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NEET Chemistry Notes Haloalkanes and Haloarenes – Stereochemical Aspects of Nucleophilic Substitution Reactions

NEET Chemistry Notes Haloalkanes and Haloarenes – Stereochemical Aspects of Nucleophilic Substitution Reactions

Stereochemical Aspects of Nucleophilic Substitution Reactions

Stereochemical Aspects of Nucleophilic Substitution Reactions

For SN1 and SN2,  we need to learn some basic stereochemical principles and notations.
NEET Chemistry Notes Haloalkanes and Haloarenes - Stereochemical Aspects of Nucleophilic Substitution Reactions 1
If plane polarised light rotates in clockwise direction, when passed through a substance, the substance is known as dextrorotatory or (+) or (d). If it rotates in anticlockwise, the substance is known as laevorotatory or (-) or (I).

  • Chiral carbon It has four different groups or atoms attached to it.
  • Chiral molecule Optically active molecule is known as chiral molecule. It mostly possesses atleast 1 chiral carbon atom
  • Racemic mixture Equimolar mixture of enantiomers (dextro and laevo) is known as racemic mixture. Racemic mixture is optically inactive as the rotation due to one enantiomer will be cancelled by the rotation due to the other enantiomer. This phenomenon is known as racemisation.
    Inversion of configuration The back side attack by a nucleophile in SN2 reactions gives rise to a product whose configuration is opposite to the reactant. Therefore, during SN2 reaction, inversion of configuration occurs. This 100% inversion of configuration is known as Walden inversion.
  • When haloalkane having NEET Chemistry Notes Haloalkanes and Haloarenes - Stereochemical Aspects of Nucleophilic Substitution Reactions 3 hydrogen atom is heated with alcoholic KOH, NEET Chemistry Notes Haloalkanes and Haloarenes - Stereochemical Aspects of Nucleophilic Substitution Reactions 3 elimination takes place, e.g.
    NEET Chemistry Notes Haloalkanes and Haloarenes - Stereochemical Aspects of Nucleophilic Substitution Reactions 2
  • If there is possibility of formation of more than one alkene due to availability of more than one a-H-atoms, the elimination of HZ takes place according to Saytzeff rule. It states, “in dehydrohalogenation reactions, the preferred product is that alkene which has the greater number of alkyl groups attached to the doubly bonded carbon atoms,
    NEET Chemistry Notes Haloalkanes and Haloarenes - Stereochemical Aspects of Nucleophilic Substitution Reactions 4
    NEET Chemistry Notes Haloalkanes and Haloarenes - Stereochemical Aspects of Nucleophilic Substitution Reactions 5

Chemical Properties of Haloarenes

Haloarenes give three types of reactions, which are explained as follws:

  • Nucleophilic Substitution Reactions

Haloarenes are less reactive than haloalkanes towards nucleophilic substitution reactions
NEET Chemistry Notes Haloalkanes and Haloarenes - Stereochemical Aspects of Nucleophilic Substitution Reactions 6

  • Electrophilic Substitution Reactions

Haloarenes undergo usual electrophilic substitution reactions of benzene nucleus such as
NEET Chemistry Notes Haloalkanes and Haloarenes - Stereochemical Aspects of Nucleophilic Substitution Reactions 7
NEET Chemistry Notes Haloalkanes and Haloarenes - Stereochemical Aspects of Nucleophilic Substitution Reactions 8

Reactions with Metals

(i) Wurtz-Fittig Reaction

A mixture of haloarene and haloalkane, when treated with Na in dry ether gives alkyl arene
NEET Chemistry Notes Haloalkanes and Haloarenes - Stereochemical Aspects of Nucleophilic Substitution Reactions 9

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