IB DP Chemistry: SL复习笔记10.1.10 3-D modelling

3-D Modelling

  • Representing 3-D molecules on a 2D surface is not easy and the best way to understand 3-D structures is to use modelling kits or 3-D modelling software, such as ACD Labs ChemSketch
  • For simplification, complex organic molecules are shown with 90o bond angles that give the minimum information of which atoms are connected together as in this representation of hexane, C6H14


A simplified displayed structure for hexane

  • The true structure of hexane looks very different when viewed in 3-D modelling software
  • Free rotation of the single bonds gives rise to structures that look different on paper:


Different 3-D structures for hexane

  • By convention, when showing using 3-D models or drawings, carbon is black, hydrogen is white and oxygen is red
  • These structures may not contain accurate atomic radiibond angles or bond lengths (modelling software usually allows you to manipulate these), but they convey information about the orientation of atoms that is very important in stereochemistry
  • Stereochemistry is the study of the relative spatial arrangements of atoms in molecules

Stereochemical drawings

  • To simplify 3-D drawings, chemists use a convention of drawing 'wedge' bonds to show bonds coming out of the plane of the paper or receding away from the plane
    • A single solid line indicates the bond is in the same plane as the paper
    • The solid wedge shows the bond is coming towards you and the hatched or partial wedge bond is going away from you
  • The stereochemical drawing for hexane is shown below:


A stereochemical drawing for hexane

  • Stereochemical drawings are particularly useful for representing isomers and complex biomolecules such as carbohydrates and proteins

Exam Tip

You are not expected to be able to draw 3-D molecules, but in an exam you may be presented with 3-D drawings from which you have to extract information such as the molecular formula or functional group