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Cyclobutadiene is an organic compound with the formula C4H4 It is very reactive owing to its tendency to dimerize Althou
Cyclobutadiene
Cyclobutadiene is an organic compound with the formula C4H4. It is very reactive owing to its tendency to dimerize. Although the parent compound has not been isolated, some substituted derivatives are robust and a single molecule of cyclobutadiene is quite stable. Since the compound degrades by a bimolecular process, the species can be observed by matrix isolation techniques at temperatures below 35 K. It is thought to adopt a rectangular structure.
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| Preferred IUPAC name Cyclobuta-1,3-diene | |||
| Other names 1,3-Cyclobutadiene Cyclobutadiene [4]Annulene | |||
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| Properties | |||
Chemical formula | C4H4 | ||
| Molar mass | 52.076 g·mol−1 | ||
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).  verify (what is Infobox references | |||
Structure and reactivity
The compound is the prototypical antiaromatic hydrocarbon with 4 pi electrons (or π electrons). It is the smallest [n]-annulene ([4]-annulene). Its rectangular structure is the result of a pseudo- (or second order) Jahn–Teller effect, which distorts the molecule and lowers its symmetry, converting the triplet to a singlet ground state. The electronic states of cyclobutadiene have been explored with a variety of computational methods. The rectangular structure is consistent with the existence of two different 1,2-dideutero-1,3-cyclobutadiene valence isomers. This distortion indicates that the pi electrons are localized, in agreement with Hückel's rule which predicts that a π-system of 4 electrons is not aromatic.
In principle, another situation is possible. Namely, cyclobutadiene could assume an undistorted square geometry, if it adopts a triplet spin state. In this case the molecule will be aromatic in agreement with Baird's rule. While a theoretical possibility, the triplet form of the parent cyclobutadiene and its substituted derivatives remained elusive for decades. However, in 2017, the square triplet excited state of 1,2,3,4-tetrakis(trimethylsilyl)-1,3-cyclobutadiene was observed spectroscopically, and a singlet-triplet gap of EST = 13.9 kcal/mol (or 0.6 eV per molecule) was measured for this compound.
Synthesis
Several cyclobutadiene derivatives have been isolated with steric bulky substituents. Orange tetrakis (tert-butyl)cyclobutadiene arises by thermolysis of its isomer tetra-tert-butyltetrahedrane. Although the cyclobutadiene derivative is stable (with respect to dimerization), it decomposes upon contact with O2.
Trapping
Samples of cyclobutadiene are unstable since the compound dimerizes at temperatures above 35 K by a Diels-Alder reaction. By suppressing bimolecular decomposition pathways, cyclobutadiene is well-behaved. Thus it has been generated in a hemicarceplex. The inclusion compound is generated by photodecarboxylation of bicyclopyran-2-one. When released from the host–guest complex, cyclobutadiene dimerizes and then converts to cyclooctatetraene.
After numerous attempts, cyclobutadiene was first generated by oxidative degradation of cyclobutadieneiron tricarbonyl with ammonium cerium(IV) nitrate. When liberated from the iron complex, cyclobutadiene reacts with electron-deficient alkynes to form a Dewar benzene:
The Dewar benzene converts to dimethyl phthalate on heating at 90 °C.
One cyclobutadiene derivative is also accessible through a [2+2]cycloaddition of a di-alkyne. In this particular reaction, the trapping reagent is 2,3,4,5-tetraphenylcyclopenta-2,4-dienone and one of the final products (after expulsion of carbon monoxide) is a cyclooctatetraene:
See also
- Butadiene
- Cyclobutene
References
- Kollmar, H.; Staemmler, V. (1977). "A theoretical study of the structure of cyclobutadiene". Journal of the American Chemical Society. 99 (11): 3583–3587. doi:10.1021/ja00453a009.
- Cram, Donald J.; Tanner, Martin E.; Thomas, Robert (1991). "The Taming of Cyclobutadiene". Angewandte Chemie International Edition in English. 30 (8): 1024–1027. doi:10.1002/anie.199110241.
- Albright, Burdett and Whangbo, Orbital Interactions in Chemistry 2nd ed. pp 282ff.
- Peter Senn (1992). "A Simple Quantum Mechanical Model that Illustrates the Jahn-Teller Effect". J. Chem. Educ. 69 (10): 819. Bibcode:1992JChEd..69..819S. doi:10.1021/ed069p819.
- Balkova, A.; Bartlett, R. J. J. Chem. Phys. 1994, 101, 8972–8987.
- Kostenko, Arseni; Tumanskii, Boris; Kobayashi, Yuzuru; Nakamoto, Masaaki; Sekiguchi, Akira; Apeloig, Yitzhak (2017-07-03). "Spectroscopic Observation of the Triplet Diradical State of a Cyclobutadiene". Angewandte Chemie International Edition. 56 (34): 10183–10187. doi:10.1002/anie.201705228. ISSN 1433-7851. PMID 28635054.
- Günther Maier; Stephan Pfriem; Ulrich Schäfer; Rudolf Matusch (1978). "Tetra-tert-butyltetrahedrane". Angew. Chem. Int. Ed. Engl. 17 (7): 520. doi:10.1002/anie.197805201.
- Hermann Irngartinger; Norbert Riegler; Klaus-Dieter Malsch; Klaus-Albert Schneider; Günther Maier (1980). "Structure of Tetra-tert-butylcyclobutadiene". Angewandte Chemie International Edition in English. 19 (3): 211–212. doi:10.1002/anie.198002111.
- Carey, Francis A.; Sundberg, Richard J. (2007). Advanced Organic Chemistry: Part A: Structure and Mechanisms (5th ed.). Springer. p. 725. ISBN 978-0-387-44897-8.
- E. J. Corey, Jacques Streith (1964). "Internal Photoaddtion Reactions of 2-Pyrone and N-Methyl-2-pyridone: A New Synthetic Approach to Cyclobutadiene". J. Am. Chem. Soc. 86 (5): 950–951. doi:10.1021/ja01059a059.
- G. F. Emerson; L. Watts; R. Pettit (1965). "Cyclobutadiene- and Benzocyclobutadiene-Iron Tricarbonyl Complexes". J. Am. Chem. Soc. 87: 131–133. doi:10.1021/ja01079a032.
- R. Pettit; J. Henery (1970). "Cyclobutadieneiron tricarbonyl". Organic Syntheses. 50: 21. doi:10.15227/orgsyn.050.0021.
- L. Watts; J. D. Fitzpatrick; R. Pettit (1965). "Cyclobutadiene". J. Am. Chem. Soc. 87 (14): 3253–3254. doi:10.1021/ja01092a049.
- Chung-Chieh Lee; Man-kit Leung; Gene-Hsiang Lee; Yi-Hung Liu; Shie-Ming Peng (2006). "Revisit of the Dessy-White Intramolecular Acetylene-Acetylene [2 + 2] Cycloadditions" (PDF). J. Org. Chem. 71 (22): 8417–8423. doi:10.1021/jo061334v. PMID 17064014. S2CID 10744108.
Further reading
- Wu, Judy I-Chia; Mo, Yirong; Evangelista, Francesco Alfredo; von Ragué Schleyer, Paul (2012). "Is cyclobutadiene really highly destabilized by antiaromaticity?". Chemical Communications. 48 (67): 8437–9. doi:10.1039/c2cc33521b. ISSN 1359-7345. PMID 22801355. S2CID 19920318.
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Cyclobutadiene is an organic compound with the formula C4H4 It is very reactive owing to its tendency to dimerize Although the parent compound has not been isolated some substituted derivatives are robust and a single molecule of cyclobutadiene is quite stable Since the compound degrades by a bimolecular process the species can be observed by matrix isolation techniques at temperatures below 35 K It is thought to adopt a rectangular structure Cyclobutadiene Cyclobutadiene Space filling model NamesPreferred IUPAC name Cyclobuta 1 3 dieneOther names 1 3 Cyclobutadiene Cyclobutadiene 4 AnnuleneIdentifiersCAS Number 1120 53 2 Y3D model JSmol Interactive imageChEBI CHEBI 33657 YChemSpider 120626 YPubChem CID 136879UNII I5G5583N4F YCompTox Dashboard EPA DTXSID00149807InChI InChI 1S C4H4 c1 2 4 3 1 h1 4H YKey HWEQKSVYKBUIIK UHFFFAOYSA N YInChI 1 C4H4 c1 2 4 3 1 h1 4HKey HWEQKSVYKBUIIK UHFFFAOYAISMILES C1 CC C1PropertiesChemical formula C 4H 4Molar mass 52 076 g mol 1Except where otherwise noted data are given for materials in their standard state at 25 C 77 F 100 kPa Y verify what is Y N Infobox referencesStructure and reactivityThe compound is the prototypical antiaromatic hydrocarbon with 4 pi electrons or p electrons It is the smallest n annulene 4 annulene Its rectangular structure is the result of a pseudo or second order Jahn Teller effect which distorts the molecule and lowers its symmetry converting the triplet to a singlet ground state The electronic states of cyclobutadiene have been explored with a variety of computational methods The rectangular structure is consistent with the existence of two different 1 2 dideutero 1 3 cyclobutadiene valence isomers This distortion indicates that the pi electrons are localized in agreement with Huckel s rule which predicts that a p system of 4 electrons is not aromatic In principle another situation is possible Namely cyclobutadiene could assume an undistorted square geometry if it adopts a triplet spin state In this case the molecule will be aromatic in agreement with Baird s rule While a theoretical possibility the triplet form of the parent cyclobutadiene and its substituted derivatives remained elusive for decades However in 2017 the square triplet excited state of 1 2 3 4 tetrakis trimethylsilyl 1 3 cyclobutadiene was observed spectroscopically and a singlet triplet gap of EST 13 9 kcal mol or 0 6 eV per molecule was measured for this compound SynthesisSeveral cyclobutadiene derivatives have been isolated with steric bulky substituents Orange tetrakis tert butyl cyclobutadiene arises by thermolysis of its isomer tetra tert butyltetrahedrane Although the cyclobutadiene derivative is stable with respect to dimerization it decomposes upon contact with O2 TrappingSamples of cyclobutadiene are unstable since the compound dimerizes at temperatures above 35 K by a Diels Alder reaction By suppressing bimolecular decomposition pathways cyclobutadiene is well behaved Thus it has been generated in a hemicarceplex The inclusion compound is generated by photodecarboxylation of bicyclopyran 2 one When released from the host guest complex cyclobutadiene dimerizes and then converts to cyclooctatetraene After numerous attempts cyclobutadiene was first generated by oxidative degradation of cyclobutadieneiron tricarbonyl with ammonium cerium IV nitrate When liberated from the iron complex cyclobutadiene reacts with electron deficient alkynes to form a Dewar benzene The Dewar benzene converts to dimethyl phthalate on heating at 90 C One cyclobutadiene derivative is also accessible through a 2 2 cycloaddition of a di alkyne In this particular reaction the trapping reagent is 2 3 4 5 tetraphenylcyclopenta 2 4 dienone and one of the final products after expulsion of carbon monoxide is a cyclooctatetraene See alsoButadiene CyclobuteneReferencesKollmar H Staemmler V 1977 A theoretical study of the structure of cyclobutadiene Journal of the American Chemical Society 99 11 3583 3587 doi 10 1021 ja00453a009 Cram Donald J Tanner Martin E Thomas Robert 1991 The Taming of Cyclobutadiene Angewandte Chemie International Edition in English 30 8 1024 1027 doi 10 1002 anie 199110241 Albright Burdett and Whangbo Orbital Interactions in Chemistry 2nd ed pp 282ff Peter Senn 1992 A Simple Quantum Mechanical Model that Illustrates the Jahn Teller Effect J Chem Educ 69 10 819 Bibcode 1992JChEd 69 819S doi 10 1021 ed069p819 Balkova A Bartlett R J J Chem Phys 1994 101 8972 8987 Kostenko Arseni Tumanskii Boris Kobayashi Yuzuru Nakamoto Masaaki Sekiguchi Akira Apeloig Yitzhak 2017 07 03 Spectroscopic Observation of the Triplet Diradical State of a Cyclobutadiene Angewandte Chemie International Edition 56 34 10183 10187 doi 10 1002 anie 201705228 ISSN 1433 7851 PMID 28635054 Gunther Maier Stephan Pfriem Ulrich Schafer Rudolf Matusch 1978 Tetra tert butyltetrahedrane Angew Chem Int Ed Engl 17 7 520 doi 10 1002 anie 197805201 Hermann Irngartinger Norbert Riegler Klaus Dieter Malsch Klaus Albert Schneider Gunther Maier 1980 Structure of Tetra tert butylcyclobutadiene Angewandte Chemie International Edition in English 19 3 211 212 doi 10 1002 anie 198002111 Carey Francis A Sundberg Richard J 2007 Advanced Organic Chemistry Part A Structure and Mechanisms 5th ed Springer p 725 ISBN 978 0 387 44897 8 E J Corey Jacques Streith 1964 Internal Photoaddtion Reactions of 2 Pyrone and N Methyl 2 pyridone A New Synthetic Approach to Cyclobutadiene J Am Chem Soc 86 5 950 951 doi 10 1021 ja01059a059 G F Emerson L Watts R Pettit 1965 Cyclobutadiene and Benzocyclobutadiene Iron Tricarbonyl Complexes J Am Chem Soc 87 131 133 doi 10 1021 ja01079a032 R Pettit J Henery 1970 Cyclobutadieneiron tricarbonyl Organic Syntheses 50 21 doi 10 15227 orgsyn 050 0021 L Watts J D Fitzpatrick R Pettit 1965 Cyclobutadiene J Am Chem Soc 87 14 3253 3254 doi 10 1021 ja01092a049 Chung Chieh Lee Man kit Leung Gene Hsiang Lee Yi Hung Liu Shie Ming Peng 2006 Revisit of the Dessy White Intramolecular Acetylene Acetylene 2 2 Cycloadditions PDF J Org Chem 71 22 8417 8423 doi 10 1021 jo061334v PMID 17064014 S2CID 10744108 Further readingWu Judy I Chia Mo Yirong Evangelista Francesco Alfredo von Rague Schleyer Paul 2012 Is cyclobutadiene really highly destabilized by antiaromaticity Chemical Communications 48 67 8437 9 doi 10 1039 c2cc33521b ISSN 1359 7345 PMID 22801355 S2CID 19920318