Gauche effect 4248491 210415435 2008-05-05T21:02:07Z Rjwilmsi 203434 accessdate compliance to [[Template:Cite_web#Required_parameters|Template:Cite_web]], Replaced: accessdate = February 27, 2008 → accessdate = 2008-02-27 using [[Project:AutoWikiBrowser|AWB]] :''Main article: [[alkane stereochemistry]]'' The term "gauche" refers to [[Conformational isomerism| conformational isomers]] (conformers) where two [[vicinal (chemistry)|vicinal]] groups are separated by a 60° torsion angle. IUPAC defines groups is gauche if they have a "synclinal alignment of groups attached to adjacent atoms".<ref name="goldbook">{{GoldBookRef|title=gauche|file=G02593| accessdate = 2008-02-27 }}</ref> In [[stereochemistry]], '''gauche''' interactions hinder [[chemical bond|bond]] rotation. For example, sighting along the C2-C3 bond in staggered [[butane]], there are two possible relative [[potential energy|potential energies]]. The two [[methyl]] groups can be in an anti-bonding relationship, or offset at sixty degree dihedral angles. In the latter configuration, the two methyls are said to be in a gauche relationship, and the relative potential energy of each methyl-methyl gauche interaction is 0.9 [[kilocalorie]]s per [[Mole (unit)|mole]] (4 [[kilojoule|kJ]]/mol). In general a gauche rotamer is less stable than an anti-rotamer. == Gauche effect == The '''Gauche effect''' characterizes any gauche [[rotamer]] which is actually more stable than the anti rotamer <ref name=Craig>''Contribution to the Study of the Gauche Effect. The Complete Structure of the Anti Rotamer of 1,2-Difluoroethane'' Norman C. Craig, Anthony Chen, Ki Hwan Suh, Stefan Klee, Georg C. Mellau, Brenda P. Winnewisser, and Manfred Winnewisser [[J. Am. Chem. Soc.]]; '''1997'''; 119(20) pp 4789 - 4790; (Communication) {{DOI|10.1021/ja963819e}}</ref>. This effect is present in 1,2-difluoroethane (H<sub>2</sub>FCCFH<sub>2</sub>) for which the gauche conformation is more stable by 2.4 to 3.4 kJ/mole in the gas phase. Another example is 1,2-dimethoxyethane. [[Image:GaucheEffect.png|300px|Gauche effect in 1,2-difluoroethane, left: anti and right: gauche]] There are two main explanations for the gauche effect: [[hyperconjugation]] and [[bent bond]]s. In the hyperconjugation model, the donation of electron density from the C–H σ bonding orbital to the C–F σ<sup>*</sup> antibonding orbital is considered the source of stabilization in the gauche isomer. Due to the greater electronegativity of fluorine, the C–H σ orbital is a better electron donor than the C–F σ orbital, while the C–F σ<sup>*</sup> orbital is a better electron acceptor than the C–H σ<sup>*</sup> orbital. Only the gauche conformation allows good overlap between the better donor and the better acceptor. <!-- figure needed, something like http://www.rsc.org/ej/CS/2008/b711844a/b711844a-f14.gif --> Key in the bent bond explanation of the gauche effect in difluoroethane is the increased [[p orbital]] character of both C-F bonds due to the large electronegativity of fluorine. As a result electron density builds up above and below to the left and right of the central C-C bond. The resulting reduced orbital overlap can be partially compensated when a gauche conformation is assumed, forming a bent bond. Of these two models, hyperconjugation is generally considered the principal cause behind the gauche effect in difluoroethane.<ref>Goodman, L.; Gu, H.; Pophristic, V.. Gauche Effect in 1,2-Difluoroethane. Hyperconjugation, Bent Bonds, Steric Repulsion. ''J. Phys. Chem. A.'' '''2005''', ''109'', 1223-1229. {{doi|10.1021/jp046290d}}</ref><ref name=ohagan>David O'Hagan. Understanding organofluorine chemistry. An introduction to the C–F bond. ''Chem. Soc. Rev.'' '''2008''' {{doi|10.1039/b711844a}}</ref> The [[molecular geometry]] of both rotamers can be obtained experimentally by high resolution [[infrared spectroscopy]] augmented with [[in silico]] work <ref name=Craig/>. In accordance with the model described above, the carbon - carbon [[bond length]] is higher for the anti-rotamer (151.4 [[picometer|pm]] vs. 150). The steric repulsion between the fluorine atoms in the gauche rotamer causes increased CCF [[bond angle]]s (by 3.2°) and increased FCCF [[dihedral angle]]s (from the default 60° to 71°). In the related ''1,2-difluorodiphenylethane'' (two protons replaced by phenyl) the [[threo]] isomer is found (by [[X-ray diffraction]] and from NMR [[coupling constant]]s) to have an [[anti conformation]] between the two [[phenyl]] groups and the two [[fluorine]] groups and a gauche conformation is found for both groups for the [[erythro]] isomer <ref>''The vicinal difluoro motif: The synthesis and conformation of erythro- and threo- diastereoisomers of 1,2-difluorodiphenylethanes, 2,3-difluorosuccinic acids and their derivatives'' O'Hagan D, Rzepa H, Schuler M, Slawin A [[Beilstein Journal of Organic Chemistry]], '''2006''' 2:19 ( 2 October 2006 ) {{doi|10.1186/1860-5397-2-19}}</ref>. According to [[in silico]] results this conformation is more stable by 0.21 kcal/mol (880 J/mol). A gauche effect has also been reported for a molecule featuring 4 successive fluor syn substituents, the last one introduced by [[deoxofluorinating agent]] ''bis(2-methoxyethyl)aminosulfur trifluoride'' <ref>''Enantioselective Synthesis of an All-syn Four Vicinal Fluorine Motif'' Luke Hunter, David O'Hagan, and Alexandra M. Z. Slawin [[J. Am. Chem. Soc.]]; '''2006'''; 128(51) pp 16422 - 16423; (Communication) {{DOI|10.1021/ja066188p}}</ref> : [[Image:MoreGaucheEffects.png|center|400px|Synthesis of an All-syn Four Vicinal Fluorine Motif]] The gauche effect is very sensitive to [[solvent effect]]s, due to the large difference in polarity of the two conformers. For example, 2,3-dinitro-2,3-dimetylbutane exists only the gauche conformation in the solid state, prefers the gauche conformer in [[benzene]] solution by a ratio of 79:21, but in [[carbon tetrachloride]] it prefers the anti conformer by a ratio of 58:42.<ref>Smith, Michael. B.; March, J. March's Advanced Organic Chemistry, 5th edition. Wiley, 2001. ISBN 0-471-58589-0</ref> Another case is trans-1,2 difluorocyclohexane, which has a larger preference for the eq/eq conformer in more polar solvents.<ref name=ohagan/> A related effect is the [[cis effect]] in alkenes. ==See also== *[[Anomeric effect]] == References == {{reflist}} [[Category:Stereochemistry]] [[ja:ゴーシュ型]]