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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.

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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.

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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

2. 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

3. 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

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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).

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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.

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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.

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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.