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Arsabenzene
Other names Arsenin
Identifiers
CAS Number 289-31-6
Molecular Formula C5H5As
Appearance Very air sensitive liquid, with a distinctive onion odor. It will turn a bright red on exposure to air.
Melting Point -54 oC
Boiling Point <25 oC


Arsabenzene is an organoarsenic compound with the chemical formula C5H5As. It belongs to a group of compounds called heteroarenes that have the general formula C5H5E (E= N, P, As, Sb, Bi). [1]

This air sensitive liquid has an onion odor, [2] and it decomposes on heating. [3] Arsabenzene is also an ambidentate ligand, and it prefers to coordinate using η1(P)- or η6(π)-routes. [4]

The study of arsabenzene and related compounds was an important step in the understanding of compounds that contain multiple bonds between carbon and heavier elements. [5]

The study of heteroarenes was begun by Märkl, with the synthesis of 2, 4, 6-triphenlphosphabenzene. This is achieved by treating 2, 4, 6-trisubstituted pyrylium salt with phosphanes. [6] The first derivative of arsabenzene was 9-arsaanthracene prepared by Jutzi and Bickelhaupt. [7]


Synthesis

Arsabenzene is synthesized in a two step process from 1,4-pentadiyne. The diyne reacts with dibutylstannane to give 1,1-dibutylstannacyclohexa-2,5-diene. [8] The organotin compound undergoes As/Sn exchange with arsenic trichloride to give 1-chloroacyclohexadiene, which loses a HCl upon heating, forming the arsabenzene. [9]

CH2(CHCH)2SnBu2 + AsCl3 → CH2(CHCH)2AsCl + Bu2SnCl2
CH2(CHCH)2AsCl → C5H5As + HCl


Reactions

Arsabenzene undergoes electrophilic aromatic substitution at its ortho and para positions. It is also capable of undergoing Friedel Crafts acylation. [10]

While pyridine cannot undergo a Diels-Alder reaction, arsabenzene does behave as a diene and will readily react with hexafluoro-2-butyne at room temperature, in contrast phosphabenzene will only undergo this reaction at 100 oC and benzene has to be forced (200 oC) to undergo a Diels Alder reaction with hexafluorobut-2-yne. Thus the ability of these heterobenzenes to undergo the Diels Alder reaction with this electrophile increases down the periodic table. Bismabenzene is so reactive that it is capable of dimerizing with itself and it exists in equilibrium with its dimer. [11]

Arsabenzene is far less basic than pyridine and unreactive with Lewis acids. There is no detectable protonation of the molecule even with trifluoroacetic or other strong acids. [12]


Strucure

Arsabenzene is planar. The C-C bond distances of 1.39 Å, the As-C bond has a length of 1.85 Å, this is 6.6% shorter than the normal As-C single bond. [13]

NMR spectroscopy carried out on arsabenzene indicates that it has a diamagnetic ring current. [14]


References

  1. ^ Elschenbroich, C. Organometallics, 3. 229-230. 2006 WILEY-VCH Weinheim, Germany. ISBN 3527293906
  2. ^ Cadogan, J. I. G.; Buckingham, J.; Macdonald, F. Dictionary of Organic Compounds, 10 (6). 491. 1997 CRC Press ISBN 0412541106
  3. ^ Elschenbroich, C. Organometallics, 3. 229-230. 2006 WILEY-VCH Weinheim, Germany. ISBN 3527293906
  4. ^ Sadimenko, A. P. Organometallic Complexes of B-, Si- (Ge-), and P- (As-, Sb-) Analogues of Pyridine. Advances in Heterocyclic Chemistry, 89, 125-157. DOI: 10.1016/S0065-2725(05)89003-8
  5. ^ Eicher, T.; Hauptmann, S.; Suschitzky, H.; Suschitzky, J. The chemistry of heterocycles: structure, reactions, syntheses, and applications (2). 368. WILEY-VCH Weinheim, Germany. ISBN 3527307206
  6. ^ Eicher, T.; Hauptmann, S.; Suschitzky, H.; Suschitzky, J. The chemistry of heterocycles: structure, reactions, syntheses, and applications (2). 368. WILEY-VCH Weinheim, Germany. ISBN 3527307206
  7. ^ Ashe, A. J. The Group 5 heterobenzenes. Accounts of Chemical Research, 1978, 11(4) 153-157. DOI: 10.1021/ar50124a005
  8. ^ Elschenbroich, C. Organometallics, 3. 229-230. 2006 WILEY-VCH Weinheim, Germany. ISBN 3527293906
  9. ^ Elschenbroich, C. Organometallics, 3. 229-230. 2006 WILEY-VCH Weinheim, Germany. ISBN 3527293906
  10. ^ Cadogan, J. I. G.; Buckingham, J.; Macdonald, F. Dictionary of Organic Compounds, 10 (6). 491. 1997 CRC Press ISBN 0412541106
  11. ^ Ashe, A. J. The Group 5 heterobenzenes. Accounts of Chemical Research, 1978, 11(4) 153-157. DOI: 10.1021/ar50124a005
  12. ^ Ashe, A. J. The Group 5 heterobenzenes. Accounts of Chemical Research, 1978, 11(4) 153-157. DOI: 10.1021/ar50124a005
  13. ^ Elschenbroich, C. Organometallics, 3. 229-230. 2006 WILEY-VCH Weinheim, Germany. ISBN 3527293906
  14. ^ Ashe, A. J.; Chan, W.; Smith, T. W.; Taba, K. M. Electrophilic aromatic substitution reactions of arsabenzene. The Journal of Organic Chemistry, 1981, 46 (5), 881-885. DOI: 10.1021/jo00318a012
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