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Stereochemistry Notes

Section I - Short Answer Questions

1. Definition of Stereochemistry
  • Stereochemistry is the branch of chemistry that deals with the spatial arrangement of atoms within molecules and the effect of this arrangement on the properties and reactions of compounds.
2. Types of Stereoisomers
  • Stereoisomers are classified into:
    • Enantiomers: Stereoisomers that are non-superimposable mirror images of each other.
    • Diastereomers: Stereoisomers that are not mirror images of each other.
    • Geometric Isomers (Cis/Trans): Stereoisomers differing in arrangement around a double bond or a cyclic structure.
3. Definition of Enantiomers
  • Enantiomers are pairs of molecules that are mirror images of each other and cannot be superimposed.
  • Examples of Enantiomers:
    • Lactic acid (D-lactic acid and L-lactic acid)
    • 2-butanol (R-2-butanol and S-2-butanol)
4. Definition of Diastereomers
  • Diastereomers are stereoisomers that are not mirror images of each other.
  • Examples of Diastereomers:
    • 2,3-butanediol (meso compound vs. optically active enantiomer)
    • Maleic acid (cis) vs. fumaric acid (trans).
5. Short Note on Geometrical Isomers
  • Geometrical Isomers are a type of stereoisomerism that occurs due to restricted rotation around a bond. Most commonly associated with double bonds or cyclic structures.
    • Commonly classified as cis (same side) or trans (opposite sides).
6. Examples of Geometrical Isomers in Stereochemistry
  • 1,2-dichloroethene ( C2H2Cl_2):
    • Cis isomer: Chlorines on the same side.
    • Trans isomer: Chlorines on opposite sides.
  • Butenedioic Acid (Maleic and Fumaric acids)
7. Specific Rotation and Its Calculation Formula
  • Specific rotation is defined as the rotation of plane polarized light by a chiral compound in a solution at a specified temperature and wavelength.
  • Formula to calculate specific rotation:[heta]=hetacimesl[ heta] = \frac{ heta}{c imes l} Where:
    • [heta][ heta] = specific rotation (°/g/mL)
    • hetaheta = observed angle of rotation (°)
    • cc = concentration of the solution (g/mL)
    • ll = path length in decimeters (dm).
8. Definition of Homotopic Atoms (or Homotopic Groups)
  • Homotopic atoms/groups are those that can be interchanged by a rotation of the molecule that leaves its overall configuration unchanged, leading to the same molecule.
9. Definition of Enantiotopic Atoms (or Enantiotopic Groups)
  • Enantiotopic atoms/groups are those that, when replaced, result in enantiomers. They are connected to the same atom and have different environments.
10. Definition of Diastereotopic Atoms (or Diastereotopic Groups)
  • Diastereotopic atoms/groups are those that yield diastereomers when one is replaced, thus not being related as mirror images.
11. Definition of Prochirality
  • Prochirality refers to a situation where an achiral molecule can become chiral by the replacement of one of its identical atoms or groups.
  • Examples of Prochirality:
    • Maleic acid when one hydrogen is replaced (giving fumaric acid) gives a chiral center.
    • (Z)-2-pentene converting to (R)-2-bromopentane.
12. Definition of Prochiral Drugs and Examples
  • Prochiral drugs are drugs containing a prochiral center that can lead to different enantiomeric forms with different biological activity.
  • Examples of Prochiral drugs:
    • Ibuprofen (R and S forms)
    • Thalidomide (R-form is sedative; S-form is teratogenic).
13. Definition of Racemic Mixture
  • Racemic mixture is a mixture containing equal amounts of enantiomers, resulting in no net optical activity.
  • Example of Racemic mixture:
    • Racemic ibuprofen contains equal parts of R-ibuprofen and S-ibuprofen.
14. Definition of Chirality
  • Chirality is a property of a molecule that makes it non-superimposable on its mirror image. A chiral molecule typically has at least one stereocenter or chiral center.
  • Examples of Chiral Compounds:
    • Lactic acid
    • 2-butanol.
15. Definition of Prochiral Environment
  • Prochiral Environment refers to the specific spatial arrangement of atoms/groups around a prochiral center that can lead to the creation of chiral products upon some transformations.

Section II — Long Answer Questions

1. Optical Isomerism in Stereochemistry
  • Optical isomerism arises due to the ability of chiral substances to rotate plane-polarized light. It can be classified into:
    • R & S system: Utilizes Cahn-Ingold-Prelog rules to assign priority and determine configuration.
    • D & L system: Based on the arrangement of molecules relative to glyceraldehyde.
  • Suitable examples include:
    • R- and S-2-butanol (rigorously chiral)
    • Lactic acid's two enantiomers (D-, L-).
2. Characteristics of Enantiomers and Diastereomers
  • Enantiomers possess identical physical properties except for their interaction with polarized light and reactions with other chiral substances; they have identical boiling points and melting points.
  • Diastereomers, however, have different physical properties and behave differently in chemical reactions leading to different boiling points and solubility.
3. Distinction Between Enantiomers and Diastereomers
  • Enantiomers are mirror images and share similar properties but react differently with other chiral compounds, whereas diastereomers are not mirror images and can have vastly different chemical and physical properties.
4. Detailed Note on Homotopic Atoms (or groups) and Faces in Stereochemistry
  • Homotopic groups: When two groups can be exchanged without altering the overall molecule.
  • Example: In methane (CH₄), replacing any hydrogen leads to identical compounds (homotopic).
5. Detailed Note on Enantiotopic Atoms (or groups) and Faces in Stereochemistry
  • Enantiotopic groups result in enantiomers if one is replaced.
  • Example: In 2-propanol, replacing one methyl group produces (R)- and (S)-2-propanol.
6. Detailed Note on Diastereotopic Atoms (or groups) and Faces in Stereochemistry
  • Diastereotopic groups yield diastereomers upon exchange.
  • Example: 2-bromobutane contains two types of hydrogen leading to different configurations upon substitution.
7. Classification of Prochiral Hydrogens in Detail
  • Prochiral hydrogens can be differentiated based on the surrounding structure:
    • Identical Hydrogens: When an atom can be replaced producing stereoisomers, e.g., in butyric acid.
    • Different environments when considering the connectivity in cyclic compounds.

Section III - Reactive Intermediates and Rearrangements

1. Order of Stability of Carbocations
  • Carbocations are ranked based on their stability:
    • Tertiary > Secondary > Primary > Methyl.
    • Stability is derived from hyperconjugation and the inductive effect.
2. Order of Stability of Carbanions
  • Carbanions also have stability orders:
    • Tertiary < Secondary < Primary < Methyl (opposite to carbocations).
3. Order of Stability of Free Radicals
  • The stability of free radicals trend is:
    • Tertiary > Secondary > Primary > Methyl.
4. Definition of Carbocations and Their Types
  • Carbocations are positively charged intermediates.
    • Types include:
    • Tertiary (3°): Carbocation bonded to three alkyl groups.
    • Secondary (2°): Carbocation bonded to two alkyl groups.
    • Primary (1°): Carbocation bonded to one alkyl group.
    • Methyl (0°): A carbon atom with a positive charge not bonded to any alkyl group.
5. Definition of Carbanions and Their Types
  • Carbanions are negatively charged intermediates.
    • Types include:
      • Tertiary (3°): More stable due to hindered sterics.
      • Secondary (2°): Moderate stability.
      • Primary (1°): Least stable, due to lesser electron-donating alkyl groups.
      • Methyl (0°): No stabilization from alkyl groups.
6. Definition of Free Radicals and Their Types
  • Free radicals are species with an unpaired electron.
    • Types include:
      • Alkyl radicals
      • Aromatic radicals
      • Allylic radical.
7. Hybridization of Carbocations
  • The hybridization of carbocations involves:
    • sp² hybridization (Trigonal planar geometry) due to three sigma bonds.
8. Hybridization of Carbanions
  • The hybridization of carbanions is:
    • sp³ hybridization (Tetrahedral geometry) due to four substituents at the carbon atom.
9. Hybridization of Free Radicals
  • Free radicals generally exhibit:
    • sp² hybridization due to the presence of one unpaired electron and two bonds.
10. Explaining Rearrangements with Mechanisms
a. Pinacol-Pinacolone Rearrangement
  • Involves the conversion of pinacol to pinacolone under acidic conditions through carbocation rearrangement.
  • Mechanism:
    1. Protonation of the hydroxyl group to form a better leaving group.
    2. Carbocation formation followed by a rearrangement and loss of water.
    3. Deprotonation to yield pinacolone.
b. Favorskii Rearrangement
  • The rearrangement of α-halo ketones to give carboxylic acids.
  • Mechanism:
    1. Cyclization of the haloketone to form an intermediate.
    2. Rearrangement to yield the carboxylic acid.
c. Wagner-Meerwein Rearrangement
  • Rearrangement of carbocations leading to different structural isomers.
  • Mechanism: Changing skeletal structure through hydride shifts or alkyl shifts, maintaining the same molecular formula.
d. Curtius Rearrangement
  • Involves the thermal rearrangement of acyl azides to amines.
  • Mechanism:
    1. Formation of a nitrogen gas with a neutral acyl radical.
    2. Further reaction leading to primary amine.
e. Schmidt Rearrangement
  • Leads to the formation of amides from carboxylic acids.
  • Mechanism: Acids into cyanides, formation of isocyanates.
f. Lossen Rearrangement
  • Involves saponification of an N-hydroxyamide to an amine and an aldehyde.
  • Mechanism: Formation of an unstable intermediate leading to rearrangement to the resulting products.