1,3-Cyclohexadiene is a highly flammable cycloalkene

Cycloalkene

A cycloalkene or cycloolefin is a type of alkene hydrocarbon which contains a closed ring of carbon atoms, but has no aromatic character. Some cycloalkenes, such as cyclobutene and cyclopentene, can be used as monomers to produce polymer chains. Unless the rings are very large, cycloalkenes are...

 that occurs as a colorless clear liquid. Its refractive index is 1.475 (20 °C, D).

It can be used as a hydrogen donor in transfer hydrogenation

Transfer hydrogenation

Transfer hydrogenation is the addition of hydrogen to a molecule from a source other than gaseous H2. It is applied in industry and in organic synthesis, in part because of the inconvenience and expense of using gaseous H2...

, since its conversion to benzene + hydrogen is in fact exothermic (about 20 kJ/mol in the gas phase, as indicated by heats of hydrogenation).
Despite this apparent instability with respect to benzene, the 1,3-cyclohexadiene structure is found in several natural products such as α-terpinene

Terpinene

The terpinenes are three isomeric hydrocarbons that are classified as terpenes. They each have the same molecular formula and carbon framework, but they differ in the position of carbon-carbon double bonds. α-Terpinene has been isolated from cardamom and marjoram oils, and from other natural...

.

Compared with 1,4-cyclohexadiene

1,4-Cyclohexadiene

1,4-Cyclohexadiene is a highly flammable cycloalkene that occurs as a colorless clear liquid.1,4-Cyclohexadiene and related compounds may be prepared from benzene using lithium or sodium in liquid ammonia, this process being known as a Birch reduction. However 1,4-cyclohexadiene is easily oxidised...

, 1,3-cyclohexadiene is about 1.6 kJ/mol less stable, due to the less favourable electronic distribution.