- Alkane stereochemistry
Alkane stereochemistry concerns the
stereochemistryof 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.
staggeredconformation is a chemical conformationthat exists in any open chain single chemical bondconnecting two sp3 hybridised atoms as a conformational "energy minimum".
eclipsedconformation is a chemical conformation that exists in any open chain single chemical bond connecting two sp3 hybridised atoms as a conformational "energy maximum". This is believed to be due to steric hindrance, although a role for hyperconjugationis proposed by a competing theory.
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, exists as an infinite number of conformations with respect to rotation around the C-C bond; two of these are recognised as energy minimum (staggered) and energy maximum (eclipsed) forms. The importance of these is seen by extension of these concepts to more complex molecules for which stable conformations may be predicted as minimum energy forms.
In the example of staggered
ethanein 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 hindranceis minimised. The staggered conformation is more stable by 12.5 kJ/mol than the eclipsedconformation, which is the energy maximum for ethane. In the eclipsed conformation the torsional angle is minimized.
butane, the two staggered conformations are no longer equivalent and represent two distinct conformers:the anti-conformation (left-most, below) and the gauche conformation (right-most, below).
Both conformations are free of torsional strain, but, in the gauche conformation, the two
methylgroups are in closer proximity than the sum of 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 energystored in butane conformers with greater steric hindrance than the 'anti'-conformer ground state is given by these values:
* Gauche, conformer - 3.8 kJ/mol
* Eclipsed H and CH3 - 16 kJ/mol
* Eclipsed CH3 and CH3 - 19 kJ/mol.The eclipsed
methyl groups exert a greater steric strain because of their greater electron densitycompared to lone hydrogenatoms.
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 lengthof 154 pm and a Van der Waals radiusfor hydrogen of 120 pm, the hydrogen atoms in ethane are never in each other's way. The question of whether steric hindrance is responsible for the eclipsed energy maximum is a topic of debate to this day. One alternative to the steric hindrance explanation is based on hyperconjugationas analyzed within the Natural Bond Orbital framework. ["Hyperconjugation not steric repulsion leads to the staggered structure of ethane." Pophristic, V. & Goodman, L. Nature 411, 565–568 (2001) [http://www.nature.com/nature/journal/v411/n6837/abs/411565a0.html Abstract] ] ["Chemistry: A new twist on molecular shape" Frank Weinhold Nature 411, 539-541 (31 May 2001)] ["Rebuttal to the Bickelhaupt-Baerends case for steric repulsion causing the staggered conformation of ethane." Weinhold, F. Angew. Chem. Int. Ed. 42, 4188-4194 (2003)] In the staggered conformation, one C-H sigma bonding orbitaldonates electron density to the antibonding orbitalof the other C-H bond. The energetic stabilization of this effect is maximized when the two orbitals have maximal overlap, occurring in the staggered conformation. There is no overlap in the eclipsed conformation, leading to a disfavored energy maximum. On the other hand, an analysis within quantitative molecular orbital theoryshows that 2-orbital-4-electron (steric) repulsions are dominant over hyperconjugation. ["The case for steric repulsion causing the staggered conformation of ethane." Bickelhaupt, F.M. & Baerends, E.J. Angew. Chem. Int. Ed. 42, 4183-4188 (2003)] A valence bond theorystudy also emphasizes the importance of steric effects. ["The magnitude of hyperconjugation in ethane: A perspective from ab initio valence bond theory." Mo, Y.R. et al. Angew. Chem. Int. Ed. 43, 1986-1990 (2004)]
Many definitions that describe a specific conformation (IUPAC Gold Book) exist:
* a torsion angle of ±60° is called gauche [ Anslyn, Eric V. and Dougherty, Dennis A. "Modern Physical Organic Chemistry". University Science (July 15, 2005), 1083 pp. ISBN-10: 1891389319 ]
* 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)GoldBookRef|title=gauche|file=G02593| accessdate = 2008-02-27 ]
* 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.
n-pentane, the terminal methylgroups experience additional pentane interference.
Replacing hydrogen by
fluorinein polytetrafluoroethylenechanges the stereochemistry from the zigzag geometry to that of a helixdue 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 crystallographyand from NMR spectroscopyand circular dichroismin 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; [http://dx.doi.org/10.1021/ja0602041 Graphical abstract] ]
*More alkane conformations exist in cyclic alkanes; see
*More on the impact of gauche interactions; see
* "Pitzer strain" in the
Gold Book[http://www.iupac.org/goldbook/E01886.pdf Link]
* "Torsion angle" in the
Gold Book[http://www.iupac.org/goldbook/T06406.pdf Link]
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