CBSE Class 12 Chemistry Haloalkanes and Haloarenes MCQs Set 07

Read the given passage and answer the questions that follow:

The substitution reaction of alkyl halides occurs in S_N1 or S_N2 mechanism whatever mechanism alkyl halide follow for substitution reaction to occur, the polarity of the carbon-halogen bond is responsible for the substitution reaction. The rate of SN¹ reactions are governed by the stability of carbocation where as for S_N2 reactions steric factor is the deciding factor. If the starting material is a chiral compound, we may end up with an inverted product or racemic mixture depending upon the type of mechanism followed by alkyl halide. Cleavage of ethers with HI is also governed by steric factor and stability of carbocation which indicates that in organic chemistry, these two major factors help us in deciding the kind of product formed.

Question. The order of reactivities of the following alkyl halides for a S_N2 reaction is
(a) CH₃F > CH₃Cl > CH₃Br > CH₃I
(b) CH₃F > CH₃Br > CH₃Cl > CH₃-I
(c) CH₃Cl > CH₃Br > CH₃F > CH₃I
(d) CH₃I > CH₃Br > CH₃Cl > CH₃F
Answer: d

Question. The stereochemistry of the product formed if optically active alkyl halide undergoes substitution reaction by S_N1 mechanism is –
(a) Optically active inverted product
(b) Optically active retention product
(c) Racemic mixture
(d) None of the above
Answer: c

Question. Which one is most reactive towards S_N1 reaction ?
(a) CH₃ CH₂ Br
(b) CH₃CH(CH₃ )Br
(c) (CH₃)₃CBr
(d) CH₃Br
Answer: c

Question. n-propyl bromide on treatment with ethanolic potassium hydroxide produces
(a) propane
(b) propene
(c) propyne
(d) propanol
Answer: b

Question. Which of the following are secondary bromides?
(a) CH₃CH₂Br
(b) (CH₃)₃CCH₂Br
(c) CH₃CH(Br)CH₂CH₃
(d)(CH₃)₂CBr
Answer: c

Question. IUPAC name of (CH₃)₃CCl
(a) 3-Chlorobutane
(b) 2-Chloro-2-methylpropane
(c) t-butyl chloride
(d) n-butyl chloride
Answer: b

Question. C – X bond is strongest in
(a) CH₃F
(b) CH₃Cl
(c) CH₃Br
(d) CH₃I
Answer: a

Question. The best method for the conversion of an alcohol into an alkyl chloride is by treating the alcohol with :
(a) PCl₅
(b) dry HCl in the presence of anhydrous ZnCl₂
(c) SOCl₂ in presence of pyridine
(d) None of these
Answer: c

Question. A Grignard reagent may be made by reacting magnesium with
(a) CH₃COOH
(b) CH₃ CH₃
(c) CH₃CH₂ I
(d) CH₃CH₂OH
Answer: c

Question. Reaction used to prepare alkyl iodide from alkyl bromide by using NaI in acetone is
(a) Willamson’s reaction
(b) Swart reaction
(c) Finkelstein reaction
(d) Wurtz reaction
Answer: c

Question. Which of the reaction given below is an example of Swart Reaction
(a) C₂H₅Cl + KOH + H₂O → C₂H₅OH
(b) C₂H₅Br + AgF → C₂H5F
(c) C₂H₅Br + Na (dry ether) →n- butane
(d) CH₃CH=CH₂ + HBr → CH₃CH(Br)CH₃
Answer: b

Question. A hydrocarbon that does not gives only one monochloro product on photo chlorination is
(a) Propane
(b) Ethane
(c) Methane
(d) Cyclopentane
Answer: a

Assertion Reason Question

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: Iodoalkanes undergo nucleophilic substitution more readily than bromoalkanes 
Reason: Iodine is a better leaving group due to its larger size which makes the covalent bond weaker.
Answer: a

Question. Assertion: S_N2 reaction is a bimolecular reaction
Reason: Both alkyl halide and nucleophiles are involved in the slow step.
Answer: a

Question. Assertion: Haloalkanes react with KCN to form cyanoalkanes. 
Reason: KCN completely ionises in solution.
Answer: a

Question. Assertion: Alkyl halides are polar molecule but immiscible in water. 
Reason: Alkyl halides form hydrogen bonding with water.
Answer: a

Very short answer type Questions 

Question. Out of CH₂=CH—CH₂Cl and CH₃CH₂CH₂Cl which one undergoes S_N1 mechanism faster?
Answer. CH₂=CH—CH₂Cl undergoes faster S_N1 reaction because allyl carbocation gets stabilised by resonance

Question. Name the instrument used for measuring the angle by which the plane polarised light is rotated.
Answer. Polarimeter

Question. What is racemic mixture?
Answer. Mixture in which two enantiomers are present in equal proportion with net zero optical rotation.

Short Answer type Question: 

Question. Among the isomeric alkyl bromide of molecular formula C₄H₉Br
(i) Write the compound which is optically active
(ii) Write the compound which is more reactive towards SN² reaction
(iii) Write the compounds which is more reactive towards SN¹ reaction.
Answer.  (i) CH₃CH₂CH(Br)CH₃ , Contains a chiral centre at carbon containing Br
(ii) CH₃CH₂CH₂CH₂Br , Primary alkyl halide
(iii) (CH₃)₃CBr , Tertiary alkyl halide

Question. Give reasons
a. C―Cl bond length in chlorobenzene is shorter than C-Cl bond length in CH₃―Cl
b. Boiling point of n-butyl chloride is more than t-butyl chloride
c. Grignard reagents should be prepared under anhydrous conditions
Answer. 
a) In chlorobenzene, there is partial double bond character b/w carbon and halogen due to+ resonance effect.
b) Strong Van-der-waal interaction due to more surface area in n-butyl chloride compared to t-butyl chloride.
c) Grignard reagent is highly reactive and its forms alkane in presence of moisture.

Question. Account for the following .
(a) The dipole moment of chlorobenzene is lower than that of cyclohexyl chloride.
(b) Alkyl halides though polar immiscible in water
(c) Sulphuric acid not used during the reaction of alcohols with KI
Answer. 
(a) Due to resonance chlorine gets partial positive charge in chlorobenzene resulting in lowering of dipole moment
(b) The force of interaction between alkyl halide and water is less compared to dipole interaction between alkyl halide and hydrogen bonding in water.
(c) HI formed will get oxidized to I₂ by concentrated Sulphuric acid which is an oxidizing agent.