Cyclohexa-1,3-diene

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.

Synthesis
Cyclohexadiene is prepared by the double dehydrobromination of 1,2-dibromocyclohexane:
 * (CH2)4(CHBr)2 + 2 NaH → (CH2)2(CH)4 + 2 NaBr + 2 H2

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

Conversion of cyclohexa-1,3-diene to benzene + hydrogen is exothermic by about 25 kJ/mol in the gas phase.
 * 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.

Cyclohexadiene and its derivatives form (diene)iron tricarbonyl 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]+. Cyclohexadienes react with ruthenium trichloride to give (Benzene)ruthenium dichloride dimer.

Occurrence
Cyclohexa-1,3-diene is a fairly common motif in nature. 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.