Cyclohexa-1,3-diene

Cyclohexa-1,3-diene is an organic compound with the formula (CH2)2(CH)4. It is a colorless, flammable liquid. Its refractive index is 1.475 (20 °C, D). A naturally occurring derivative of 1,3-cyclohexadiene is terpinene, a component of pine oil.

Cyclohexa-1,3-diene
Names
IUPAC name
Cyclohexa-1,3-diene
Other names
1,3-Cyclohexadiene, 1,2-Dihydrobenzene, 1,3-CHD
Identifiers
3D model (JSmol)
506024
ChEBI
ChemSpider
ECHA InfoCard 100.008.878
EC Number
  • 209-764-1
1657
RTECS number
  • GU4702350
UNII
UN number 1993
Properties
C6H8
Molar mass 80.13 g/mol
Appearance Colorless liquid
Density 0.841 g/cm3
Melting point −98 °C (−144 °F; 175 K)
Boiling point 80 °C (176 °F; 353 K)
-48.6·10−6 cm3/mol
Hazards
GHS pictograms
GHS Signal word Danger
GHS hazard statements
 H225, H335
P210, P233, P240, P241, P242, P243, P261, P271, P280, P303+361+353, P304+340, P312, P370+378, P403+233, P403+235, P405, P501
Flash point 26 °C (79 °F; 299 K) c.c.
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Synthesis

Cyclohexadiene is prepared by the dehydrobromination of 1,2-dibromocyclohexane:[1]

(CH2)4(CHBr)2 + 2 NaH → (CH2)2(CH)4 + 2 NaBr + 2 H2

1,3-Cyclohexadiene can also be prepared by the electrocyclic reaction from benzene either photochemically or at temperatures above 110 °C.[2]

Reactions

Useful reactions of this diene are cycloadditions, such as the Diels-Alder reaction.[3]

Conversion of cyclohexa-1,3-diene to benzene + hydrogen is exothermic by about 25 kJ/mol (gas phase).[4][5]

cyclohexane → cyclohexa-1,3-diene + 2 H2 ΔH +231.5 kJ/mol (endothermic)
cyclohexane → benzene + 3 H2 ΔH +205 kJ (endothermic)
cyclohexa-1,3-diene → benzene + H2 ΔH -26.5 kJ (exothermic)

Compared with its isomer cyclohexa-1,4-diene, cyclohexa-1,3-diene is about 1.6 kJ/mol more stable.[6]

Cyclohexadiene and its derivatives form metal-alkene complexes. Illustrative is [C6H8)Fe(CO)3], an orange liquid. This complex reacts with hydride-abstracting reagents to give the cyclohexadienyl derivative [C6H7)Fe(CO)3]+.[7] Cyclohexadienes react with ruthenium trichloride to give (Benzene)ruthenium dichloride dimer.

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See also

References
  1. John P. Schaefer and Leland Endres (1967). "1,3-Cyclohexadiene". Organic Syntheses. 47: 31.; Collective Volume, 5, p. 285
  2. Roberts, Stanley (1995). Comprehensive Organic Functional Group Transformations: Synthesis: Carbon with No Attached Heteroatoms. Cambridge, UK: Elsevier Science Ltd. p. 419. ISBN 0080423221.
  3. Sanjeeva Rao Guppi, George A. O'Doherty, "1,3-Cyclohexadiene" Encyclopedia of Reagents for Organic Synthesis, 2008 John Wiley & Sons. doi:10.1002/047084289X.rn00921
  4. US National Institute of Standards and Technology, NIST Chemistry WebBook 1,3-Cyclohexadiene Benzene
  5. J. Sherman The heats of hydrogenation of unsaturated hydrocarbons Archived 2011-07-14 at the Wayback Machine Journal of the American Oil Chemists' Society; Volume 16, Number 2 / February, 1939
  6. NIST Chemistry WebBook 1,4-Cyclohexadiene
  7. Pearson, Anthony J.; Sun, Huikai (2008). "Cyclohexadieneiron Tricarbonyl". e-EROS Encyclopedia of Reagents for Organic Synthesis. doi:10.1002/047084289X.rn00791.
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