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Haloalkanes and Haloarenes — Chemistry STD 12 Science — Question

Gujarat BoardEnglish MediumSTD 12 ScienceChemistryHaloalkanes and Haloarenes4 Marks
Question

Read the passage given below and answer the following questions:
The aryl halides are relatively less reactive towards nucleophilic substitution reactions as compared to alkyl halides. This low reactivity can be attributed to the following factors:
  • The $C - X$ bond in halobenzene has a partial double bond character due to involvement of halogen electrons in resonance with benzene ring.
  • The $C - X$ bond in aryl halides is less polar as compared to that in alkyl halides as $sp^2$ hyridised carbon is more electronegative than $sp^3$ hybridised carbon.
In these questions (Q. No. i-Iv), a statement of assertion followed by a statement of reason is given. Choose the correct answer out of the following choices.
  1. Assertion and reason both are correct statements and reason is correct explanation for assertion.
  2. Assertion and reason both are correct statements but reason is not correct explanation for assertion.
  3. Assertion is correct statement but reason is wrong statement.
  4. Assertion is wrong statement but reason is correct statement.
  1. Assertion: Primary benzylic halides are more reactive than primary alkyl halides towards $S_N1$ reactions.
Reason: Reactivity depends upon the nature of the nucleophile and the solvent.
  1. Assertion: is more reactive than towards nucleophilic substitution reactions.
Reason: Tertiary alkyl halides react predominantly by $S_N1$ mechanism.
  1. Assertion: Chlorobenzene is more reactive than p-chloroanisole to nucleophilic substitution reactions.
Reason: Greater the stability of carbanion, greater is its ease of formation and hence, more reactive is the aryl halide.
  1. Assertion: $4-$Nitrochlorobenzene undergoes nucleophilic substitution more readily than chlorobenzene.
Reason: Chlorobenzene undergoes nucleophilic substitution by elimination-addition mechanism while 4-nitrochlorobenzene undergoes nucleophilic substitution by addition-elimination mechanism.
  1. Assertion: Chlorobenzene is less reactive than benzene towards the electrophilic substitution reaction.
Reason: Resonance destabilises the carbocation.
✓

Answer

  1. (b) Assertion and reason both are correct statements but reason is not correct explanation for assertion.
​​​​​​​Explanation:
Primary benzylic halides show higher reactivity in $S_N1$ reactions than primary alkyl halides. This is due to the greater stabilisation of the benzylic carbocation intermediates by resonance.
  1. (a) Assertion and reason both are correct statements and reason is correct explanation for assertion.
  2. (a) Assertion and reason both are correct statements and reason is correct explanation for assertion.
​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​Explanation:
On comparing the relative stabilities of carbanion of chlorobenzene and p-chloroanisole,

The electron donating group $(OCH_3)$ in anisole tends to intensify the negative charge relative to carbanion in chlorobenzene. Thus, p-chloroanisole is less reactive than chlorobenzene.
  1. (b) Assertion and reason both are correct statements but reason is not correct explanation for assertion.
​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​Explanation:
As compared to chlorobenzene, the intermediate carbanion resulting from $4-$nitrochlorobenzene is stabilized by$-R-$effect of the N02 group.
  1. (c) Assertion is correct statement but reason is wrong statement.
​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​Explanation:
Chlorobenzene is less reactive than benzene towards the electrophilic substitution reactions due to -I effect.

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  2. 2
  3. 3
  4. 4
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Read the passage given below and answer the following questions:
Haloarenes are less reactive than haloalkanes. The low reactivity of haloarenes can be attributed to:
  • Resonance effect.
  • $sp^2$ hybridisation of C - X bond.
  • Polarity of C - X bond
  • Instability of phenyl cation (formed by self-ionisation of haloarene).
  • Repulsion between the electron rich attacking nucleophiles and electron rich arenes.
Reactivity of haloarenes can be increased or decreased by the presence of certain groups at certain positions for example, nitro ($-NO_{^2}$) group at o/ p positions increases the reactivity of haloarenes towards nucleophilc substitution reactions.
The following questions are multiple choice questions Choose the most appropriate answer:
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  1. The formation of less stable carbonium ion.
  2. Resonance stabilisation.
  3. Larger carbon-halogen bond.
  4. Inductive effect.
  1. Which of the following aryl halides is the most reactive towards nucleophilic substitution?
  1. Which one of the following will react fastest with aqueous $NaOH$?
  1. Which chloro derivative of benzene among the followings would undergo hydrolysis most readily with aqueous sodium hydroxide to furnish the corresponding hydroxy derivative?
  1. $C_6H_5Cl$
  1. The reactivity of the compounds (i) $MeBr$, (ii) $PhCH_2Br$, (iii) $MeCI$, (iv) $p-MeOC_6H_4Br$ decreases as:
  1. (i) > (ii) > (iii) > (iv)
  2. (iv) > (ii) > (i) > (iii)
  3. (iv) > (iii) > (i) > (ii)
  4. (ii) > (i) > (iii) > (iv)
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The following questions are multiple choice questions. Choose the most appropriate answer:
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  3. Nucleophilic addition reaction.
  4. Electrophilic addition reaction.
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  2. aq. $NaOH + CH_2Cl_2$
  3. aq. $NaOH + CHCl_3$
  4. aq. $NaOH + CCl_4$
  1. The electrophile in this reaction (A) is:
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  2. $^+CHCl_2$
  3. : $CCl_2$
  4. .$CCl_3$
  1. The structure of the intermediate (A) is:
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The following questions are multiple choice questions. Choose the most appropriate answer:
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  2. $0.02s^{-1}$
  3. $0.01s^{-1}$
  4. $0.3s^{-1}$
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  2. $2\text{N}_2\text{O}_{5(\text{g})}\rightarrow4\text{NO}_{2(\text{g})}+\text{O}_{2(\text{g})}$
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Ionic solids: These solids consist of positively and negatively charged ions arranged in a regular fashion throughout the solid. These solids are very hard and brittle, have very high melting points and have high enthalpies of vaporisation, e.g., NaCl, MgO, KCl, LiCl etc.
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In these questions (Q. No. i-iv), a statement of assertion followed by a statement of reason is given. Choose the correct answer out of the following choices.
  1. Assertion and reason both are correct statements, and reason is correct explanation for assertion.
  2. Assertion and reason both are correct statements, but reason is not correct explanation for assertion.
  3. Assertion is correct statement, but reason is wrong statement.
  4. Assertion is wrong statement, but reason is correct statement.
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Reason: Molecular solids are made up of covalent molecules.
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Reason: Ionic solids have coulombic forces of attraction between their ions.
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Reason: Covalent solids are constituted by ions.
  1. Assertion: Diamond and graphite do not have the same covalent structure.
Reason: Silicon carbide is typical example of network solid.
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The following questions are multiple choice questions. Choose the most appropriate answer:
  1. Phenols are highly acidic as compare to alcohols due to:
  1. The higher molecular mass of phenols.
  2. The stronger hydrogen bonds in phenols.
  3. Alkoxide ion is a strong conjugate base.
  4. Phenoxide ion is resonance stabilised.
  1. The correct order of acidic strength among the following is:
  1. (III) > (IV) > (II) > (I)
  2. (IV) > (III) > (I) > (II)
  3. (IV) > (III) > (II) > (I)
  4. (I) > (II) > (IV) > (III)
  1. The correct decreasing order of $pK_a$ value is:
  1. II > IV > I > III
  2. IV > II > III > I
  3. II I> II > IV > I
  4. IV > I > II > III
  1. The compound that does not liberate $CO_2$, on treatment with aqueous sodium bicarbonate solution is:
  1. Benzoic acid.
  2. Benzenesulphonic acid.
  3. Salicylic acid.
  4. Carbolic acid.
  1. Most acidic amongst the following is:
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Read the passage given below and answer the following questions:
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The following questions are multiple choice questions. Choose the most appropriate answer:
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  1. Cationic vacancies equal to number of $Cd^{2+}$ ions incorporated.
  2. Cationic vacancies equal to double the number of $Cd^{2+}$ ions.
  3. Anionic vacancies.
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  1. Lattice defect per $10^{15}$ $NaCl$ is $1$. What is the number of lattice defects in a mole of $NaCl?$
  1. $6.02 \times 10^{23}$
  2. $6.02 x 10^8$
  3. $10^{14}$
  4. None of these
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  1. Non-stoichiometric defect
  2. Schottky defect
  3. Frenkel defect
  4. All of these.
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  1. $3.01 \times 10^{23}$
  2. $6.02 \times 10^{23}$
  3. $9.03 \times 10^{23}$
  4. $12.04 \times 10^{23}$
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  3. $KCl$
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The following questions are multiple choice questions. Choose the most appropriate answer:
  1. In hexagonal close packing, a sphere has coordination number of.
  1. 4
  2. 6
  3. 8
  4. 12
  1. Which of the following arrangements correctly represents hexagonal and cubic close packed structure respectively?
  1. ABCABC...and ABAB...
  2. ABAB...and ABCABC...
  3. Both have ABAB ... arrangement.
  4. Both have ABCABC... Arrangement.
  1. The arrangement of the first two layers, one above the other, in hep and ccp arrangements is.
  1. Exactly same in both cases
  2. Partly same and partly different
  3. Different from each other
  4. Nothing definite.
  1. Which of the following statements is not correct?
  1. The amorphous solids have a random, disordered arrangement of constituents.
  2. The simple cubic, face-centred and body-centred are the three lattices of the cubic system.
  3. The number of Bravais lattice in which a crystal can be categorized is 7.
  4. A metal that crystallizes in hep structure has coordination number 12.
  1. Which of the following statements about amorphous solids is incorrect?
  1. They melt over a range of temperature.
  2. There is no orderly arrangement of particles.
  3. They are anisotropic.
  4. They are rigid and incompressible.
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Read the passage given below and answer the following questions:
At $298\ K$, the vapour pressure of pure benzene, $C_6, H_6$ is $0.256$ bar and the vapour pressure of pure toluene $C_6 H_5 CH_3$ is $0.0925$ bar. Two mixtures were prepared as follows:
  1. $7.8g of C_6 H_6 + 9.2g$ of toluene
  2. $3.9g of C_6 H_6 + 13.8g$ of toluene
The following questions are multiple choice questions. Choose the most appropriate answer:
  1. The total vapour pressure (bar) of solution I is.
  1. $0.128$
  2. $0.174$
  3. $0.198$
  4. $0.258$
  1. Which of the given solutions have higher vapour pressure?
  1. $I$
  2. $II$
  3. Both have equal vapour pressure
  4. Cannot be predicted
  1. Mole fraction of benzene in vapour phase in solution I is.
  1. $0.128$
  2. $0.174$
  3. $0.734$
  4. $0.266$
  1. Which of the following statements is/are correct?
  1. Mole fraction of toluene in vapour phase is more in solution I.
  2. Mole fraction of toluene in vapour phase is less in solution I.
  3. Mole fraction of benzene in vapour phase is less in solution I.
  1. Only II
  2. Only I
  3. I and III
  4. II and III
  1. Solution I is an example of a/an.
  1. Ideal solution.
  2. Non-ideal solution with positive deviation.
  3. Non-ideal solution with negative deviation.
  4. Can't be predicted.
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