Chirality is like a molecular handshake - some molecules can't be superimposed on their mirror images, just like your left and right hands. This property affects how molecules interact with light and other chiral molecules, crucial in biological systems.

Identifying chirality centers, like carbon atoms bonded to four different groups, is key. Chiral molecules lack symmetry planes, while achiral ones have them or no chirality centers. This concept is vital in understanding molecular structures and their properties.

Chirality and Molecular Handedness

Chirality and molecular mirror images

• Chirality geometric property of molecules lacking an internal plane of symmetry
  • Chiral molecules non-superimposable mirror images of each other (enantiomers)
  • Enantiomers have identical physical properties (melting point, boiling point, solubility)
    • Differ in interaction with plane-polarized light and other chiral molecules
• Two enantiomers of a chiral molecule designated as right-handed (R) or left-handed (S)
  • Designation based on Cahn-Ingold-Prelog (CIP) priority rules
• Chirality plays a crucial role in biological systems
  • Many drugs and natural compounds are chiral (amino acids, sugars, DNA)
  • Enantiomers can have different biological activities (thalidomide, carvone)

Identification of chirality centers

• Chirality center (stereocenter) atom bonded to four different groups
  • In organic molecules, most common chirality center tetrahedral carbon atom (asymmetric carbon)
• Determining if a carbon atom is a chirality center:
  1. Check if carbon atom bonded to four different groups
  2. If any two groups are the same, carbon atom not a chirality center
• Examples of molecules with chirality centers:
  • Amino acids (alanine, serine)
  • Sugars (glucose, fructose)
  • Lactic acid
• Molecules can have multiple chirality centers
  • Number of stereoisomers increases with number of chirality centers ($2^n$ stereoisomers, where $n$ = number of chirality centers)

Chiral vs achiral molecular structures

• Chiral molecules have at least one chirality center and lack an internal plane of symmetry
  • Non-superimposable mirror images (enantiomers)
• Achiral molecules do not have chirality centers or have an internal plane of symmetry
  • Superimposable on their mirror images
• Determining if a molecule is chiral or achiral:
  1. Identify potential chirality centers
  2. Check for presence of an internal plane of symmetry
     • If plane of symmetry exists, molecule is achiral
     • If no plane of symmetry and at least one chirality center, molecule is chiral
• Meso compounds achiral despite having chirality centers
  • Internal plane of symmetry bisects the molecule
  • Chirality centers "cancel out" each other's handedness
• Examples of achiral molecules:
  • Ethanol (no chirality center)
  • trans-1,2-Dichloroethene (plane of symmetry)
• Examples of chiral molecules:
  • 2-Butanol (one chirality center)
  • Tartaric acid (two chirality centers, chiral)
  • meso-Tartaric acid (two chirality centers, achiral due to internal plane of symmetry)

Optical Activity and Stereochemistry

• Optical activity is the ability of chiral molecules to rotate plane-polarized light
• Stereochemistry studies the three-dimensional arrangement of atoms in molecules
• Molecular symmetry plays a crucial role in determining chirality and optical activity
  • Molecules with a plane of symmetry are achiral and optically inactive
  • Chiral molecules lack a plane of symmetry and are optically active