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Alkane stereochemistry concerns the stereochemistry of linear alkanes and the linear alkane conformers. The existence of more than one conformation is due to hindered rotation around sp3 hybridised carbon carbon bonds. The smallest molecule with such a chemical bond, ethane, is found to exist as two conformers, staggered and eclipsed.
- A staggered conformation is a chemical conformation that exists in any open chain single chemical bond connecting two sp3 hybridised atoms as a conformational energy minimum.
- an eclipsed conformation is a chemical conformation that exists in any open chain single chemical bond connecting two sp3 hybridised atoms as a conformational energy maximum due to steric hindrance.
Conformations
In the example of staggered ethane in Newman projection a hydrogen atom on one carbon atom has a 60° torsional angle or torsion angle with respect to the nearest hydrogen atom on the other carbon so that steric hindrance is minimised. The staggered conformation is more stable by 12.5 kJ/mole than the eclipsed conformation which is the energy maximum for ethane. In the eclipsed conformation the torsional angle is minimized.
In butane two additional staggered conformations exist the anti conformation (left) and the gauche conformation (right).
Both conformations are free of torsional strain but in the gauche conformation the two methyl groups are in closer proximity than the sum their van der Waals radii. The interaction between the two methyl groups is repulsive (van der Waals strain) and an energy barrier results.
A measure of the potential energy stored in butane conformers with greater steric hindrance than the 'anti' conformer ground state is given by these values:
- Staggered, also known as gauche, conformer - 3.8 kJ/mol
- Eclipsed H and CH3 - 16 kJ/mol
- Eclipsed CH3 and CH3 - 19 kJ/mol
The textbook explanation for the existence of the energy maximum for an eclipsed conformation in ethane is steric hindrance but with a C-C bond length of 154 pm and a Van der Waals radius for hydrogen of 120 pm both hydrogen atoms in ethane are never in each other way. This inconsistency is a topic of debate to this day. One alternative to the steric hindrance explanation is based on hyperconjugation Hyperconjugation not steric repulsion leads to the staggered structure of ethane Pophristic, V. & Goodman, L. Nature 411, 565–568 (2001)Abstract Chemistry: A new twist on molecular shape Frank Weinhold Nature 411, 539-541 (31 May 2001). In terms of molecular orbital theory in the staggered conformation one C-H sigma bonding orbital donates electrons to the antibonding orbital of the other C-H bond with maximum overlap of the same-sign lobes. This stabilizing overlap is not present in the eclipsed conformation.
Definitions
Many definitions exist that describe a specific conformation (IUPAC Gold Book):
- a torsion angle between 0° and ± 90° is called syn (s),
- a torsion angle between ± 90° and 180° is called anti (a).
- a torsion angle between 30° and 150° or between –30° and –150° is called clinal
- a torsion angle between 0° and 30° or 150° and 180° is called periplanar (p)
- a torsion angle between 0° to 30° is called synperiplanar or syn- or cis-conformation (sp)
- a torsion angle between 30° to 90° and –30° to –90° is called synclinal or gauche or skew (sc);
- a torsion angle between 90° to 150°, and –90° to –150° is called anticlinal (ac);
- a torsion angle between ± 150° to 180° is called antiperiplanar or anti or trans (ap).
Any strain resulting from torsion is also called Pitzer Strain or eclipsing strain.
Special cases
In n-pentane the terminal methyl groups experience additional pentane interference.
Replacing hydrogen by fluorine in polytetrafluoroethylene changes the stereochemistry from the zigzag geometry to that of a helix due to electrostatic repulsion of the fluorine atoms in the 1,3 positions. Evidence for the helix structure in the crystalline state is derived from X-ray crystallography and from NMR and circular dichroism in solution Conformational Analysis of Chiral Helical Perfluoroalkyl Chains by VCD Kenji Monde, Nobuaki Miura, Mai Hashimoto, Tohru Taniguchi, and Tamotsu Inabe J. Am. Chem. Soc.; 2006; 128(18) pp 6000 - 6001; Graphical abstract.
See also
- More alkane conformations exist in cyclic alkanes : see cyclohexane conformations.
External links
References
"Alkane stereochemistry"