Cycloheptatriene

Cycloheptatriene (CHT) is an organic compound with the formula C₇H₈. It is a closed ring of seven carbon atoms joined by three double bonds (as the name implies) and four single bonds. This colourless liquid has been of recurring theoretical interest in organic chemistry. It is a ligand in organometallic chemistry and a building block in organic synthesis. Cycloheptatriene is not aromatic, as reflected by the nonplanarity of the methylene bridge (-CH₂-) with respect to the other atoms; however the related tropylium cation is.

Cycloheptatriene

Names
Preferred IUPAC name Cyclohepta-1,3,5-triene[1]
Other names 1,3,5-Cycloheptatriene 1H-[7]Annulene CHT Tropilidene
Identifiers
CAS Number	544-25-2 Y
3D model (JSmol)	Interactive image
Beilstein Reference	506066
ChEBI	CHEBI:37519 Y
ChemSpider	10534 Y
ECHA InfoCard	100.008.061
EC Number	208-866-3
Gmelin Reference	1943
PubChem CID	11000
UNII	P58Q106NTF Y
UN number	2603
CompTox Dashboard (EPA)	DTXSID5073909
InChI InChI=1S/C7H8/c1-2-4-6-7-5-3-1/h1-6H,7H2 Y Key: CHVJITGCYZJHLR-UHFFFAOYSA-N Y InChI=1/C7H8/c1-2-4-6-7-5-3-1/h1-6H,7H2 Key: CHVJITGCYZJHLR-UHFFFAOYAI
SMILES C1=C\C/C=C\C=C1
Properties
Chemical formula	C7H8
Molar mass	92.141 g·mol−1
Appearance	Colourless liquid
Density	0.888 g/mL at 25 °C
Melting point	−80 °C (−112 °F; 193 K)
Boiling point	116 °C (241 °F; 389 K)
Solubility in water	Insoluble in water
Acidity (pKa)	36
Hazards
GHS labelling:
Pictograms
Signal word	Danger
Hazard statements	H225, H301, H304, H311, H315, H319, H335
Precautionary statements	P210, P233, P240, P241, P242, P243, P261, P264, P270, P271, P280, P301+P310, P302+P352, P303+P361+P353, P304+P340, P305+P351+P338, P312, P321, P322, P330, P331, P332+P313, P337+P313, P361, P362, P363, P370+P378, P403+P233, P403+P235, P405, P501
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Y verify (what is YN ?) Infobox references

Synthesis

Albert Ladenburg first generated cycloheptatriene in 1881 by the decomposition of tropine.^[2][3] The structure was finally proven by the synthesis of Richard Willstätter in 1901. This synthesis started from cycloheptanone and established the seven membered ring structure of the compound.^[4]

Cycloheptatriene can be obtained in the laboratory by photochemical reaction of benzene with diazomethane or the pyrolysis of the adduct of cyclohexene and dichlorocarbene.^[5] A related classic synthesis for cycloheptatriene derivatives, the Buchner ring enlargement, starts with the reaction of benzene with ethyl diazoacetate to give the corresponding norcaradiene ethyl ester, which then undergoes a thermally-allowed electrocyclic ring expansion to give 1,3,5-cycloheptatriene 7-carboxylic acid ethyl ester.^[6][7]

The Buchner ring expansion, a classical synthesis for cycloheptatriene derivatives

• Selected cycloheptatriene derivatives
• 
  Heptalene – composed of two fused cycloheptatriene rings.
• 
  Azulene – composed of fused cyclopentadiene and cycloheptatriene rings.
• 
  Sesquifulvalene – composed of linked cyclopentadiene and cycloheptatriene rings.
• 
  Elassovalene – composed of one cycloheptatriene and two fused cyclopentene rings.
• 
  Tropone – composed of cycloheptatriene ring and carbonyl group.
• 
  Tropolone – composed of cycloheptatriene ring, carbonyl and hydroxy groups.
• 
  Hinokitiol – composed of cycloheptatriene ring, isopropyl, carbonyl and hydroxy groups (isopropyl cycloheptatrienolone).

Reactions

Removal of a hydride ion from the methylene bridge gives the planar and aromatic cycloheptatriene cation, also called the tropylium ion. A practical route to this cation employs PCl₅ as the oxidant.^[8] CHT behaves as a diene in Diels–Alder reactions, for example with maleic anhydride:^[9]

Many metal complexes of cycloheptatriene are known, including Cr(CO)₃(C₇H₈) ^[10] and cycloheptatrienemolybdenum tricarbonyl.^[11]

Structure of Mo(C₇H₈)(CO)₃.

Cyclooctatetraene and cycloheptatriene are used as a triplet quencher for rhodamine 6G dye lasers.^[12][13]

See also

• Cyclopentadiene
• Cyclooctatetraene
• Benzene

References

1. Nomenclature of Organic Chemistry : IUPAC Recommendations and Preferred Names 2013 (Blue Book). Cambridge: The Royal Society of Chemistry. 2014. p. 223. doi:10.1039/9781849733069-FP001. ISBN 978-0-85404-182-4.
2. A. Ladenburg (1883). "Die Constitution des Atropins". Justus Liebig's Annalen der Chemie. 217 (1): 74–149. doi:10.1002/jlac.18832170107.
3. A. Ladenburg (1881). "Die Zerlegung des Tropines". Berichte der Deutschen Chemischen Gesellschaft. 14 (2): 2126–2131. doi:10.1002/cber.188101402127.
4. Richard Willstätter (1901). "Synthesen in der Tropingruppe. I. Synthese des Tropilidens". Justus Liebig's Annalen der Chemie. 317 (2): 204–265. doi:10.1002/jlac.19013170206.
5. H.E. Winberg (1959). "Synthesis of Cycloheptatriene". Journal of Organic Chemistry. 24 (2): 264–265. doi:10.1021/jo01084a635.
6. Buchner, et al., Ber., 18, 2377 (1885);
7. For a variation: Irvin Smith Lee; Tawney Pliny O (1934). "Studies on the Polymethylbenzenes. IX. Addition of Ethyl Diazoacetate to Durene". J. Am. Chem. Soc. 56 (10): 2167–2169. doi:10.1021/ja01325a054.
8. Conrow, K. (1973). "Tropylium Fluoroborate" (PDF). Organic Syntheses, Collected Volume. 5: 1138.
9. Ishitobi, Hiroyuki; Tanida, Hiroshi; Tori, Kazuo; Tsuji, Teruji (1971). "Re-examination of the Cycloaddition of Cycloheptatriene with Maleic Anhydride". Bulletin of the Chemical Society of Japan. 44 (11): 2993–3000. doi:10.1246/bcsj.44.2993.
10. James H. Rigby, Kevin R. Fales (2004). "7α-ACETOXY-(1Hβ, 6Hβ)-BICYCLO[4.4.1]UNDECA-2,4,8-TRIENE VIA CHROMIUM-MEDIATED HIGHER ORDER CYCLOADDITION". Organic Syntheses; Collected Volumes, vol. 10, p. 1.
11. Green, Malcolm L. H.; Ng, Dennis K. P. (1995). "Cycloheptatriene and -enyl Complexes of the Early Transition Metals". Chemical Reviews. 95 (2): 439–473. doi:10.1021/cr00034a006.
12. Tomi Nath Das; K. Indira Priyadarsini (1994). "Triplet of Cyclooctatetraene : Reactivity and Properties". Journal of the Chemical Society, Faraday Transactions. 90 (7): 963–968. doi:10.1039/ft9949000963.
13. R. Pappalardo; H. Samelson; A. Lempicki (1970). "Long Pulse Laser Emission From Rhodamine 6G Using Cyclooctatetraene". Applied Physics Letters. 16 (7): 267–269. Bibcode:1970ApPhL..16..267P. doi:10.1063/1.1653190.