In organic chemistry, two molecules are valence isomers when they are constitutional isomers that can interconvert through pericyclic reaction

Pericyclic reaction

In organic chemistry, a pericyclic reaction is a type of organic reaction wherein the transition state of the molecule has a cyclic geometry, and the reaction progresses in a concerted fashion. Pericyclic reactions are usually rearrangement reactions...

s.

Benzene

For example, Dewar benzene

Dewar benzene

Dewar benzene or bicyclo[2.2.0]hexa-2,5-diene is a bicyclic isomer of benzene with the molecular formula C6H6. The compound is named after James Dewar who included this structure in a list of possible C6H6 structures in 1867....

 transforms spontaneously into its valence isomer benzene

Benzene

Benzene is an organic chemical compound. It is composed of 6 carbon atoms in a ring, with 1 hydrogen atom attached to each carbon atom, with the molecular formula C6H6....

. The compounds both can be described as (CH)₆ but the arrangement of the six carbon

Carbon

Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...

-hydrogen

Hydrogen

Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

 blocks is different. Besides benzene and Dewar benzene, prismane

Prismane

Prismane is a polycyclic hydrocarbon with the formula C6H6. It is an isomer of benzene, more specific: a valence isomer. Prismane is far less stable than benzene. The carbon atoms of the prismane molecule are arranged in the shape of a six-atom triangular prism. Albert Ladenburg proposed this...

, benzvalene

Benzvalene

Benzvalene is an organic compound and one of several isomers of benzene.. It was synthesised in 1971 by Thomas J. Katz et al.The 1971 synthesis consisted of treating cyclopentadiene with methyllithium in dimethylether and then with dichloromethane and methyllithium in at -45°C. The hydrocarbon in...

 and bicyclopropenyl

Bicyclopropenyl

Bicyclopropenyl is an organic compound and one of several valence isomers of benzene. The molecule can be described as two coupled cyclopropene units. The positions of the alkene groups can vary and therefore two other isomers are known: bicycloprop-1,2-enyl and bicyclopropen-1-yl.The synthesis of...

 are also valence-isomers of the (CH)₆ family. Topologically, more arrangements in this family are possible, though they have not been attested to in chemistry. The ring strain

Ring strain

In organic chemistry, ring strain is the tendency of a cyclic molecule, such as cyclopropane, to destabilize when its atoms are in non-favorable high energy spatial orientations...

on angles or lengths of bonding prohibit their actual formation.

Cyclooctatetraene

The valence isomers are not restricted to isomers of benzene. Valence isomers are also seen in the series (CH)₈. Due to the larger number of units, the number of possible valence isomers is also greater and at least 21:

Naphthalene

Perhaps no pair of valence isomers differ more strongly in appearance than colourless naphthalene and the intensely violet azulene.