Assignments Class 12 Chemistry Haloalkanes and Haloarenes

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Haloalkanes and Haloarenes Assignments Class 12 Chemistry

Question. In which of the following molecules carbon atom marked with asterisk (*) is asymmetric?

(A) (i), (ii), (iii), (iv)
(B) (i), (ii), (iii)
(C) (ii), (iii), (iv)
(D) (i), (iii), (iv)

Question. What is ‘A’ in the following reaction?

Question. Arrange the following compounds in increasing order of their boiling points:

(A) (ii) < (i) < (iii)
(B) (i) < (ii) < (iii)
(C) (iii) < (i) < (ii)
(D) (iii) < (ii) < (i) 

Question. Which of the following alkyl halides will undergo SN¹ reaction most readily?
(A) (CH₃)₃C—F
(B) (CH₃)₃C—Cl
(C) (CH₃)₃C—Br
(D) (CH₃)₃C—I

Question. Which of the following is halogen exchange reaction?

Question. The order of reactivity of following alcohols with halogen acids is:

(A) (i) > (ii) > (iii)
(B) (iii) > (ii) > (i)
(C) (ii) > (i) > (iii)
(D) (i) > (iii) > (ii)

Question. Which reagent will you use for the following reaction?
CH₃CH₂CH₂CH₃   →    CH₃CH₂CH₂CH₂Cl + CH₃CH₂CHClCH₃
(A) Cl₂/UV light
(B) NaCl + H₂SO₄
(C) Cl₂ gas in dark
(D) Cl₂ gas in the presence of iron in dark

Question. Which of the following structures is enantiomeric with the molecule (a) given below?

Question.Arrange the following compounds in the increasing order of their densities.

(A) (i) < (ii) < (iii) < (iv)
(B) (i) < (iii) < (iv) < (ii)
(C) (iv) < (iii) < (ii) < (i)
(D) (ii) < (iv) < (iii) < (i)

Question. Identify the compound Y in the following reaction.  

Question. Reaction of C₆H₅CH₂Br with aqueous sodium hydroxide follows:
(A) SN1 mechanism
(B) SN2 mechanism
(C) Any of the above two depending upon the temperature of reaction
(D) Saytzeff rule

Question. The conversion of an alkyl halide into an alcohol by aqueous NaOH is classified as
(A) a dehydrohalogenation reaction
(B) a substitution reaction
(C) an addition reaction
(D) a dehydration reaction

Question. The IUPAC name of the compound shown below is:

(A) 2-bromo-6-chlorocyclohex-1-ene
(B) 6-bromo-2-chlorocyclohexene
(C) 3-bromo-1-chlorocyclohexene
(D) 1-bromo-3-chlorocyclohexene 

Question. Which of the following alcohols will yield the corresponding alkyl chloride on reaction with concentrated HCl at room temperature? 

Question. Chlorobenzene is formed by reaction of chlorine with benzene in the presence of AlCl₃.Which of the following species attacks the benzene ring in this reaction?
(A) Cl^–
(B) Cl⁺
(C) AlCl₃
(D) [AlCl₄]⁻

Question. Toluene reacts with a halogen in the presence of iron (III) 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

ASSERTION AND REASON BASED MCQs

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 A and R are true and R is the correct explanation of A
(B) Both A and R are true but R is NOT the correct explanation of A
(C) A is true but R is false
(D) A is false and R is True

Question. 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.

Question. 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.

Question. 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 I₂ into HI.

Question. 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.

Question. 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.

Question. Assertion (A): In monohaloarenes, further electrophilic substitution occurs at ortho and para positions.
Reason (R): Halogen atom is a ring deactivator.

Question. Assertion (A): Aryl halides undergo nucleophilic substitution reactions with ease.
Reason(R): The carbon halogen bond in aryl halides has partial double bond character.

Question. Assertion (A): Hydrolysis of (–)-2-bromooctane proceeds with inversion of configuration.
Reason (R): This reaction proceeds through the formation of a carbocation.

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 S_N¹ and S_N² 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 attacks 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; that is to say, the reactant is more easier to be substituted. The alkalinity order of halogen ion is I⁻ < 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 > RCl > RF. 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 S_N¹ mechanism. If the leaving group is not easy to leave, the reaction is based on S_N² mechanism.

Influences of solvent polarity: In S_N¹ 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 S_N² 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 (S_N¹) of tertiary chlorobutane in 25° water (dielectric constant 79) is 300000 times faster than in ethanol (dielectric constant 24). The reaction rate (S_N²) of 2-bromopropane and NaOH in ethanol containing 40% water is twice slower than in absolute ethanol. In a word, the level of solvent polarity has influence on both S_N ¹ and S_N ² reactions, but with different results. Generally speaking, weak polar solvent is favorable for S_N ² reaction, while strong polar solvent is favorable for S_N¹ 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 S_N¹ mechanism in solvents with a strong polarity (for example, ethanol containing water).

The following questions are multiple choice questions. Choose the most appropriate answer:

Question. 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

Question. S_N¹ reaction will be fastest in case of:
(A) 1-Chloro-2-methyl propane
(B) 1-Iodo-2-methyl propane
(C) 1-Chlorobutane
(D) 1-Iodobutane

Question. S_N¹ 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)

Question. S_N¹ mechanism is favoured in which of the following solvents:
(A) benzene
(B) carbon tetrachloride
(C) acetic acid
(D) carbon disulphide

Question. Polar solvents make the reaction faster as they:
(A) destabilize transition state 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

II. Read the passage given below and answer the following questions:

Alkyl/Aryl halides may be classified as mono, di or polyhalogen compounds depending on one, two or more halogen atoms in their structures. Alkyl halides are prepared by free radical halogenation of alkanes, addition of halogen acids to alkenes and replacement of –OH group of alcohols with halogens using phosphorus halides, thionyl chloride or halogen acids. Aryl halides are prepared by electrophilic substitution to arenes. The following questions are multiple choice questions. Choose the most appropriate answer:

Question. 2-Bromopentane, 2-Bromo-2-methylbutane, 1-Bromopentane Write the compound which is most reactive towards β-elimination reaction.
(A) 2-Bromopentane
(B) 1- Bromopentane
(C) 2-Bromo-2-methylbutane
(D) None of the above

Question. Name the major monohalo product of the following reaction:

(A) 1-Iodo-1-methyl cyclohexane
(B) 1-Iodomethyl cyclohexane
(C) 1-Chloro cyclohexane
(D) None of the above

Question. Complete the reaction:
H₃C-Br + AgF →
(A) H₃C-Br + AgF → H₃C-F + AgBr
(B) H₃C-Br + AgF → Br-CH₂-F + AgH
(C) H₃C-Br + AgF → [Ag(CH₃)]F + Br
(D) None of the above

Question. Which of the following is halogen exchange reaction?

III. Read the passage given below and answer the following questions:

The objects which are non-superimposable on their mirror image (like a pair of hands) are said to be chiral and this property is known as chirality. Chiral molecules are optically active, while the objects, which are, superimposable on their mirror images are called achiral. These molecules are optically inactive. The above test of molecular chirality can be applied to organic molecules by constructing models and its mirror images or by drawing three dimensional structures and attempting to superimpose them in our minds. There are other aids, however, that can assist us in recognising chiral molecules. One such aid is the presence of a single asymmetric carbon atom.
In these questions a statement of assertion followed by a statement of reason is given . Choose the correct answer out of the following choices.

(A) Assertion and Reason both are correct statements and Reason is correct explanation for Assertion.
(B) Assertion and Reason both are correct statements but Reason is not correct explanation for Assertion.
(C) Assertion is correct statement but reason is wrong statement.
(D) Assertion is wrong statement but Reason is correct statement.

Question. Assertion (A): Propan-2-ol is an achiral molecule.
Reason (R): Carbon is called asymmetric carbon or stereocentre.

Question. Assertion (A): A racemic mixture containing two enantiomers in equal proportions will have zero optical rotation.
Reason (R): This is because the rotation due to one isomer will be cancelled by the rotation due to the other isomer.

Question. Assertion (A): Butan-2-ol is a chiral molecule.
Reason (R): It has 4 different groups attached to carbon atom.

Question. Assertion (A): The stereoisomers related to each other as non-superimposable mirror images are called enantiomers
Reason (R): Enantiomers possess identical physical properties.

IV. Read the passage given below and answer the following questions:

Aryl halides are extremely less reactive towards nucleophilic substitution reactions due to the following reasons:
(i) In haloarenes, the electron pairs on halogen atom are in conjugation with π-electrons of the ring.
(ii) In haloalkane, the carbon atom attached to halogen is sp3 hybridised while in case of haloarene, the carbon atom attached to halogen is sp² -hybridised.
(iii) In case of haloarenes, the phenyl cation formed as a result of self-ionisation will not be stabilised by resonance.
The following questions are Multiple Choice

Choose the most appropriate answer:

Question. Reaction of C₆H₅CH₂Br with aqueous sodium hydroxide follows _______.
(A) SN1 mechanism
(B) S_N² mechanism
(C) Any of the above two depending upon the temperature of reaction
(D) Saytzeff rule.

Question. Which of the following alkyl halides will undergoes SN1 reaction most readily?
(A) (CH₃)₃C—F
(B) (CH₃)₃C—Cl
(C) (CH₃)₃C—Br
(D) (CH₃)₃C—I

Question. A primary alkyl halide would prefer to undergo________.
(A) S_N¹ reaction
(B) S_N² reaction
(C) α-Elimination
(D) Racemisation

Question. What is ‘A’ in the following reaction?