تشكل كيميائي

Rotation about single bond of butane to interconvert one conformation to another. The gauche conformation on the right is a conformer, while the eclipsed conformation on the left is a transition state between conformers. Above: Newman projection; below: depiction of spatial orientation.

في الكيمياء، التشكل الكيميائي هو الترتيب في الفضاء للذرات في الجزيء. فالجزيئات يمكن أن ترتبط بها الذرات بنفس الطريقة, ولكن بشكل مختلف في الفضاء, وتسمي مثل هذه الظاهرة متزامرات تشكيلية ( conformational isomers)" أو متشاكلات (conformers). المتشاكلات المختلفة يمكن لهم أن يتحولوا لبعض عن طريق الدوران حول الرابطة المفردة. بدون كسر هذه الرابطة. ونظرا لأن عمليات الدوران لها حواجز طاقة مختلفة, فإن بعض المتشاكلات يكون أثبت من الأخرى. ويمكن ملاحظة ذلك في البروتين, حيث يوج أحد أشكال البروتين ثابت وفعال, بينما الشكل الأخر عكس ذلك. أيضا هناك مثال أخر تقليدي, وهو البيوتان. فإن له 6 تشكيلات محتملة, بينما 3 فقط منهم ثابت.


الأنواع

IUPAC definition
rotamer: One of a set of conformers arising from restricted rotation about one single bond.[1]
Relative conformation energy diagram of butane as a function of dihedral angle.[2] A: antiperiplanar, anti or trans. B: synclinal or gauche. C: anticlinal or eclipsed. D: synperiplanar or cis.[3]

ويمكن رسم التشكل الكيميائيى بعدة طرق. شكل رأس الحصان حيث يتم رسم الرابطة C-C بشكل مائل. تخطيط نيومان حيث يتم رسم الرابطة C-C للأمام وللخلف على مستوى الورقة, ويتم التعبير عن ذرة الكربون الأمامية بنقطة وذرة الكربون الخلفية على هيئة دائرة.


تشكل جزيء البيوتان في تشكل متماثل مائل
Sawhorse projection butane -sc.png Newman projection butane -sc.png
مسقط (شكل)
رأس الحصان 		مسقط (شكل)
نيومان
  • يتواجد التشكل الكيميائي في عديد من أنواع الجزيئات فمثلا الألكانات يمكن أن يكون لها تشكلان بين الرابطة C-C. مقارب وهو التشكل الذى يكون فيه ذرات الهيدروجين الموجودة على الكربون لها زاوية 60 درجة من ذرات الهيدروجين المقابلة لها على ذرة الكربون الأخرى. مكسوف وهو التشكل الذى تكون فيه ذرات الهيدروجين متراكبة. ويتبع كل من المقارب والمكسوف توزيع بولتزمان من أقصى المقارب لأقصى المكسوف.

الرمزان السفليان i و j يمثلان أقل وأعلى طاقة, مقارب ومكسوف. g هى عدد التشكلات الموجودة عند هذه الطاقة, الإنحلال. N تمثل توزيع الجزيئات عند تشكل معين.

Equilibrium of conformers

Equilibrium distribution of two conformers at different temperatures given the free energy of their interconversion.

Conformers generally exist in a dynamic equilibrium[4]

Three isotherms are given in the diagram depicting the equilibrium distribution of two conformers at different temperatures. At a free energy difference of 0 kcal/mol, this gives an equilibrium constant of 1, meaning that two conformers exist in a 1:1 ratio. The two have equal free energy; neither is more stable, so neither predominates compared to the other. A negative difference in free energy means that a conformer interconverts to a thermodynamically more stable conformation, thus the equilibrium constant will always be greater than 1. For example, the ΔG° for the transformation of butane from the gauche conformer to the anti conformer is −0.47 kcal/mol at 298 K.[5] This gives an equilibrium constant is about 2.2 in favor of the anti conformer, or a 31:69 mixture of gauche:anti conformers at equilibrium. Conversely, a positive difference in free energy means the conformer already is the more stable one, so the interconversion is an unfavorable equilibrium (K < 1).

Population distribution of conformers

Boltzmann distribution % of lowest energy conformation in a two component equilibrating system at various temperatures (°C, color) and energy difference in kcal/mol (x-axis)

The fractional population distribution of different conformers follows a Boltzmann distribution:[6]

The left hand side is the proportion of conformer i in an equilibrating mixture of M conformers in thermodynamic equilibrium. On the right side, Ek (k = 1, 2, ..., M) is the energy of conformer k, R is the molar ideal gas constant (approximately equal to 8.314 J/(mol·K) or 1.987 cal/(mol·K)), and T is the absolute temperature. The denominator of the right side is the partition function.

Factors contributing to the free energy of conformers

The effects of electrostatic and steric interactions of the substituents as well as orbital interactions such as hyperconjugation are responsible for the relative stability of conformers and their transition states. The contributions of these factors vary depending on the nature of the substituents and may either contribute positively or negatively to the energy barrier. Computational studies of small molecules such as ethane suggest that electrostatic effects make the greatest contribution to the energy barrier; however, the barrier is traditionally attributed primarily to steric interactions.[7][8]

Contributions to rotational energy barrier

In the case of cyclic systems, the steric effect and contribution to the free energy can be approximated by A values, which measure the energy difference when a substituent on cyclohexane in the axial as compared to the equatorial position. In large (>14 atom) rings, there are many accessible low-energy conformations which correspond to the strain-free diamond lattice.[9]

Observation of conformers

The short timescale of interconversion precludes the separation of conformer in most cases. Atropisomers are conformational isomers which can be separated due to restricted rotation.[10] The equilibrium between conformational isomers can be observed using a variety of spectroscopic techniques.[11]

Protein folding also generates conformers which can be observed. The Karplus equation relates the dihedral angle of vicinal protons to their J-coupling constants as measured by NMR. The equation aids in the elucidation of protein folding as well as the conformations of other rigid aliphatic molecules.[12] Protein side chains exhibit rotamers, whose distribution is determined by their steric interaction with different conformations of the backbone. This effect is evident from statistical analysis of the conformations of protein side chains in the Backbone-dependent rotamer library.[13]

المطيافية

Conformational dynamics can be monitored by variable temperature NMR spectroscopy. The technique applies to barriers of 8–14 kcal/mol, and species exhibiting such dynamics are often called "fluxional". For example, in cyclohexane derivatives, the two chair conformers interconvert rapidly at room temperature. The ring-flip proceeds at a rates of approximately 105 ring-flips/sec, with an overall energy barrier of 10 kcal/mol (42 kJ/mol). This barrier precludes separation at ambient temperatures.[14] However, at low temperatures below the coalescence point one can directly monitor the equilibrium by NMR spectroscopy and by dynamic, temperature dependent NMR spectroscopy the barrier interconversion.[15]

Besides NMR spectroscopy, IR spectroscopy is used to measure conformer ratios. For the axial and equatorial conformer of bromocyclohexane, νCBr differs by almost 50 cm−1.[14]

Conformation-dependent reactions

Reaction rates are highly dependent on the conformation of the reactants. In many cases the dominant product arises from the reaction of the less prevalent conformer, by virtue of the Curtin-Hammett principle. This is typical for situations where the conformational equilibration is much faster than reaction to form the product. The dependence of a reaction on the stereochemical orientation is therefore usually only visible in Configurational analysis, in which a particular conformation is locked by substituents. Prediction of rates of many reactions involving the transition between sp2 and sp3 states, such as ketone reduction, alcohol oxidation or nucleophilic substitution is possible if all conformers and their relative stability ruled by their strain is taken into account.[16]

One example where the rotamers become significant is elimination reactions, which involve the simultaneous removal of a proton and a leaving group from vicinal or antiperiplanar positions under the influence of a base.

Base-induced bimolecular dehydrohalogenation (an E2 type reaction mechanism). The optimum geometry for the transition state requires the breaking bonds to be antiperiplanar, as they are in the appropriate staggered conformation

The mechanism requires that the departing atoms or groups follow antiparallel trajectories. For open chain substrates this geometric prerequisite is met by at least one of the three staggered conformers. For some cyclic substrates such as cyclohexane, however, an antiparallel arrangement may not be attainable depending on the substituents which might set a conformational lock.[17] Adjacent substituents on a cyclohexane ring can achieve antiperiplanarity only when they occupy trans diaxial positions (that is, both are in axial position, one going up and one going down). [بحاجة لمصدر]

One consequence of this analysis is that trans-4-tert-butylcyclohexyl chloride cannot easily eliminate but instead undergoes substitution (see diagram below) because the most stable conformation has the bulky t-Bu group in the equatorial position, therefore the chloride group is not antiperiplanar with any vicinal hydrogen (it is gauche to all four). The thermodynamically unfavored conformation has the t-Bu group in the axial position, which is higher in energy by more than 5 kcal/mol (see A value).[18] As a result, the t-Bu group "locks" the ring in the conformation where it is in the equatorial position and substitution reaction is observed. On the other hand, cis-4-tert-butylcyclohexyl chloride undergoes elimination because antiperiplanarity of Cl and H can be achieved when the t-Bu group is in the favorable equatorial position.

Thermodynamically unfavored conformation of trans-4-tert-butylcyclohexyl chloride where the t-Bu group is in the axial position exerting 7-atom interactions.
The trans isomer can attain antiperiplanarity only via the unfavored axial conformer; therefore, it does not eliminate. The cis isomer is already in the correct geometry in its most stable conformation; therefore, it eliminates easily.

The repulsion between an axial t-butyl group and hydrogen atoms in the 1,3-diaxial position is so strong that the cyclohexane ring will revert to a twisted boat conformation. The strain in cyclic structures is usually characterized by deviations from ideal bond angles (Baeyer strain), ideal torsional angles (Pitzer strain) or transannular (Prelog) interactions.

Alkane stereochemistry

Alkane conformers arise from rotation around sp3 hybridised carbon–carbon sigma bonds. 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. [بحاجة لمصدر]

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. [بحاجة لمصدر]

In the example of staggered ethane in Newman projection, a hydrogen atom on one carbon atom has a 60° torsional angle or torsion angle[19] 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/mol than the eclipsed conformation, which is the energy maximum for ethane. In the eclipsed conformation the torsional angle is minimised.

staggered conformation left, eclipsed conformation right in Newman projection
Ethane-staggered-depth-cue-3D-balls.png Ethane-eclipsed-depth-cue-3D-balls.png

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).

anti vs gauche conformations
Butane-anti-side-3D-balls.png Butane-eclipsed-side-3D-balls.png Butane-negative-gauche-side-3D-balls.png

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 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:[20]

  • 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 density compared to lone hydrogen atoms.

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.[21][22][23] In the staggered conformation, one C-H 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.[24] A valence bond theory study also emphasizes the importance of steric effects.[25]

مسميات

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:[19]

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 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[26]
  • a torsion angle between 90° and 150° or −90° and −150° is called anticlinal (ac)
  • a torsion angle between ±150° and 180° is called antiperiplanar (ap), also called anti- or trans- conformation

Torsional strain or "Pitzer strain" refers to resistance to twisting about a bond.

حالات خاصة

In n-pentane, the terminal methyl groups experience additional pentane interference. [بحاجة لمصدر]

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.[27]

انظر أيضاً

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المراجع

  1. ^ "rotamer". Gold Book. IUPAC. 2014. doi:10.1351/goldbook.R05407.
  2. ^ J, McMurry (2012). Organic chemistry (8 ed.). Belmont, CA: Brooks/Cole. p. 98. ISBN 9780840054449.
  3. ^ خطأ استشهاد: وسم <ref> غير صحيح؛ لا نص تم توفيره للمراجع المسماة :0
  4. ^ Bruzik, Karol. "Chapter 6: Conformation". University of Illinois at Chicago. Archived from the original on 11 November 2013. Retrieved 10 November 2013.
  5. ^ The standard enthalpy change ΔH° from gauche to anti is –0.88 kcal/mol. However, because there are two possible gauche forms, there is a statistical factor that needs to be taken into account as an entropic term. Thus, ΔG° = ΔH° – TΔS° = ΔH° + RT ln 2 = –0.88 kcal/mol + 0.41 kcal/mol = –0.47 kcal/mol, at 298 K.
  6. ^ Rzepa, Henry. "Conformational Analysis". Imperial College London. Retrieved 11 November 2013.
  7. ^ Liu, Shubin (7 February 2013). "Origin and Nature of Bond Rotation Barriers: A Unified View". The Journal of Physical Chemistry A. 117 (5): 962–965. Bibcode:2013JPCA..117..962L. doi:10.1021/jp312521z. PMID 23327680.
  8. ^ Carey, Francis A. (2011). Organic chemistry (8th ed.). New York: McGraw-Hill. p. 105. ISBN 978-0-07-340261-1.
  9. ^ Dragojlovic, Veljko (2015). "Conformational analysis of cycloalkanes" (PDF). Chemtexts. 1 (3): 14. Bibcode:2015ChTxt...1...14D. doi:10.1007/s40828-015-0014-0. S2CID 94348487.
  10. ^ McNaught (1997). "Atropisomers". IUPAC Compendium of Chemical Terminology. Oxford: Blackwell Scientific Publications. doi:10.1351/goldbook.A00511. ISBN 978-0967855097.
  11. ^ قالب:March6th
  12. ^ Dalton, Louisa. "Karplus Equation". Chemical and Engineering News. American Chemical Society. Retrieved 2013-10-27.
  13. ^ Dunbrack, R. L.; Cohen, F. E. (1997). "Bayesian statistical analysis of protein side-chain rotamer preferences". Protein Science. 6 (8): 1661–1681. doi:10.1002/pro.5560060807. ISSN 0961-8368. PMC 2143774. PMID 9260279.
  14. ^ أ ب Eliel, E. L.; Wilen, S. H.; Mander, L. N. (1994). Stereochemistry Of Organic Compounds. J. Wiley and Sons. ISBN 978-0-471-01670-0.
  15. ^ Jensen, Frederick R.; Bushweller, C. Hackett (1969-06-01). "Separation of conformers. II. Axial and equatorial isomers of chlorocyclohexane and trideuteriomethoxycyclohexane". Journal of the American Chemical Society. 91 (12): 3223–3225. Bibcode:1969JAChS..91.3223J. doi:10.1021/ja01040a022. ISSN 0002-7863.
  16. ^ Schneider, H.-J.; Schmidt, G.; Thomas F. J. Am. Chem. Soc., 1983, 105, 3556. https://pubs.acs.org/doi/pdf/10.1021/ja00349a031
  17. ^ Rzepa, Henry S. (2014). "Cycloalkanes". Imperial College London.
  18. ^ Dougherty, Eric V. Anslyn; Dennis, A. (2006). Modern Physical Organic Chemistry (Dodr. ed.). Sausalito, CA: University Science Books. p. 104. ISBN 978-1-891389-31-3.
  19. ^ أ ب IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version:  (2006–) "torsion angle".
  20. ^ McMurry, J.E. (2003). Organic Chemistry (6 ed.). Brooks Cole. ISBN 978-0534000134.
  21. ^ Pophristic, Vojislava; Goodman, Lionel (2001). "Hyperconjugation not steric repulsion leads to the staggered structure of ethane". Nature. 411 (6837): 565–568. Bibcode:2001Natur.411..565P. doi:10.1038/35079036. ISSN 1476-4687. PMID 11385566.
  22. ^ Weinhold, Frank (2001). "A new twist on molecular shape". Nature. Springer Science and Business Media LLC. 411 (6837): 539–541. doi:10.1038/35079225. ISSN 0028-0836. PMID 11385553.
  23. ^ Weinhold, Frank (2003-09-15). "Rebuttal to the Bickelhaupt–Baerends Case for Steric Repulsion Causing the Staggered Conformation of Ethane". Angewandte Chemie International Edition. 42 (35): 4188–4194. doi:10.1002/anie.200351777. ISSN 1433-7851.
  24. ^ Bickelhaupt, F. Matthias; Baerends, Evert Jan (2003-09-15). "The Case for Steric Repulsion Causing the Staggered Conformation of Ethane". Angewandte Chemie (in الألمانية). 115 (35): 4315–4320. Bibcode:2003AngCh.115.4315B. doi:10.1002/ange.200350947. ISSN 0044-8249.
  25. ^ Mo, Yirong; Wu, Wei; Song, Lingchun; Lin, Menghai; Zhang, Qianer; Gao, Jiali (2004-03-30). "The Magnitude of Hyperconjugation in Ethane: A Perspective from Ab Initio Valence Bond Theory". Angewandte Chemie International Edition. Wiley. 43 (15): 1986–1990. doi:10.1002/anie.200352931. ISSN 1433-7851. PMID 15065281.
  26. ^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version:  (2006–) "gauche".
  27. ^ 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


المصادر

  • ويكيبيديا الإنجليزية.
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