CBSE 12th Standard Chemistry Subject Haloalkanes and Haloarenes Chapter Case Study Questions 2021
QB365 Provides the updated CASE Study Questions for Class 12 , and also provide the detail solution for each and every case study questions . Case study questions are latest updated question pattern from NCERT, QB365 will helps to get more marks in Exams
QB365 - Question Bank Software
CBSE 12th Standard Chemistry Subject Haloalkanes and Haloarenes Case Study Questions 2021
12th Standard CBSE
-
Reg.No. :
Chemistry
-
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
(i) resonance effect
(ii) Sp2 hybridisation of C - X bond
(iii) polarity of C - X bond
(iv) instability of phenyl cation (formed by self-ionisation ofhaloarene)
(v) 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 (-NO2) group at olp positions increases the reactivity of haloarenes towards nucleophilc substitution reactions.
The following questions are multiple choice questions. Choose the most appropriate answer:
(i) Aryl halides are less reactive towards nucleophilic substitution reaction as compared to alkyl halides due to(a) the formation ofless stable carbonium ion (b) resonance stabilisation (c) larger carbon-halogen bond (d) inductive effect. (ii) Which of the following aryl halides is the most reactive towards nucleophilic substitution?
(iii) Which one of the following will react fastest with aqueous NaOH?
The reactivity of the compounds (i) MeBr, (ii) PhCH2Br, (iii) MeCI, (iv) p- MeOC6H4Br decreases as
(a) (i) > (ii) >(iii) > (iv) (b) (iv) > (ii) >(i) > (iii) (c) (iv) > (iii) >(i) > (ii) (d) (ii) > (i) > (iii) > (iv) -
Read the passage given below and answer the following questions:
In haloalkanes, when a nucleophile stronger than the halide ion approaches the positively charged carbon atom of an alkyl halide, the halogen atom along with its bonding electron pair gets displaced and a new bond with the carbon and the nucleophile is formed. These reactions are called nucleophilic substitution reactions.
In these reactions the atom or group of atoms which loses its bond from carbon and takes on an additional pair of electrons is called leaving group. Halide ions are good leaving groups. Some important nucleophilic substitution reactions of haloalkanes with common nucleophiles are given in the table below.Reagent Nudeophile (Nu-) Substitution product
R-NuClass of main product 1. NaOH or KOH or moist Ag2O -OH ROH Alcohol 2. H2O H2O ROH Alcohol 3. NaI I- R-I Alkyl iodide 4. R'NH 2 \(R^{\prime} \ddot{\mathrm{N}} \mathrm{H}_{2}\) RNHR' Sec. amine 5. KCN \(\overline{\mathrm{C}} \equiv \mathrm{N}:\) RCN Nitrile (cyanide) 6. KNO2 O=N-O- R-O-N=O Alkyl nitrite In these questions (i-iv), 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.
(i) Assertion: Alkyl halides are hydrolysed to alcohols by moist silver oxide.
Reason: RCI is hydrolysed to ROH easily but reactions slow down on addition of KI.
(ii) Assertion : Alkyl halides form alkenes when heated above 300°C.
Reason: CH3CH2I reacts slowly with strong base as compared to CD3CH2I.
(iii) Assertion : RBr reacts with AgNO2 to give nitro alkane.
Reason: Silver nitrite (AgNO2) is an ionic compound, therefore the negative charge on nitrogen is the attacking site.
(iv) Assertion: The nucleophilic substitution of vinyl chloride is difficult than ethyl chloride.
Reason: Vinyl group is electron donating group. -
Read the passage given below and answer the following questions:
When a chemical reaction involves bond cleavage or bond formation at an asymmetric carbon atom, three different products may be formed. For example, during the substitution of a group X by Y in the following reaction, the three possible products may be shown below:
(i) If B is the only product, the process is called retention of configuration because B has the same configuration as the starting reactant (A).
(ii) If C is the only product, the process is called inversion of configuration because C has the configuration opposite to the starting reactant (A).
(iii) If an equimolar mixture of B and C (i.e., a 50 : 50 mixture) is formed, then the process is called racemisation and the product is optically inactive because one isomer will rotate the light in the direction opposite to another.
In these questions ( i-iv), 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 correctstatements 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.
(i) Assertion: A reaction is said to be stereospecific if a particular stereoisomer of the reactant produces a specific stereoisomer of the product.
Reason: Bromination of cis-2-butene gives meso-2, 3-dibromobutane which is stereospecific
(ii) Assertion: Addition of Br2 to cis-but-2-ene is stereoselective.
Reason: SN2 reactions are stereospecific as well as stereoselective.
(iii) Assertion: Optically active 2-iodobutane on treatment with NaI in acetone undergoes recemization.
Reason: Repeated Walden inversions on the reactant and its product eventually gives a racemic mixture.
(iv) Assertion: SN2 reaction of an optically active alkyl halide with an aqueous solution of KOH always gives an alcohol with opposite sign of rotation.
Reason: SN2 reactions always proceed with inversion of configuration. -
Read the passage given below and answer the following questions:
The order of reactivity towards SN1 reaction depends upon the stability of carbocation in the first step. Greater the stability of the carbocation, greater will be its ease of formation from alkyl halide and hence faster will be the rate of the reaction. As we know, 3° carbocation is most stable, therefore, the tert-alkyl that halides will undergo SN1 reaction very fast. For example, it has been observed that the reaction (CH3)3CBr with OH- ion to give 2-methyl-2-propanol is about 1 million times as fast as the corresponding reaction of the methyl bromide to give methanol.
The primary alkyl halides always react predominantly by SN2 mechanism. On the other hand, the tertiary alkyl halides react predominantly by SN1 mechanism. Secondary alkyl halides may react by either mechanism or by both the mechanisms without much preference depending upon the nature of the nucleophile and solvent.
In these questions ( i-iv), 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.
(i) Assertion: Low concentration of nucleophile favours SN1mechanism.
Reason: 2° alkyl halides are less reactive than 1° towards SN1 reactions.
(ii) Assertion: Polar solvent slows down SN2reactions.
Reason: CH3-Br is less reactive than CH3Cl.
(iii) Assertion: Benzyl bromide when kept in acetone- water it produces benzyl alcohol.
Reason: The reaction follows SN2 mechanism.
(iv) Assertion: Rate of hydrolysis of methyl chloride to methanol is higher in DMF than in water.
Reason: Hydrolysis of methyl chloride follows second order kinetics. -
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:
1. The C - X bond in halobenzene has a partial double bond character due to involvement of halogen electrons in resonance with benzene ring.
2. The C - X bond in aryl halides is less polar as compared to that in alkyl halides as sp2 hyridised carbon is more electronegative than sp3 hybridised carbon.
In these questions ( i-iv), 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.
(i) Assertion: Primary benzylic halides are more reactive than primary alkyl halides towards SN1 reactions.
Reason: Reactivity depends upon the nature of the nucleophile and the solvent.
(ii) Assertion :
is more reactive than 
towards nucleophilic substitution reactions.
Reason: Tertiary alkyl halides react predominantly by SN1 mechanism.
(iii) 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.
(iv) Assertion: Chlorobenzene is less reactive than benzene towards the electrophilic substitution reaction.
Reason: Resonance destabilises the carbo cation.
*****************************************
CBSE 12th Standard Chemistry Subject Haloalkanes and Haloarenes Case Study Questions 2021 Answer Keys
-
(i) (b)
(ii) (d): When in aryl halides the electron withdrawing groups are attached at ortho and para positions to the chlorine atom then the removal of chlorine atom as CI- ion becomes easy, therefore, 2,4,6- trinitro chlorobenzene is the most reactive among given aryl halides.
(d) : The order of reactivity follows the sequence: benzyl halides> alkyl halides> aryl halides. Out of chlorides and bromides, bromides are more reactive. Therefore, the correct order of reactivity is PhCH2Br(ii) > MeBr(i) > MeCI(iii) >p-MeOC6H4Br (iv). -
(i) (c) : KI reacts with RCI to form RI. This RI molecule now hydrolysed easily to give ROH because alkyl iodide are more reactive than alkyl chloride. Thus, reaction becomes faster on addition of KI.
(ii) (c): CH3CH2I reacts more rapidly with strong base in comparison to CD3CH2I. The elimination of HI (or DI) in presence of strong base shows E2 elimination. The rate determining step involves the breaking up of C - H (or C - D) bond. The C - D bond being stronger than C - H bond is difficult to break.
(iii) (c) : Silver nitrite is a covalent compound and the bond between Ag - O is covalent. Therefore, it does not have a negative charge on the oxygen atom. Hence, the nucleophillic attack occurs through the lone pair on nitrogen forming nitro alkanes (R - NO2).
(iv) (c): The carbon-halogen bond in vinyl halides has some double bond character and hence little difficult to break. -
(i) (c): Bromination of cis-2-butene give (±) 2,3-dibromobutane.
(ii) (b)
(iii) (a)
(iv) (a) -
(i) (c) : Ability to accommodate a positive charge determines the ease of heterolysis leading to SN1 mechanism. This ability to accommodate positive charge is more in the 20 alkyl halide since it has two alkyl groups as compared to one in 10 alkyl halide .
(ii) (c): CH3 - Br is more reactive than CH3 - Cl. The C - Br has bond dissociation energy of 293 kJ mol-1 while C - CI bond has its dissociation energy of 351 kJ mol-1. As the bond dissociation energy increases, the ease of breaking of C - X bond decreases and hence the reactivity of haloalkanes decreases.
(iv) (a) -
(i) (b): Primary benzylic halides show higher reactivity in SN1reactions than primary alkyl halides. This is due to the greater stabilisation of the benzylic carbocation intermediates by resonance.
(ii) (a)
(iii) (a) : On comparing the relative stabilities of carbanion of chlorobenzene and p-chloroanisole.
the electron donating group (OCH3) in anisole tends to intensify the negative charge relative to carbanion in chlorobenzene. Thus, p-chloroanisole is less reactive than chlorobenzene.
(iv) (c): Chlorobenzene is less reactive than benzene towards the electrophilic substitution reactions due to -I effect.
