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Markovnikov’s Rule and Anti-Markovnikov’s Rule (Kharash Effect)

These rules describe the regioselectivity of addition reactions to unsymmetrical alkenes (alkenes with different groups on each carbon of the double bond). The rules predict where the new atoms or groups will add during the reaction.

1. Markovnikov’s Rule

  • Definition:
  • In the addition of a protic acid (e.g., (HX)) or water ((H_2O)) to an unsymmetrical alkene, the hydrogen ((H)) attaches to the carbon with the greater number of hydrogen atoms, and the halide ((X)) or hydroxyl ((OH)) attaches to the carbon with the fewer hydrogen atoms.
  • Mechanism:
  • The reaction proceeds via the formation of the more stable carbocation intermediate.
  • The more substituted carbocation (tertiary > secondary > primary) is more stable due to hyperconjugation and inductive effects.
  • Example:
  • Addition of (HBr) to propene:
    [
    CH_3-CH=CH_2 + HBr \rightarrow CH_3-CH(Br)-CH_3
    ]
    • Hydrogen adds to the carbon with more hydrogens ((CH_2)), and bromine adds to the carbon with fewer hydrogens ((CH)).
  • Summary:
  • “The rich get richer”: The carbon with more hydrogens gets the hydrogen.

2. Anti-Markovnikov’s Rule (Kharash Effect)

  • Definition:
  • In the presence of peroxides ((ROOR)), the addition of (HBr) to an unsymmetrical alkene follows Anti-Markovnikov’s rule. Here, the hydrogen ((H)) attaches to the carbon with the fewer hydrogen atoms, and the bromine ((Br)) attaches to the carbon with the greater number of hydrogen atoms.
  • Mechanism:
  • The reaction proceeds via a free radical mechanism initiated by peroxides.
  • The bromine radical ((Br^•)) adds to the less substituted carbon, forming a more stable radical intermediate.
  • Example:
  • Addition of (HBr) to propene in the presence of peroxides:
    [
    CH_3-CH=CH_2 + HBr \xrightarrow{ROOR} CH_3-CH_2-CH_2Br
    ]
    • Hydrogen adds to the carbon with fewer hydrogens ((CH)), and bromine adds to the carbon with more hydrogens ((CH_2)).
  • Summary:
  • “The poor get richer”: The carbon with fewer hydrogens gets the hydrogen.

Key Differences:

FeatureMarkovnikov’s RuleAnti-Markovnikov’s Rule
RegioselectivityHydrogen adds to the carbon with more hydrogens.Hydrogen adds to the carbon with fewer hydrogens.
MechanismCarbocation intermediate.Free radical intermediate.
ConditionsProtic acid ((HX), (H_2O)).(HBr) in the presence of peroxides.
Example(CH_3-CH(Br)-CH_3) (2-bromopropane).(CH_3-CH_2-CH_2Br) (1-bromopropane).

Mechanistic Explanation:

Markovnikov’s Rule:

  1. The alkene reacts with (HX), and the proton ((H^+)) adds to the carbon with more hydrogens, forming a carbocation.
  2. The more stable carbocation intermediate is formed (tertiary > secondary > primary).
  3. The halide ((X^-)) attacks the carbocation, completing the addition.

Anti-Markovnikov’s Rule:

  1. Peroxides generate free radicals, initiating a radical chain reaction.
  2. The bromine radical ((Br^•)) adds to the less substituted carbon, forming a carbon radical.
  3. The carbon radical abstracts a hydrogen from (HBr), forming the final product.

Summary:

  • Markovnikov’s Rule: Hydrogen adds to the carbon with more hydrogens (carbocation mechanism).
  • Anti-Markovnikov’s Rule: Hydrogen adds to the carbon with fewer hydrogens (radical mechanism, requires peroxides).

Practice Problems:

  1. Markovnikov’s Rule:
  • Predict the product of the addition of (HCl) to 2-methylpropene: [ (CH_3)_2C=CH_2 + HCl \rightarrow ? ]
    • Answer: ((CH_3)_2C(Cl)-CH_3) (2-chloro-2-methylpropane).
  1. Anti-Markovnikov’s Rule:
  • Predict the product of the addition of (HBr) to 1-butene in the presence of peroxides: [ CH_2=CH-CH_2-CH_3 + HBr \xrightarrow{ROOR} ? ]
    • Answer: (CH_3-CH_2-CH_2-CH_2Br) (1-bromobutane).

These rules are essential for predicting the outcomes of addition reactions in organic chemistry.

Chirality and Chiral Molecules

1. Definition of Chirality:

  • Chirality is a property of a molecule that makes it non-superimposable on its mirror image. A molecule is chiral if it cannot be superimposed on its mirror image, much like how your left and right hands are mirror images but not identical.
  • Chiral Center: A carbon atom bonded to four different groups is called a chiral center (or stereocenter). Molecules with one or more chiral centers are often chiral.

2. Key Features of Chiral Molecules:

  • Non-Superimposable Mirror Images: Chiral molecules have mirror images that cannot be perfectly aligned with the original molecule.
  • Optical Activity: Chiral molecules rotate plane-polarized light, a property known as optical activity.
  • Enantiomers: The two non-superimposable mirror images of a chiral molecule are called enantiomers. They have identical physical and chemical properties but differ in their interaction with plane-polarized light and other chiral molecules.

3. Examples of Chiral Molecules:

  1. Lactic Acid:
  • Lactic acid has a chiral center at the second carbon, which is bonded to:
    • (OH) (hydroxyl group)
    • (COOH) (carboxyl group)
    • (CH_3) (methyl group)
    • (H) (hydrogen)
  • The two enantiomers of lactic acid are:
    • L-lactic acid (found in muscles)
    • D-lactic acid (produced by bacteria)
    Structure:
       COOH             COOH
        |                |
   H - C - OH        HO - C - H
        |                |
       CH3              CH3
  1. Alanine (Amino Acid):
  • Alanine is a chiral amino acid with a chiral center at the second carbon, bonded to:
    • (NH_2) (amino group)
    • (COOH) (carboxyl group)
    • (CH_3) (methyl group)
    • (H) (hydrogen)
  • The two enantiomers are:
    • L-alanine (found in proteins)
    • D-alanine (rare in nature)
    Structure:
       COOH             COOH
        |                |
   H - C - NH2       H2N - C - H
        |                |
       CH3              CH3
  1. 2-Butanol:
  • 2-Butanol has a chiral center at the second carbon, bonded to:
    • (OH) (hydroxyl group)
    • (CH_3) (methyl group)
    • (CH_2CH_3) (ethyl group)
    • (H) (hydrogen)
  • The two enantiomers are:
    • R-2-butanol
    • S-2-butanol
    Structure:
       CH3             CH3
        |                |
   H - C - OH        HO - C - H
        |                |
      CH2CH3           CH2CH3

4. Identifying Chirality:

  • A molecule is chiral if it has:
  • At least one chiral center (carbon with four different groups).
  • No internal plane of symmetry.
  • Example of an Achiral Molecule:
  • Ethanol ((CH_3-CH_2-OH)) is achiral because it has no chiral center (the second carbon is bonded to two hydrogens).

5. Importance of Chirality:

  • Biological Systems: Most biological molecules (e.g., amino acids, sugars) are chiral. Only one enantiomer is typically active in biological processes.
  • Drug Design: The two enantiomers of a drug may have different effects. For example:
  • Thalidomide: One enantiomer treats morning sickness, while the other causes birth defects.
  • Optical Activity: Chiral molecules rotate plane-polarized light, which is used to study their properties.

6. Practice Problem:

Identify whether the following molecule is chiral or achiral:
[
CH_3-CH(Cl)-CH_2-CH_3
]

  • The second carbon is bonded to:
  • (Cl) (chlorine)
  • (CH_3) (methyl)
  • (CH_2CH_3) (ethyl)
  • (H) (hydrogen)
  • Since the second carbon has four different groups, the molecule is chiral.

Summary:

  • Chirality: A molecule is chiral if it is non-superimposable on its mirror image.
  • Chiral Center: A carbon atom bonded to four different groups.
  • Enantiomers: Non-superimposable mirror images of a chiral molecule.
  • Examples: Lactic acid, alanine, 2-butanol.

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