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Organic Molecule Structures

High schoolChemistryValence-complete idealised ball-and-stick structures with a Fourier butane torsion model

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About this experiment

Build short carbon chains, compare structures with the same molecular formula, and rotate a single bond without changing connectivity. Generated hydrogens make every displayed valence explicit.

Learning goals

  • Relate carbon valence to single, double and triple bonds.
  • Distinguish structural isomers from conformers.
  • Recognise alcohol, ether, alkene, alkyne and aromatic structural motifs.

How to use it

  1. 1Add carbons to a straight alkane and record its molecular formula.
  2. 2Compare straight and branched butane; check that both have the same formula.
  3. 3Compare ethanol with dimethyl ether and inspect the oxygen neighbours.
  4. 4Return to butane and scan the central dihedral through a full turn while following the illustrative torsion energy.

The model

An acyclic saturated carbon chain has this formula.
Bond orders, including bonds to hydrogen atoms, complete carbon valence.
The teaching Fourier energy is in kJ per mole and is not a fitted force field.

Assumptions

  • Bond lengths and local tetrahedral, trigonal or linear geometries are idealised. Structures are not geometry-optimised quantum calculations.
  • Oxygen is shown with an ideal bent local geometry. Hydrogens complete valence; nonbonded steric relaxation is not solved.
  • Benzene uses alternating bond notation for its delocalised aromatic bonding. Space-filling radii are illustrative van der Waals radii.

Try this in class

  1. Q1Why does rotating butane not create a new connectivity isomer?
  2. Q2How can identical atom counts give different functional groups?

Frequently asked questions

Can every displayed conformation be a stable minimum?

No. A structure may be imposed at any dihedral angle. Only a simple teaching torsion curve is provided for butane.

Does hiding hydrogens remove them chemically?

No. The molecular formula, mass and valence counts always include all generated hydrogens.