Editing Rotamer (section)

==Alkane stereochemistry==
Alkane conformers arise from rotation around [[Sp³ bond|sp<sup>3</sup>]] hybridised carbon–carbon [[sigma bond]]s. The smallest alkane 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 conformation]]) and energy maximum ([[eclipsed conformation]]) forms. The existence of specific conformations is due to hindered rotation around sigma bonds, although a role for [[hyperconjugation]] is proposed by a competing theory. {{citation needed|date=January 2025}}

The importance of energy minima and energy maxima is seen by extension of these concepts to more complex molecules for which stable conformations may be predicted as minimum-energy forms.  The determination of stable conformations has also played a large role in the establishment of the concept of [[asymmetric induction]] and the ability to predict the [[stereochemistry]] of reactions controlled by steric effects. {{citation needed|date=January 2025}}

In the example of staggered [[ethane]] in [[Newman projection]], a hydrogen atom on one carbon atom has a 60° '''torsional angle''' or '''torsion angle'''<ref name = torsion/> 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 [[joule|kJ]]/[[mole (unit)|mol]] than the [[eclipsed]] conformation, which is the energy maximum for ethane. In the eclipsed conformation the torsional angle is minimised.

[[File:Staggered and eclipsed.svg|center|338px|staggered conformation left, eclipsed conformation right in [[Newman projection]]]]

<div class="center">[[Image:Ethane-staggered-depth-cue-3D-balls.png|150px]] [[Image:Ethane-eclipsed-depth-cue-3D-balls.png|150px]]</div>

In [[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).

[[File:Anti gauche.svg|center|558px|anti vs gauche conformations]]

<div class="center">[[Image:Butane-anti-side-3D-balls.png|150px]] [[Image:Butane-eclipsed-side-3D-balls.png|150px]] [[Image:Butane-negative-gauche-side-3D-balls.png|150px]]</div>

Both conformations are free of torsional strain, but, in the gauche conformation, the two [[methyl]] groups 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 [[activation energy|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:<ref>{{cite book|title =Organic Chemistry|edition = 6|last1= McMurry|first1= J.E.|publisher= Brooks Cole |year=2003|isbn = 978-0534000134}}</ref>
* Gauche, conformer – 3.8 kJ/mol
* Eclipsed H and CH<sub>3</sub> – 16 kJ/mol
* Eclipsed CH<sub>3</sub> and CH<sub>3</sub> – 19 kJ/mol.
The eclipsed [[methyl group]]s exert a greater steric strain because of their greater [[electron density]] compared to lone [[hydrogen]] atoms.

[[image:Butane conformers.svg|400px|thumb|center|Relative energies of conformations of butane with respect to rotation of the central C-C bond.]]

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, 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 [[hyperconjugation]] as analyzed within the Natural Bond Orbital framework.<ref>{{cite journal | last1=Pophristic | first1=Vojislava | last2=Goodman | first2=Lionel | title=Hyperconjugation not steric repulsion leads to the staggered structure of ethane | journal=Nature | volume=411 | issue=6837 | date=2001 | issn=1476-4687 | doi=10.1038/35079036 | pages=565–568 | pmid=11385566 | bibcode=2001Natur.411..565P | url=http://www.nature.com/nature/journal/v411/n6837/abs/411565a0.html | url-access=subscription }}</ref><ref>{{cite journal | last=Weinhold | first=Frank | title=A new twist on molecular shape | journal=Nature | publisher=Springer Science and Business Media LLC | volume=411 | issue=6837 | year=2001 | issn=0028-0836 | doi=10.1038/35079225 | pages=539–541| pmid=11385553 }}</ref><ref>{{cite journal | last=Weinhold | first=Frank | title=Rebuttal to the Bickelhaupt–Baerends Case for Steric Repulsion Causing the Staggered Conformation of Ethane | journal=Angewandte Chemie International Edition | volume=42 | issue=35 | date=2003-09-15 | issn=1433-7851 | doi=10.1002/anie.200351777 | pages=4188–4194 |url=https://www.researchgate.net/publication/229639507}}</ref> In the staggered conformation, one C-H [[sigma bond|sigma]] [[bonding orbital]] donates electron density to the [[antibonding orbital]] of 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 theory]] shows that 2-orbital-4-electron (steric) repulsions are dominant over hyperconjugation.<ref>{{cite journal | last1=Bickelhaupt | first1=F. Matthias | last2=Baerends | first2=Evert Jan | title=The Case for Steric Repulsion Causing the Staggered Conformation of Ethane | journal=Angewandte Chemie | volume=115 | issue=35 | date=2003-09-15 | issn=0044-8249 | doi=10.1002/ange.200350947 | pages=4315–4320 | bibcode=2003AngCh.115.4315B | language=de}}</ref> A [[valence bond theory]] study also emphasizes the importance of steric effects.<ref>{{cite journal | last1=Mo | first1=Yirong | last2=Wu | first2=Wei | last3=Song | first3=Lingchun | last4=Lin | first4=Menghai | last5=Zhang | first5=Qianer | last6=Gao | first6=Jiali | title=The Magnitude of Hyperconjugation in Ethane: A Perspective from Ab Initio Valence Bond Theory | journal=Angewandte Chemie International Edition | publisher=Wiley | volume=43 | issue=15 | date=2004-03-30 | issn=1433-7851 | doi=10.1002/anie.200352931 | pages=1986–1990| pmid=15065281 }}</ref>

===Nomenclature===
Naming alkanes per standards listed in the [[IUPAC Gold Book]] is done according to the [[Klyne–Prelog system]] for specifying angles (called either torsional or [[dihedral angles]]) between substituents around a single bond:<ref name =torsion>{{GoldBookRef|title=torsion angle|file=T06406| accessdate = 2015-10-29 }}</ref>
[[Image:Synantipericlinal.svg|200px|right|syn/anti peri/clinal]]
* 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''' (c)
* a torsion angle between 0° and ±30° or ±150° and 180° is called '''periplanar''' (p)
* a torsion angle between 0° and ±30° is called '''[[Anti-periplanar|synperiplanar]]''' (sp), also called '''syn-''' or '''cis-''' conformation
* a torsion angle between 30° to 90° and −30° to −90° is called '''synclinal''' (sc), also called '''gauche''' or '''skew'''<ref name="GoldbookGauche">{{GoldBookRef|title=gauche|file=G02593| accessdate = 2008-02-27 }}</ref> 
* a torsion angle between 90° and 150° or −90° and −150° is called '''anticlinal''' (ac)
* a torsion angle between ±150° and 180° is called '''[[Anti-periplanar|antiperiplanar]]''' (ap), also called '''anti-''' or '''trans-''' conformation
[[Strain (chemistry)#Torsional strain|Torsional strain]] or "Pitzer strain" refers to resistance to twisting about a bond.

===Special cases===
In [[n-pentane|''n''-pentane]], the terminal [[methyl]] groups experience additional [[pentane interference]]. {{citation needed|date=January 2025}}

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 spectroscopy]] and [[circular dichroism]] in solution.<ref>''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; [https://dx.doi.org/10.1021/ja0602041 Graphical abstract]</ref>