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Introduction to Conformational Isomerism

The manner in which the atoms of a molecule are bonded together (its constitution) is displayed by Structural formulas, but do not usually explain the 3-d shape of a molecule, except special bonding notations (example hatched and wedge lines) are used. The significance of such type of three-dimensional descriptive formulas became understandable in while discussing Configurational stereoisomerism, where in space the relative orientation of atoms is fixed by a molecule's bonding constitution (example rings and double-bonds). It was noted here too that nomenclature prefixes must be employed when naming particular stereoisomers. In this section we shall expand our 3-D view of molecular structure to comprise compounds that usually assume an array of equilibrating 3-d spatial orientations, which together characterize similar isolable compound. We call these distinct spatial orientations of the atoms of a molecule that result from rotations or twisting about single bonds conformations.

We have an unbranched chain of six carbons in case of hexane, which is often written as a linear formula: CH3CH2CH2CH2CH2CH3. It is well known that this is not firmly true, because the carbon atoms all have a tetrahedral configuration. Therefore the concrete shape of the extended chain is zig-zag in nature. Though, about the carbon-carbon bonds there is facile rotation and the six-carbon chain simply coils up to assume a rather different shape. Various conformations of hexane are possible and two are demonstrated below in the diagram.

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Types of conformational isomerism

Butane has three rotamers: that are two gauche conformers, enantiomeric and an anti conformer, where four carbon centres are coplanar. Three eclipsed conformations with dihedral angles of the 0°, 120° and 240° are not considered to be rotamers but are instead transition states.

Some major examples of conformational isomerism includes:

1.      Linear alkane conformations with the gauche, staggered and eclipsed conformers, and

2.      Ring conformation

  • Cyclohexane conformations with the boat and chair conformers.
  • Carbohydrate conformation

3.      Atropisomerism- the molecule can become chiral due to restricted rotation about a bond.

4.      Folding of molecules, where some shapes are functional and stable but others are not.

Isolation or observation of conformational isomers

  • Atropoisomers can be fairly stable depending on the steric effects and were the first conformational isomers to be recognized. In biphenylic system atropisomerism is especially prevalent, examples are binaphthol.
  • In cyclohexane derivatives, the two chair conformers interconvert with rates on the order of 105ring flips/sec, which is clearly precludes their separation. Equatorial conformer crystallizes selectively and when these crystals are dissolved at the very low temperatures, one can directly monitor approach to the equilibrium by NMR spectroscopy.

Techniques for conformational isomerism

Several information on the conformational isomerism comes from single crystal X-ray diffraction studies. IR spectroscopy is usually used to measure conformer ratios. For equatorial and axial conformer of bromocyclohexane, the νCBr differs by almost 50 cm-1.

Dynamics of the conformational (and other kinds of) isomerism can be watched by NMR spectroscopy at varying temperatures. The techniques apply to barriers of 8-14 kcal/mol and species exhibiting such type of dynamics are frequently called "fluxional".

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