, which is higher than that of bromomethane. Thus, the boiling point increases as :
   Chloromethane < Bromomethane < Dibromomethane < Bromoform.
2. For the same halogen, boiling point increases with increase in size of the alkyl group due to increase in van der Waals forces of attraction. Therefore, boiling point of 1-chlorobutane is more than that of 1-chloropropane. Further, the boiling point decreases as branching increases so that the boiling point of 1-chloropropane is higher than that of isopropyl chloride. Thus, the boiling point increases as :
   Isopropyl chloride < 1-chloropropane < 1-chlorobutane

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The reactivity of alkyl halide towards S_N1 reaction depends upon the stability of the carbocation formed as 3$°$ > 2$°$ > 1$°$.

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The compound is saturated C₄H_4×2+1Br. It has following four isomers :

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The nucleophiles which can attack through two different sites are called ambident nucleophiles. For example, cyanide is an ambident nucleophile because it can attack through C or N because of the following resonance structures :
–:C$\equiv$$\equiv$N: $\leftrightarrow$ :C==N:–

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1. In chlorobenzene, the C of C—Cl bond is sp² hybridised while the C of C—Cl bond in cyclohexyl chloride is sp³-hybridised.

   

2. Alkyl halides are polar molecules and therefore, their molecules are held together by dipole-dipole forces. On the other hand, the molecules of H₂O are held together by hydrogen bonds. When alkyl halides are added to water, the new forces of attraction between water and alkyl halide molecules are weaker than the forces of attraction already existing between alkyl halide-alkyl halide molecules and water-water molecules. Hence, alkyl halides are immiscible in water.
3. Grignard reagents are very reactive. They react with the moisture present in the apparatus or the starting materials (RX or Mg).
   R MgX + HOH $\to$ R—H + Mg(OH)X
   Therefore, Grignard reagents must be prepared in anhydrous conditions.

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Uses of Freon-12

1. Freons are used as refrigerants in refrigerators and air conditioners.
2. These have also been used extensively as propellants for aerosols and foams to spray out deodorants, cleansers, shaving creams, hair sprays and insecticides.
   However, the use of freon is restricted or banned in many countries because they are responsible for ozone depletion.

Uses of DDT

1. D.D.T. is almost insoluble in water but it is moderately soluble in polar solvents.
2. D.D.T. is a powerful insecticide. It is widely used as an insecticide for killing mosquitoes and other insects.

Uses of CC_l4

1. It is used as a solvent for oils, fats, waxes, etc.
2. It is used as a fire extinguisher under the name pyrene.
3. It is used in dry cleaning.
4. It is used for the manufacture of freon.

Uses of Iodoform

1. It is used as an antiseptic and this nature is due to iodine that it liberates. However, because of its very unpleasant smell, it has now been replaced by better antiseptics.
2. It is used in the manufacture of pharmaceuticals.

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KCN has nucleophile CN– ion which is ambident nucleophile because of the following two contributing structures:
–:C$\equiv$$\equiv$$\equiv$N: $\leftrightarrow$:C===N:–
Therefore, it can attack the carbon atom of C—Br bond in n-BuBr either through C or N. Since C—C bond is stronger than C—N bond, therefore, the attack occurs through C to form n-butyl cyanide as given below :

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The reactivity in S_N2 reaction depends upon steric hindrance. Lesser the steric hindrance more the reactivity. Therefore, the reactivity of different alkyl halides towards S_N2 reactions is 1$°$ > 2$°$ > 3$°$.

Thus, reactivity decreases with steric hindrance as :
1-Bromobutane > 1-Bromo-3-methylbutane > 1-Bromo-2-methylbutane > 1-Bromo-2, 2-dimethylpropane.

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The melting point of p-isomer of dichlorobenzene is higher than that of o- and m-isomers. This is because, p-isomer has symmetrical structure and therefore, its molecules can easily pack closely in crystal lattice. Hence, it has stronger intermolecular forces of attraction than o- and m-isomer. Therefore, greater energy is required to break the intermolecular forces to melt or dissolve the p-isomer than the corresponding o- and m-isomers. In other words, the melting point of p-isomer is higher than the corresponding o- and m-isomers.

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In aqueous KOH, KOH is almost completely ionized to give OH^– ions. These being strong nucleophiles result into substitution reaction on alkyl chlorides to form alcohols. Moreover, in aqueous solution, the OH^– ions are highly hydrated (solvated). The hydration reduces the basic character of OH– ions which therefore, fails to abstract a hydrogen from the $\mathrm{\beta }$-carbon of the alkyl chloride to form an alkene. On the other hand, an alcoholic solution of KOH contains alkoxide (RO^–) ions which being stronger base than OH^– ions preferentially eliminates a molecule of HCl from an alkyl halide to form alkenes.

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(i) Two primary alkyl halides having the molecular formula C₄H₉Br are possible. These are :

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