The isotoluenes in organic chemistry

Organic chemistry

Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, composition, reactions, and preparation of carbon-based compounds, hydrocarbons, and their derivatives...

 are the non-aromatic

Aromaticity

In organic chemistry, Aromaticity is a chemical property in which a conjugated ring of unsaturated bonds, lone pairs, or empty orbitals exhibit a stabilization stronger than would be expected by the stabilization of conjugation alone. The earliest use of the term was in an article by August...

 toluene

Toluene

Toluene, formerly known as toluol, is a clear, water-insoluble liquid with the typical smell of paint thinners. It is a mono-substituted benzene derivative, i.e., one in which a single hydrogen atom from the benzene molecule has been replaced by a univalent group, in this case CH3.It is an aromatic...

 isomer

Isomer

In chemistry, isomers are compounds with the same molecular formula but different structural formulas. Isomers do not necessarily share similar properties, unless they also have the same functional groups. There are many different classes of isomers, like stereoisomers, enantiomers, geometrical...

s with an exocyclic double bond

Double bond

A double bond in chemistry is a chemical bond between two chemical elements involving four bonding electrons instead of the usual two. The most common double bond, that between two carbon atoms, can be found in alkenes. Many types of double bonds between two different elements exist, for example in...

. They are of some academic interest in relation to aromaticity and isomerisation

Isomerisation

In chemistry isomerisation is the process by which one molecule is transformed into another molecule which has exactly the same atoms, but the atoms are rearranged e.g. A-B-C → B-A-C . In some molecules and under some conditions, isomerisation occurs spontaneously...

 mechanisms.

The three basic isotoluenes are ortho-isotoluene or 5-methylene-1,3-cyclohexadiene (here labelled 1); para-isotoluene (2); and meta-isotoluene (3). Another structural isomer is the bicyclic compound 5-methylenebicyclo[2.2.0] hexene (4).

The o- and p-isotoluenes isomerise to toluene, a reaction driven by aromatic stabilisation. It is estimated that these compounds are 23 kcal/mol less stable.

The isomerisation of p-isotoluene to toluene takes place at 100 °C in 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....

 with bimolecular reaction kinetics by an intermolecular free radical reaction

Free radical reaction

A free radical reaction is any chemical reaction involving free radicals. This reaction type is abundant in organic reactions.Two pioneering studies into free radical reactions have been the discovery of the triphenylmethyl radical by Moses Gomberg and the lead-mirror experiment described by...

. The intramolecular isomerisation, a 1,3-sigmatropic reaction

Sigmatropic reaction

A sigmatropic reaction in organic chemistry is a pericyclic reaction wherein the net result is one σ-bond is changed to another σ-bond in an uncatalyzed intramolecular process. The name sigmatropic is the result of a compounding of the long-established sigma designation from single carbon-carbon...

 , is unfavorable because a antarafacial mode is enforced. Other dimer radical reaction products are formed as well.

The ortho-isomer is found to isomerise at 60°C in benzene, also in a second order reaction. The proposed reaction mechanism

Reaction mechanism

In chemistry, a reaction mechanism is the step by step sequence of elementary reactions by which overall chemical change occurs.Although only the net chemical change is directly observable for most chemical reactions, experiments can often be designed that suggest the possible sequence of steps in...

 is a concerted

Concerted reaction

In chemistry, a concerted reaction is a chemical reaction in which all bond breaking and bond making occurs in a single step. Reactive intermediates or other unstable high energy intermediates are not involved. Concerted reaction rates tend not to depend on solvent polarity ruling out large buildup...

 intermolecular ene reaction

Ene reaction

The Ene reaction is a chemical reaction between an alkene with an allylic hydrogen and a compound containing a multiple bond , in order to form a new σ-bond with migration of the ene double bond and 1,5 hydrogen shift. The product is a substituted alkene with the double bond shifted to the...

. The reaction product is either toluene or a mixture of dimerized ene reaction products, depending on the exact reaction conditions.
Ortho-isotoluene has been researched in connection with the mechanism of initiator

Radical initiator

In chemistry, radical initiators are substances that can produce radical species under mild conditions and promote radical reactions . These substances generally possess weak bonds—bonds that have small bond dissociation energies. Radical initiators are utilized in industrial processes such...

-free polymerization

Polymerization

In polymer chemistry, polymerization is a process of reacting monomer molecules together in a chemical reaction to form three-dimensional networks or polymer chains...

 of polystyrene

Polystyrene

Polystyrene ) also known as Thermocole, abbreviated following ISO Standard PS, is an aromatic polymer made from the monomer styrene, a liquid hydrocarbon that is manufactured from petroleum by the chemical industry...

.