Question Set C

cbse 12th Standard Chemistry Haloalkanes and Haloarenes Model Questions Papers Study Material - QB365

Download CBSE Class 12th Standard CBSE Chemistry question papers, sample papers, important questions, and previous year solved papers in PDF format. Get free study materials, NCERT solutions, and exam preparation resources for Class 12th Standard CBSE Chemistry

Questions:

Generic 2

5 x 2 = 10

What happpens when :
(i) CH₃ -CI is treated with aqueous KOH?
(ii) CH₃ -CI is treated with KCN?
(iii) CH₃ -Br is treated with Mg in the presence of dry ether ?

Amongst the isomeric alkanes of molecular formula C₅H₁₂ , identify the one that on photochemical chlorination yields a single monochloride.

Identify all the possible monochloro structural isomers that would be expected to form on free radical chlorination of (CH₃)₂CHCH₂CH₃

Which compound in each of the following pairs will react faste in S_N2 reaction with OH^-?
(a) CH₃Br or CH₃I
(b) (CH₃)₃CCl or CH₃Cl

Write the IUPAC name of the following compound:

Generic 3

5 x 3 = 15

Which one of the following has the highest dipole moment?
(i) CH₂Cl₂
(ii) CHCl₃
(iii) CCl₄

Give reasons for the following observations.
(i) p-dibromobenzene has higher melting point than those of o- and m-Isomers.
(ii) Out of chlorobenzene and chloromethane, which is more reactive towards nucleophilic substitution reaction?
(iii) Racemisation occurs in S_N1reaction.

How do you convert:
(i) Chlorobenzene to biphenyl,
(ii) Propene to I-iodopropane,
(iii) 2-bromobutane to but-2-ene.

Write the products of the following reactions:

10.

Complete the following chemical equations:

Generic 5

3 x 5 = 15

11.

Some halogen containing compounds are useful in daily life. Some compounds of this class are responsible for exposure of flora and fauna to more and more of UV light which causes destruction to a great extent. Name the class of these halo compounds. In your opinion, what should be done to minimise harmful effects of these compounds.

12.

Although chlorine is an electron-withdrawing group, yet it is ortho-para-directing in electrophilic aromatic substitution reactions. why ?

13.

Explain the following in one or two sentences
(i) Displacement of cyanic and amide ion is never observed in nucleophilic substitution reactions.
(ii) RCI is hydrolysed to ROH slowly but the reaction is rapid if a catalytic amount of KI is added to the reaction mixture.

Multiple Choice Question

5 x 1 = 5

14.

The increasing order of reactivity towards S_N1 mechanism is

(a)

III < II < I

(b)

II< I< III

(c)

I < III < II

(d)

II< III < I

15.

With which mixture carbylamine test is performed in the presence of alc. KOH?

(a)

Chloroform and silver powder

(b)

Tri halogenated methane and one primary amine

(c)

One alkyl halide and one 1⁰ amine

(d)

One alkyl cyanide and one 1⁰ amine

16.

(a)

(i) < (ii) < (iii)

(b)

(i) < (iii) < (ii)

(c)

(iii) < (ii) < (i)

(d)

(ii) < (iii) < (i)

17.

Which of the carbon atoms present in the molecule given below are asymmetric?

(a)

1, 2, 3, 4

(b)

2, 3

(c)

1,4

(d)

1, 2, 3

18.

Arrange the following : CH₃CH₂CH₂CI (I), CH₃CH₂CHCICH₃(II), (CH₃)₂CHCH₂CI (III) and(CH₃)₃C^__CI (IV) in order of decreasing tensency towards S_N2 reactions

(a)

I > III > II > IV

(b)

III > IV > II > I

(c)

II > I > III > IV

(d)

IV > III > II > I

Assertion and reason

1 x 1 = 1

19.

Assertion: Vinyl chloride is less reactive than alkyl chloride.
Reason: Stability of alkyl halide decreases as the strength of C-X bond decreases.
Codes:
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.

Case Study Questions

1 x 4 = 4

20.

A primary alkyl halide (A) C₄H₉Br reacted with alcoholic KOH to give compound (B). Compound (B) is reacted with HBr to give compound (C) which is an isomer of (A). When (A) reacted with sodium metal, it gave a compound (D) C₈H₁₈that is different than the compound obtained when n-butyl bromide reacted with sodium metal.
(i) Which type of isomerism is present in compounds (A) and (C)?
(a) Positional
(b) Functional
(c) Chain
(d) Both (a) and (c)
(ii) IUPAC name of compound (D) is
(a) n-octane
(c) 2-dimethyl heptane
(c) 2-methylheptane
(d) 3,4-dimethyl hexane.

Visit QB365-SLMS - https://www.qb365.in/slms/

Answers Key:

Generic 2

5 x 2 = 10

(i) CH₃ - CI + KOH\(\rightarrow\)CH₃OH + KCI
(aq) Methanol
(ii) CH₃ - CI + KCM \(\rightarrow\) CH₃ CN + KCI
Methyl cyanide
(iii) CH₃ Br + Mg \(\overset { Dry\ ether }{ \longrightarrow } \)CH₃MgBr
Methyl magnesium bromide

Neopentane or 2, 2-Dimethylpropane

In the given molecule, there are four different types of hydrogen atoms. Replacement of these hydrogen atoms will give the following four monochloro derivatives.
(i) (CH₃)₂CHCH₂CH₂Cl
(ii) (CH₃)₂CHCH(Cl)CH₃
(iii) (CH₃)₂C(Cl)CH₂CH₃
(iv) CH₃CH(CH₂Cl)CH₂CH₃

In the S_N2 mechanism, the reactivity of halides for same halides group increase down the group. Because increase in size increase, the halide becomes a better leaving group. Therefore, CH₃I will react faster than CH₃Br in S_N2 reactions with OH^-.

1-Chloro-2, 2-dimethylpropane.

Generic 3

5 x 3 = 15

Dichloromethane has highest dipole moment among CH₂Cl₂, CHCl₃ and CCl₄. The decreasing order of dipole moments is CH₂Cl₂>CHCl₃>CCl₄. These molecules have tetrahedral geometry due to sp3 hybridization of carbon atom.In CCl4, the individual C−Cl bond dipoles cancel each other which results in zero dipole moment.
Hence, CCl₄ is non polar.

(i) In case of dichlorobenzenes, para isomer is more symmetrical than ortho and meta isomers. Hence, in the crystal lattice, para isomer fits more closely than ortho and meta isomers. Due to this, more energy is required to break the crystal lattice of para isomer. Hence, p-Dichlorobenzene has higher melting point than those of o- and m-isomers.
(ii) Out of chlorobenzene and chloromethane, chloromethane is more reactive towards nucleophilic substitution reactions since it is an alkyl halide. Chlorobenzene is less reactive towards nucleophilic substitution reaction because of the resonance effect and polarity of C−Cl bond.
(iii) Racemisation occurs in S_N1 reaction. both inversion and retention of the configuration.

(i) Chlorobenzene to biphenyl: When two chlorobenzene combine with sodium metal in the presence of dry ether it forms biphenyl.
(ii) Propene to 1-iodopropane :
CH₃CH = CH₂ + HI \(\overset { Peroxide }{ \underset { Anti-Markovnokov's\ rule }{ \longrightarrow } } \)CH₃CH₂I + CH₃CHICH₃
(Major) (Minor)
CH₃CH = CH₂\(\overset { HBr/Peroxide }{ \longrightarrow } \)CH₃CH₂CH₂Br\(\overset { NaI/acetone }{ \longrightarrow } \)CH₃CH₂CH₂I
(iii) 2 - bromobutance to but-2-ene
\({ H }_{ 3 }C-{ CH }_{ 2 }-\underset { \overset { | }{ Br } }{ CH } -{ CH }_{ 3 }\overset { Alc.KO }{ \longrightarrow } \quad { H }_{ 3 }C-\underset { But-2-ene\\ \quad \quad (80%) }{ CH } =CH-{ CH }_{ 3 }+{ CH }_{ 3 }-\underset { But-1-ene\\ \quad \quad (20%) }{ { CH }_{ 2 } } -CH={ CH }_{ 2 }\)

10.

Generic 5

3 x 5 = 15

11.

CFC (chlorofluoro carbons) are responsible for the destruction of ozone layer and UV light can reach earth which may cause skin cancer and affect flora and fauna.
These compounds should be banned and their use should be made to minimum extent.

12.

Chlorine withdraws electrons through inductive effect and releases electrons through resonance. Through inductive effect, chlorine destabilises the intermediate carbocation formed during the electrophilic substitution.

Through resonance, halogen tends to stabilise the carbocation and the effect is more pronounced at ortho- and para- positions. The inductive effect is stronger than resonance and causes net electron withdrawal and thus causes net deactivation. The resonance effect tends to oppose the inductive effect for the attack at ortho- and parapositions and hence makes the deactivation less for ortho- and paraattack. Reactivity is thus controlled by the stronger inductive effect and orientation is controlled by resonance effect.

13.

(i) HCN (PK_a 10) and NH3 (PK_a 11·25) are very weak: acids. Therefore, their conjugate bases, i.e., CN^- ion and NH₂^- are very strong bases. Since strong bases are bad leaving groups, their displacement in nucleophilic substitution reactions is never observed,
(ii) Iodide ion is a powerful nucleophile and hence reacts rapidly with RCI to form RI,
Further because 1- ion is a better leaving group than Cl- ion, therefore, RI is more rapidly hydrolysed than RCI to form ROH,
The I^- ion thus regenerated recycles in the above reaction thereby explaining its catalytic effect.

Multiple Choice Question

5 x 1 = 5

14.