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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.

It retains its configuration.

B.

It undergoes racemisation.

C.

It inverts its configuration.

D.

It forms a carbocation intermediate.
Correct Answer: C

Solution:

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

Chapter Concept:

Classification 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.

1-Chlorobutane

B.

2-Chlorobutane

C.

3-Chlorobutane

D.

1-Chloro-2-methylpropane
Correct Answer: B

Solution:

The IUPAC name for 'sec-Butyl chloride' is 2-Chlorobutane, as the chlorine atom is attached to the second carbon in the butane chain.

Chapter Concept:

Nomenclature of Haloalkanes and Haloarenes

A.

The C-X bond is non-polar because carbon and halogen have similar electronegativities.

B.

The C-X bond length decreases as you move down the halogen group.

C.

The C-X bond enthalpy decreases as you move down the halogen group.

D.

The dipole moment of the C-X bond is independent of bond length.
Correct Answer: C

Solution:

The C-X bond enthalpy decreases as you move down the halogen group because the bond length increases, making the bond weaker.

Chapter Concept:

Nature of Carbon-Halogen Bond

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.

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.

Presence of a strong base

B.

Use of a polar protic solvent

C.

High temperature and pressure

D.

Presence of a Lewis acid catalyst
Correct Answer: D

Solution:

The presence of a Lewis acid catalyst is necessary for the electrophilic substitution of haloarenes, as it helps to generate the electrophile needed for the reaction.

Chapter Concept:

Electrophilic Substitution in Haloarenes

A.

It is retained.

B.

It is inverted.

C.

It is racemized.

D.

It remains unchanged.
Correct Answer: B

Solution:

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

Chapter Concept:

Nomenclature of Haloalkanes and Haloarenes

A.

The nucleophile attacks from the same side as the leaving group.

B.

The carbocation intermediate is planar, allowing attack from either side.

C.

The reaction involves inversion of configuration.

D.

The leaving group remains attached to the molecule.
Correct Answer: B

Solution:

In SN1 reactions, racemisation occurs because the carbocation intermediate is planar, allowing the nucleophile to attack from either side, leading to a mixture of enantiomers.

Chapter Concept:

Nomenclature of Haloalkanes and Haloarenes

A.

Inversion of configuration

B.

Racemisation

C.

Retention of configuration

D.

Formation of a carbocation intermediate
Correct Answer: D

Solution:

An SN1SN1 reaction involves the formation of a carbocation intermediate, which is planar and can be attacked by the nucleophile from either side, leading to racemisation.

Chapter Concept:

Preparation of Haloalkanes

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Haloalkanes and Haloarenes

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True or False

Correct Answer: True

Solution:

The C−XC-X bond is polar due to the difference in electronegativity between carbon and halogen atoms. Halogens are more electronegative, which causes the carbon to have a partial positive charge and the halogen a partial negative charge.

Chapter Concept :

Nature of Carbon-Halogen Bond

Correct Answer: False

Solution:

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

Chapter Concept :

Stereochemical Aspects of Reactions

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:

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

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: True

Solution:

Halogens in haloarenes exhibit a -I (inductive) effect, which is deactivating, but they also have a resonance effect that directs incoming electrophiles to the ortho and para positions.

Chapter Concept :

Electrophilic Substitution in Haloarenes

Correct Answer: True

Solution:

In an SN1S_N1 reaction, the intermediate carbocation is planar and achiral, allowing the nucleophile to attack from either side, leading to a racemic mixture of enantiomers.

Chapter Concept :

Chirality and Optical Activity

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: 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

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