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Class 12 Science (PCM)

Chemistry Part 2

Haloalkanes and Haloarenes

Haloalkanes and Haloarenes

Action PlanNotesQuestionsMock-Test
Study Action Plan (Sorted by Difficulty)
Polyhalogen Compounds and Environmental Effects
Accuracy: 40%
Total Attempted25
Total Correct10
Total Incorrect15
Difficulty Level:
High
Electrophilic Substitution in Haloarenes
Accuracy: 46%
Total Attempted98
Total Correct45
Total Incorrect53
Difficulty Level:
High
Nature of Carbon-Halogen Bond
Accuracy: 47%
Total Attempted53
Total Correct25
Total Incorrect28
Difficulty Level:
High
Nucleophilic Substitution in Haloarenes
Accuracy: 49%
Total Attempted73
Total Correct36
Total Incorrect37
Difficulty Level:
High
Physical Properties of Haloalkanes
Accuracy: 49%
Total Attempted97
Total Correct48
Total Incorrect49
Difficulty Level:
High
Elimination Reactions
Accuracy: 51%
Total Attempted81
Total Correct41
Total Incorrect40
Difficulty Level:
Medium
Nomenclature of Haloalkanes and Haloarenes
Accuracy: 55%
Total Attempted56
Total Correct31
Total Incorrect25
Difficulty Level:
Medium
Preparation of Haloarenes
Accuracy: 56%
Total Attempted45
Total Correct25
Total Incorrect20
Difficulty Level:
Medium
Nucleophilic Substitution Reactions in Haloalkanes
Accuracy: 58%
Total Attempted72
Total Correct42
Total Incorrect30
Difficulty Level:
Medium
Stereochemical Aspects of Reactions
Accuracy: 59%
Total Attempted58
Total Correct34
Total Incorrect24
Difficulty Level:
Medium
Chirality and Optical Activity
Accuracy: 63%
Total Attempted32
Total Correct20
Total Incorrect12
Difficulty Level:
Medium
Reactions of Haloalkanes with Metals
Accuracy: 63%
Total Attempted30
Total Correct19
Total Incorrect11
Difficulty Level:
Medium
Reactivity Trends in Halogen Compounds
Accuracy: 66%
Total Attempted41
Total Correct27
Total Incorrect14
Difficulty Level:
Medium
Ambident Nucleophiles in Haloalkane Substitution
Accuracy: 71%
Total Attempted21
Total Correct15
Total Incorrect6
Difficulty Level:
Medium
Classification of Haloalkanes and Haloarenes
Accuracy: 79%
Total Attempted92
Total Correct73
Total Incorrect19
Difficulty Level:
Low
Preparation of Haloalkanes
Accuracy: 86%
Total Attempted72
Total Correct62
Total Incorrect10
Difficulty Level:
Low

Learning Objectives

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Learning Objectives

  • Name haloalkanes and haloarenes according to the IUPAC system of nomenclature from their given structures.
  • Describe the reactions involved in the preparation of haloalkanes and haloarenes and understand various reactions that they undergo.
  • Correlate the structures of haloalkanes and haloarenes with various types of reactions.
  • Use stereochemistry as a tool for understanding the reaction mechanism.
  • Appreciate the applications of organic compounds.
  • Highlight the environmental effects of polyhalogen compounds.

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Revision Notes & Summary

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Notes on Haloalkanes and Haloarenes

Objectives

  • Name haloalkanes and haloarenes according to the IUPAC system of nomenclature from their given structures.
  • Describe the reactions involved in the preparation of haloalkanes and haloarenes and understand various reactions that they undergo.
  • Correlate the structures of haloalkanes and haloarenes with various types of reactions.
  • Use stereochemistry as a tool for understanding the reaction mechanism.
  • Appreciate the applications of organic compounds.
  • Highlight the environmental effects of polyhalogen compounds.

Classification of Haloalkanes and Haloarenes

1. Based on Number of Halogen Atoms

  • Monohaloalkane: Contains one halogen atom (e.g., CH₂X)
  • Dihaloalkane: Contains two halogen atoms (e.g., C₂H₅X)
  • Trihaloalkane: Contains three halogen atoms (e.g., CHX)

2. Based on Hybridization of Carbon Atom

  • Alkyl Halides (R-X): Halogen bonded to an sp³ hybridized carbon.
    • Primary (1°): Halogen attached to a primary carbon.
    • Secondary (2°): Halogen attached to a secondary carbon.
    • Tertiary (3°): Halogen attached to a tertiary carbon.
  • Allylic Halides: Halogen bonded to an sp³ hybridized carbon adjacent to a double bond.
  • Benzylic Halides: Halogen bonded to an sp³ hybridized carbon attached to an aromatic ring.

Physical Properties

  • Organohalogen compounds have higher boiling points than corresponding hydrocarbons due to strong dipole-dipole and van der Waals forces.
  • Slightly soluble in water but completely soluble in organic solvents.

Chemical Reactions

Nucleophilic Substitution Reactions

  • Categorized into SN1 and SN2 based on kinetic properties.
  • SN2 Reactions: Characterized by inversion of configuration.
  • SN1 Reactions: Characterized by racemization.

Applications of Polyhalogen Compounds

  • Dichloromethane, Chloroform, Iodoform, Carbon Tetrachloride, Freon, DDT: Industrial applications but may cause environmental hazards such as ozone layer depletion.

Environmental Effects

  • Some halogenated compounds persist in the environment due to resistance to breakdown by soil bacteria.
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Practice Test – MCQs, True/False

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Multiple Choice Questions

A.

Retention of configuration

B.

Inversion of configuration

C.

Racemization

D.

No change in configuration
Correct Answer: B

Solution:

In an SN2S_N2 reaction, the nucleophile attacks from the opposite side of the leaving group, resulting in inversion of configuration.

Chapter Concept:

Nucleophilic Substitution Reactions in Haloalkanes

A.

They do not stabilize the carbanion intermediate

B.

They increase the electron density on the benzene ring

C.

They form a stronger C–X bond

D.

They cause steric hindrance
Correct Answer: A

Solution:

Meta -NO₂ groups do not increase the reactivity because they do not stabilize the carbanion intermediate as effectively as ortho and para -NO₂ groups.

Chapter Concept:

Nucleophilic Substitution in Haloarenes

A.

Inductive effect

B.

Resonance effect

C.

Steric hindrance

D.

Hyperconjugation
Correct Answer: B

Solution:

The orientation in electrophilic substitution of haloarenes is controlled by the resonance effect, which directs substitution to ortho/para positions.

Chapter Concept:

Electrophilic Substitution in Haloarenes

A.

They have identical physical properties except for the direction in which they rotate plane-polarized light.

B.

They are superimposable mirror images.

C.

They have different molecular formulas.

D.

They cannot exist in chiral molecules.
Correct Answer: A

Solution:

Enantiomers are non-superimposable mirror images that have identical physical properties but differ in the direction they rotate plane-polarized light.

Chapter Concept:

Chirality and Optical Activity

A.

Retention of configuration

B.

Inversion of configuration

C.

Racemisation

D.

Formation of a single enantiomer
Correct Answer: C

Solution:

S_N1 reactions lead to racemisation because the carbocation intermediate is planar, allowing nucleophilic attack from either side.

Chapter Concept:

Chirality and Optical Activity

A.

CH3CH2CH2CH2BrCH_3CH_2CH_2CH_2Br

B.

(CH3)2CHCH2Br(CH_3)_2CHCH_2Br

C.

CH3CH2CH(Br)CH3CH_3CH_2CH(Br)CH_3

D.

(CH3)3CBr(CH_3)_3CBr
Correct Answer: A

Solution:

Primary halides like CH3CH2CH2CH2BrCH_3CH_2CH_2CH_2Br undergo SN2S_N2 reactions more rapidly due to less steric hindrance compared to secondary and tertiary halides.

Chapter Concept:

Reactivity Trends in Halogen Compounds

A.

1-Bromopropane

B.

2-Bromopropane

C.

1-Bromo-2-methylpropane

D.

2-Bromo-2-methylpropane
Correct Answer: A

Solution:

The IUPAC name for 'n-Propyl bromide' is 1-Bromopropane as it indicates the bromine atom is attached to the first carbon of the propane chain.

Chapter Concept:

Nomenclature of Haloalkanes and Haloarenes

A.

Inductive effect

B.

Resonance effect

C.

Hyperconjugation

D.

Steric hindrance
Correct Answer: B

Solution:

The resonance effect primarily controls the orientation of electrophilic substitution in haloarenes, directing the substitution to the ortho and para positions.

Chapter Concept:

Electrophilic Substitution in Haloarenes

A.

CH3CH2CH2ClCH_3CH_2CH_2Cl

B.

(CH3)2CHCl(CH_3)_2CHCl

C.

(CH3)3CCl(CH_3)_3CCl

D.

CH2=CHCH2ClCH_2=CHCH_2Cl
Correct Answer: A

Solution:

CH3CH2CH2ClCH_3CH_2CH_2Cl is a primary alkyl halide because the chlorine atom is attached to a primary carbon atom, which is bonded to only one other carbon atom.

Chapter Concept:

Classification of Haloalkanes and Haloarenes

A.

Chlorine

B.

Bromine

C.

Iodine

D.

Fluorine
Correct Answer: D

Solution:

Fluorine is not typically used in the electrophilic substitution of haloarenes due to its high reactivity, which can lead to uncontrollable reactions.

Chapter Concept:

Electrophilic Substitution in Haloarenes

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Haloalkanes and Haloarenes

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True or False

Correct Answer: True

Solution:

Haloalkanes are classified into primary, secondary, and tertiary based on whether the carbon atom bonded to the halogen is attached to one, two, or three other carbon atoms, respectively.

Chapter Concept :

Classification of Haloalkanes and Haloarenes

Correct Answer: False

Solution:

The Saytzeff rule predicts that the major product of a β-elimination reaction is the more substituted alkene, not the less substituted one.

Chapter Concept :

Elimination Reactions

Correct Answer: False

Solution:

In the IUPAC system, dihalogen derivatives of benzene are named using the numerals 1,2-; 1,3-; and 1,4- instead of the prefixes o-, m-, and p- which are used in the common naming system.

Chapter Concept :

Nomenclature of Haloalkanes and Haloarenes

Correct Answer: True

Solution:

In SN1 reactions, the carbocation intermediate is planar and achiral, allowing the nucleophile to attack from either side, resulting in a mixture of products with both configurations, thus forming a racemic mixture.

Chapter Concept :

Classification of Haloalkanes and Haloarenes

Correct Answer: True

Solution:

In haloalkanes, increased branching reduces the surface area, leading to weaker van der Waals forces and thus lower boiling points.

Chapter Concept :

Physical Properties of Haloalkanes

Correct Answer: False

Solution:

Haloarenes undergo electrophilic substitution reactions under more drastic conditions than benzene due to the deactivating effect of halogens, despite being ortho/para-directing.

Chapter Concept :

Electrophilic Substitution in Haloarenes

Correct Answer: True

Solution:

In an SN2 reaction, the nucleophile attacks from the side opposite to the leaving group, resulting in an inversion of configuration at the stereocenter.

Chapter Concept :

Chirality and Optical Activity

Correct Answer: True

Solution:

The SN2 mechanism involves a backside attack by the nucleophile, leading to the inversion of configuration at the chiral center, similar to an umbrella turning inside out.

Chapter Concept :

Nucleophilic Substitution Reactions in Haloalkanes

Correct Answer: True

Solution:

The preparation of haloarenes involves the electrophilic substitution of arenes with Cl2Cl_2 or Br2Br_2 using Lewis acids like FeFe or FeCl3FeCl_3 as catalysts. This method is effective for chlorination and bromination but not for fluorination due to the high reactivity of fluorine.

Chapter Concept :

Preparation of Haloarenes

Correct Answer: True

Solution:

Grignard reagents (RMgXRMgX) react readily with water, alcohols, and amines to form hydrocarbons, hence they must be kept away from moisture.

Chapter Concept :

Reactions of Haloalkanes with Metals

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