Anti-periplanar

{{Short description|Bond arrangement in organic chemistry}} {{Use dmy dates|date=November 2017}}

In organic chemistry, '''anti-periplanar''', or '''antiperiplanar''', describes the {{chem2|A\sB\sC\sD}} bond angle in a molecule. In this conformer, the dihedral angle of the {{chem2|A\sB}} bond and the {{chem2|C\sD}} bond is greater than +150° or less than −150°<ref>{{cite book|last1=Eliel|first1=Ernest|last2=Wilen|first2=Samuel|last3=Mander|first3=Lewis|title=Stereochemistry of Organic Compounds|date=September 1994|publisher=Wiley-Scientific|location=New York|isbn=978-0-471-01670-0}}</ref> (Figures 1 and 2). Anti-periplanar is often used in textbooks to mean strictly anti-coplanar,<ref>{{cite journal|last1=Kane|first1=Saul|last2=Hersh|first2=William|title=Periplanar or Coplanar?|journal=Journal of Chemical Education|date=1 October 2000|volume=77|issue=10|page=1366|doi=10.1021/ed077p1366|bibcode=2000JChEd..77.1366K }}</ref> with an {{chem2|A\sB}} {{chem2|C\sD}} dihedral angle of 180° (Figure 3). In a Newman projection, the molecule will be in a staggered arrangement with the anti-periplanar functional groups pointing up and down, 180° away from each other (see Figure 4). Figure 5 shows 2-chloro-2,3-dimethylbutane in a sawhorse projection with chlorine and a hydrogen anti-periplanar to each other.

'''Syn-periplanar''' or '''synperiplanar''' is similar to anti-periplanar. In the syn-periplanar conformer, the A and D are on the same side of the plane of the bond, with the dihedral angle of {{chem2|A\sB}} and {{chem2|C\sD}} between +30° and −30° (see Figure 2).

{|style="margin: 0 auto;" | thumb|Figure 1: Functional groups A and D are anti-periplanar | thumb|Figure 2: Functional groups are considered periplanar if they have a dihedral angle less than −150° or greater than +150° or −30° to +30°. Adapted from a figure by Dreamtheater published on Wikimedia Commons.<ref>{{Citation|last=Wikipedia|first=Dreamtheater at English|title=English: An illustration of the syn/anti peri/clinal nomenclature of molecular torsional conformations. To be used on the page Alkane stereochemistry.|date=9 August 2012|url=https://commons.wikimedia.org/wiki/File:Synantipericlinal.svg|accessdate=2017-03-17}}</ref> | thumb|Figure 3: Representation of a strictly anti-coplanar conformation. A, B, C, and D are in the same plane and the dihedral angle between A–B and C–D is 180°. | thumb|Figure 4: Newman projection showing A and D anti-periplanar. | thumb|Figure 5: Sawhorse projection of 2-chloro-2,3-dimethylbutane showing Cl and H anti-periplanar. |}

== Molecular orbitals== An important factor in the antiperiplanar conformer is the interaction between molecular orbitals. Anti-periplanar geometry will put a bonding orbital and an anti-bonding orbital approximately parallel to each other, or syn-periplanar. Figure 6 is another representation of 2-chloro-2,3-dimethylbutane (Figure 5), showing the C–H bonding orbital, σ_C–H, and the C–Cl anti-bonding orbital, σ*_C–Cl, syn-periplanar. The parallel orbitals can overlap and become involved in hyperconjugation. If the bonding orbital is an electron donor and the anti-bonding orbital is an electron acceptor, then the bonding orbital will be able to donate electronegativity into the anti-bonding orbital. This filled-to-unfilled donor-acceptor interaction has an overall stabilizing effect on the molecule. However, donation from a bonding orbital into an anti-bonding orbital will also result in the weakening of both of those bonds. In Figure 6, 2-chloro-2,3-dimethylbutane is stabilized through hyperconjugation from electron donation from σ_C-H into σ*_C-Cl, but both C–H and C–Cl bonds are weakened. A molecular orbital diagram shows that the mixing of σ_C–H and σ*_C–Cl in 2-chloro-2,3-dimethylbutane lowers the energy of both the orbitals (Figure 7).

{|style="margin: 0 auto;" | thumb|Figure 6: The C–H bonding orbital is aligned with the anti-bonding orbital of C–Cl and can donate into the anti-bonding orbital through hyperconjugation. | thumb|Figure 7: The energy of both the C–H bonding orbital and the C–Cl anti-bonding orbital lower when they mix. |}

== Examples of anti-periplanar geometry in mechanisms== === E₂ mechanism=== A bimolecular elimination reaction will occur in a molecule where the breaking carbon-hydrogen bond and the leaving group are anti-periplanar<ref>{{cite book|last1=Wade|first1=Leroy|title=Organic Chemistry|url=https://archive.org/details/organicchemistry00wade_531|url-access=limited|date=6 January 2012|publisher=Pearson|isbn=978-0321768414|pages=[https://archive.org/details/organicchemistry00wade_531/page/n304 267]–268|edition=8th}}</ref><ref>{{cite book|last1=Carey|first1=Francis|last2=Sundberg|first2=Richard|title=Advanced Organic Chemistry: Part A: Structure and Mechanisms|url=https://archive.org/details/advancedorganicc00care_636|url-access=limited|date=27 May 2008|publisher=Springer|isbn=978-0387683461|pages=[https://archive.org/details/advancedorganicc00care_636/page/n583 558]–563|edition=5th}}</ref><ref>{{cite journal|last1=Deslongchamps|first1=Ghislain|last2=Deslongchamps|first2=Pierre|title=Bent bonds, the antiperiplanar hypothesis and the theory of resonance. A simple model to understand reactivity in organic chemistry|journal=Organic & Biomolecular Chemistry|date=12 May 2011|volume=9|issue=15|pages=5321–5333|doi=10.1039/C1OB05393K|pmid=21687842}}</ref><ref>{{cite web|last1=Hunt|first1=Ian|last2=Spinney|first2=Rick|title=Chapter 5: Structure and Preparation of Alkenes. Elimination Reactions|url=http://www.chem.ucalgary.ca/courses/351/Carey5th/Ch05/ch5-6-1.html|accessdate=13 March 2017}}</ref> (Figure 8). This geometry is preferred because it aligns σ_C-H and σ*_C-X orbitals.<ref>{{cite book|last1=Anslyn|first1=Eric|last2=Dougherty|first2=Dennis|title=Modern Physical Organic Chemistry|url=https://archive.org/details/modernphysicalor00ansl|url-access=limited|date=15 July 2005|publisher=University Science|isbn=978-1891389313|pages=[https://archive.org/details/modernphysicalor00ansl/page/n617 590]–592}}</ref><ref>{{cite web|last1=Rzepa|first1=Henry|title=An orbtial analysis of the stereochemistry of the E2 elimination reaction|url=http://www.ch.imperial.ac.uk/rzepa/blog/?p=6205|accessdate=13 March 2017|date=2012-02-04}}</ref> Figure 9 shows the σ_C-H orbital and the σ*_C-X orbital parallel to each other, allowing the σ_C-H orbital to donate into the σ*_C-X anti-bonding orbital through hyperconjugation. This serves to weaken C-H and C-X bond, both of which are broken in an E₂ reaction. It also sets up the molecule to more easily move its σ_C-H electrons into a π_C-C orbital (Figure 10).

{|style="margin: 0 auto;" | thumb|Figure 8: In an E₂ mechanism, the breaking C–H bond and the leaving group are often anti-periplanar. In the Figure B is a general base and X is a leaving group. | thumb|Figure 9: The C–H bonding orbital is mixing with the C–X anti-bonding orbital through hyperconjugation. | thumb|Figure 10: In an E₂ mechanism molecules generally prefer an anti-periplanar geometry because it aligns molecular orbitals and sets up the molecule to move electrons in a C–H bonding orbital into a π_C-C bonding orbital. |}

=== Pinacol rearrangement=== thumb|Figure 11: Mechanism of a pinacol rearrangement. The C–C bonding orbital is aligned with the C–O anti-bonding orbital, which facilitates the methyl shift. H–A is a generic acid. In the pinacol rearrangement, a methyl group is found anti-periplanar to an activated alcohol functional group.<ref>{{cite book|last1=Anslyn|first1=Eric|last2=Dougherty|first2=Dennis|title=Modern Physical Organic Chemistry|url=https://archive.org/details/modernphysicalor00ansl|url-access=limited|date=15 July 2005|publisher=University Science|isbn=978-1891389313|pages=[https://archive.org/details/modernphysicalor00ansl/page/n703 676]–677}}</ref><ref>{{cite book|last1=Carey|first1=Francis|last2=Sundberg|first2=Richard|title=Advanced Organic Chemistry: Part B: Reactions and Synthesis|url=https://archive.org/details/advancedorganicc00care_201|url-access=limited|date=30 December 2010|publisher=Springer|isbn=978-0387683546|pages=[https://archive.org/details/advancedorganicc00care_201/page/n898 883]–886|edition=5th}}</ref> This places the σ_C–C orbital of the methyl group parallel with the σ*_C–O orbital of the activated alcohol. Before the activated alcohol leaves as H₂O the methyl bonding orbital donates into the C–O antibonding orbital, weakening both bonds. This hyperconjugation facilitates the 1,2-methyl shift that occurs to remove water. See Figure 11 for the mechanism.

== History, etymology, and misuse== The term anti-periplanar was first coined by Klyne and Prelog in their work entitled "Description of steric relationships across single bonds", published in 1960.<ref>{{cite journal|last1=Klyne|first1=William|last2=Prelog|first2=Vladimir|title=Description of steric relationships across single bonds|journal=Experientia|date=1 December 1960|volume=16|issue=12|pages=521–523|doi=10.1007/BF02158433|s2cid=48829 }}</ref> ‘Anti’ refers to the two functional groups lying on opposite sides of the plane of the bond. ‘Peri’ comes from the Greek word for ‘near’ and so periplanar means “approximately planar”.<ref>{{cite journal|last1=Kane|first1=Saul|last2=Hersh|first2=William|title=Periplanar or Coplanar?|journal=Journal of Chemical Education|date=1 October 2000|volume=77|issue=10|page=1366|doi=10.1021/ed077p1366|bibcode=2000JChEd..77.1366K }}</ref> In their article “Periplanar or Coplanar?” Kane and Hersh point out that many organic textbooks use anti-periplanar to mean completely anti-planar, or anti-coplanar, which is technically incorrect.<ref>{{cite journal|last1=Kane|first1=Saul|last2=Hersh|first2=William|title=Periplanar or Coplanar?|journal=Journal of Chemical Education|date=1 October 2000|volume=77|issue=10|page=1366|doi=10.1021/ed077p1366|bibcode=2000JChEd..77.1366K }}</ref>

==References== {{reflist}}

Category:Organic chemistry