Cycloheptatriene

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Cycloheptatriene

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Template:Longitem	Template:Unbulleted list
Template:Longitem	506066
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EC Number	Template:Unbulleted list
Template:Longitem	1943
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UN number	2603
Template:Longitem	Template:Chembox Elements/molecular formula
Molar mass	Template:Chem molar mass
Appearance	Colourless liquid
Density	0.888 g/mL at 25 °C
Melting point	Template:Chembox CalcTemperatures
Boiling point	Template:Chembox CalcTemperatures
Acidity (pKa)	36

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 (Template:Chem2) with respect to the other atoms; however the related tropylium cation is, rendering the compound relatively suspectible to oxidation.

Synthesis

Albert Ladenburg first generated cycloheptatriene in 1881 by the decomposition of tropine. 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.

Cycloheptatriene can be obtained in the laboratory by photochemical reaction of benzene with diazomethane or the pyrolysis of the adduct of cyclohexene and dichlorocarbene. 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.

File:TOC ACS1996v2.svg

The Buchner ring expansion, a classical synthesis for cycloheptatriene derivatives

• Selected cycloheptatriene derivatives
• Heptalene – composed of two fused cycloheptatriene rings.
  Heptalene – composed of two fused cycloheptatriene rings.
• Azulene – composed of fused cyclopentadiene and cycloheptatriene rings.
  Azulene – composed of fused cyclopentadiene and cycloheptatriene rings.
• Sesquifulvalene – composed of linked cyclopentadiene and cycloheptatriene rings.
  Sesquifulvalene – composed of linked cyclopentadiene and cycloheptatriene rings.
• Elassovalene – composed of one cycloheptatriene and two fused cyclopentene rings.
  Elassovalene – composed of one cycloheptatriene and two fused cyclopentene rings.
• Tropone – composed of cycloheptatriene ring and carbonyl group.
  Tropone – composed of cycloheptatriene ring and carbonyl group.
• Tropolone – composed of cycloheptatriene ring, carbonyl and hydroxy groups.
  Tropolone – composed of cycloheptatriene ring, carbonyl and hydroxy groups.
• Hinokitiol – composed of cycloheptatriene ring, isopropyl, carbonyl and hydroxy groups (isopropyl cycloheptatrienolone).
  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. CHT behaves as a diene in Diels–Alder reactions, for example with maleic anhydride:

File:Cycloheptatriene Diels-Alder reaction.svg

Many metal complexes of cycloheptatriene are known, including Cr(CO)₃(C₇H₈) and cycloheptatrienemolybdenum tricarbonyl.

File:CHTMo(CO)3.png

Structure of Mo(C₇H₈)(CO)₃.

Cyclooctatetraene and cycloheptatriene are used as a triplet quencher for rhodamine 6G dye lasers.

See also

• Cyclopentadiene
• Cyclooctatetraene
• Benzene