Chemistry · MU Sofia

1.4. Isomerism

Covers constitutional isomers, distinguished by chain and positional differences, with pentane and methylpentane examples, then stereoisomerism including wedge-and-dash drawings, sigma-diastereomers with multiple chiral centres, and cis-trans geometric isomers. It moves on to conformers with Newman projections, anti, gauche and eclipsed forms, and optical isomerism covering enantiomers, racemic mixtures and meso compounds. Butane and chair-form cyclohexane are used as conformational examples.

Inside: 7 sections · 10 labelled pictures · 17 flashcards · 4 min

Topic 5 of 42 in the MU Sofia syllabus for Chemistry (Dental Medicine, Year 1). The free AceMed note covers it in 7 sections, about 615 words (4 min of reading), with 17 flashcards.

In this note

  1. Constitutional Isomerism (Chain and Positional):
  2. Stereoisomerism:
  3. Pi-Diastereomers (Geometric Isomerism)
  4. Conformational isomerism Newman projection formulae. Basic types of conformations.
  5. High-Yield Additions: Optical Isomerism & Conformational Stability
  6. Optical Isomerism
  7. Conformational Analysis

Constitutional Isomerism (Chain and Positional):

  • Definition: Compounds with the same molecular formula but different arrangements of atoms or the positions of functional groups.
  • Chain Isomerism:
  • Differ in the arrangement of carbon atoms within a chain, either linear or branched.
  • Example: Pentane (C5H12) has a straight chain, while 2-methylbutane (C5H12) has a branched chain.
  • Positional Isomerism:
  • Differ in the position of functional groups or substituents within the same carbon chain.
  • Example: 2-methylpentane (C6H14) and 3-methylpentane (C6H14) both have a methyl group attached to a pentane chain, but at different positions.

Stereoisomerism:

projections and wedge – and - dash structural formulas.

  • A wedge and dash projection is a drawing, a means of representing a molecule in which three types of lines are used in order to represent the three-dimensional structure:
  • Solid lines to represent bonds that are in the plane of the paper. Dashed lines to represent bonds that extend away from the viewer.
  • Sigma - diastereomerism of molecules with more than chiral centers VS Pi - diastereomers (geometric isomerism).

Sigma-Diastereomerism of Molecules with More Than One Chiral Center

  • When a molecule has more than one chiral center, This type of diastereomerism is called sigma-diastereomerism because it arises from differences in configuration around single (sigma) bonds between chiral centers.
  • Concept: Multiple chiral centers allow for different combinations of R/S configurations, leading to diastereomers even when they are not mirror images.
  • Number of Diastereomers: The maximum number of diastereomers for a molecule with n chiral centers is 2^n. However, this is a theoretical maximum, and some molecules may have fewer diastereomers due to symmetry or other factors.
  • Examples:
  • Erythrose and threose: Both are monosaccharides with two chiral centers. They are diastereomers due to the different configurations at C2 and C3 (D-erythrose: RR, D-threose: RS).

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