Cyclohexa-1,3-diene (also known as Benzane) is an organic compound with the formula (C2H4)(CH)4. It is a colorless, flammable liquid. Its refractive index is 1.475 (20 °C, D). It is one of two isomers of cyclohexadiene, the other being 1,4-cyclohexadiene.
Cyclohexadiene is prepared by the double dehydrobromination of 1,2-dibromocyclohexane:[1]
(CH2)4(CHBr)2 + 2 NaH → (CH2)2(CH)4 + 2 NaBr + 2 H2
Useful reactions of this diene are cycloadditions, such as the Diels-Alder reaction.[2]
Conversion of cyclohexa-1,3-diene to benzene + hydrogen is exothermic by about 25 kJ/mol in the gas phase.[3] [4]
cyclohexane → cyclohexa-1,3-diene + 2 H2 (ΔH = +231.5 kJ/mol; endothermic)
cyclohexane → benzene + 3 H2 (ΔH = +205 kJ/mol; endothermic)
cyclohexa-1,3-diene → benzene + H2 (ΔH = -26.5 kJ/mol; exothermic)
Compared with its isomer cyclohexa-1,4-diene, cyclohexa-1,3-diene is about 1.6 kJ/mol more stable.[5]
Cyclohexadiene and its derivatives form (diene)iron tricarbonyl complexes. Illustrative is [(C<sub>6</sub>H<sub>8</sub>)Fe(CO)<sub>3</sub>], an orange liquid. This complex reacts with hydride-abstracting reagents to give the cyclohexadienyl derivative [(C<sub>6</sub>H<sub>7</sub>)Fe(CO)<sub>3</sub>]+.[6] Cyclohexadienes react with ruthenium trichloride to give (Benzene)ruthenium dichloride dimer.[7]
Cyclohexa-1,3-diene is a fairly common motif in nature.[8] One example is chorismic acid, an intermediate in the shikimic acid pathway. Of the several examples of the terpenoids and terpenes, a prominent example is phellandrene.
Coenzyme A-conjugated to the 2-position of cyclohexadiene-2-carboxylic acid is an intermediate in the biodegradation of aromatic carboxylic acids.[9]