Toluene

Structure, Properties, Preparation and Reactions of Toluene

Toluene is also called as toluol, is an aromatic hydrocarbon. 

It is first isolated in 1837 by distillation of pine oil by a Polish chemist named Filip Walter. 

It is colorless, water-insoluble and has a smell associated with paint thinners.

Its chemical formula is C7H8; it's molar mass is 92.14 g mol-1. The molecule is a phenyl ring (from benzene), and a methyl group is present as a substituent.

The molecule is planar due to the necessity of sp2 hybridization carbon to support the aromatic effect of the ring. 

The chemical structure can be written as shown below: -



Its IUPAC systematic name is methylbenzene. Toluene is mainly used as an industrial feedstock and solvent.

Toluene can also be used as a recreational inhalant sometimes, and it also has the potential of causing severe neurological harm.


Physical properties: -

Molecular Weight 92.14
Boiling Point 110.7 °C
Melting Point -95 °C
Vapor Density 3.2 (air = 1)
Vapor Pressure 36.7 mm Hg at 30 °C
Density/Specific Gravity 0.866 at 20/4 °C (water = 1)

It is a colorless liquid, highly flammable, and has a sweet, pungent aromatic odor.

Toluene is less dense than water. Therefore, it floats on water. Toluene vapors are heavier than air.
It is present in crude oils and is a product of oil-refining processes; it is used in aviation and automotive fuels. It is also used as a solvent to make other chemicals. 

Toluene is liquid at standard conditions. However, if heated it becomes gas when cooled it becomes a solid. The phase diagram for toluene shows the phase behavior with changes in pressure and temperature. 

Temperature [ °C]

At the critical point, no change of state is present when heat is added or even if the pressure is increased.
 The triple point of a substance can be defined as the temperature and pressure at which the three phases (solid, liquid, and gas) of the substance coexists at thermodynamic equilibrium.

Chemical properties

Reactions of Toluene are similar to the reactions of normal aromatic hydrocarbon in electrophilic aromatic substitution. The reason behind this is because the methyl group present in toluene has greater electron-releasing properties than hydrogen atom present in the same position. The methyl group is more reactive than benzene towards electrophiles. It undergoes sulfonation to produce p-toluene sulfonic acid, and chlorination by Cl2 in the presence of FeCl2 to give para and ortho isomers of chlorotoluene.

Most importantly, the methyl side chain in toluene is susceptible to oxidation. It reacts with Potassium permanganate to give benzoic acid, and with chromyl chloride to yield benzaldehyde, i.e., Étard reaction.
Under free radical conditions, the methyl group undergoes halogenation. For example, N-Bromo succinimide (NBS) when heated with toluene in the presence of AIBN leads to benzyl bromide. Toluene can also be brominated by treating it with HBr and H2O2 in the presence of light.

C6H5CH3 + Br2 → C6H5CH2Br + HBr

 C6H5CH2Br + Br2 → C6H 5CHBr2 + HBr

The methyl group in it undergoes deprotonation with very strong bases; the pKa is estimated to be approximately 41. Hydrogenation of it gives methylcyclohexane. The reaction requires high pressure of hydrogen and catalyst.

 Production 

It is present at low levels in crude oil. Toluene may also be found as a by-product of gasoline. It is also a by-product of the production of coke from coal. Final purification and separation of it are done by any of the solvent or distillation extraction processes used for BTX aromatics (Benzene, Toluene, and Xylene isomers).


Laboratory preparation

Toluene is inexpensively produced industrially that it is not prepared in the laboratory. prepared by a, For example, benzene reacts with methyl chloride in the presence of a Lewis acid such as aluminum chloride to give toluene:

C6H5H + CH3Cl → C6H5CH3 + HCl

Such reactions are complex ones by polymethylation as toluene is more susceptible to alkylation than is benzene.
It is also prepared by alkylation of benzene, using methanol:



COMMERCIAL PREPARATION OF TOLUENE

From coal tar: -

light oil fraction of coal-tar is The primary source of commercial production of toluene. The light oil fraction is washed with conc.H2SO4in order to remove the bases present in it, then with NaOH to remove acidic substances and finally with water.

It is subjected to fractional distillation. The vapors collected between 80 – 110oC is 90% benzol it contains 70 - 80% benzene and 14 - 24% toluene. 90% benzol is again distilled, and the part distilling between 108 – 1100C is collected as toluene.

PREPARATION OF TOLUENE FROM METHYLCYCLOHEXANE AND N- HEPTANE

It is also obtained by cyclization of n-heptane followed by an aromatization.


  Reactions of toluene

  • 1. Oxidation of Toluene

  • As toluene is an aromatic compound, it is less susceptible towards oxidation reaction. The methyl group of toluene is a side chain in the aromatic ring structure and is oxidized to the carboxyl group in the presence of a strong oxidizing agent.
    The oxidation of toluene forms benzaldehyde which can further be oxidized to form benzoic acid. There are many oxidizing agents like potassium permanganate.



  • 2. Bromination of toluene

  • The reaction of toluene with bromine is known as bromination of toluene. The bromination of it can take place either on the side chain or an aromatic ring. 

    Both bromination reactions occur with a different mechanism. Generally, side chain by free radical mechanism and aromatic bromination follows electrophilic substitution mechanism.


    Nitration of Toluene

    Introduction of a nitro group into toluene forms ortho-toluene & para-toluene and the reaction is called as nitration of toluene.

    The reaction follows the electrophilic substitution mechanism, and the mixture of concentrated sulfuric and nitric acid behaves as a nitrating agent.

    In this case, concentrated sulfuric acid acts as a catalyst and generates nitronium ion which behaves as an electrophile.

    Nitronium ion attacks on aromatic ring majorly at ortho and para position which further form ortho and para-products.



    Due to the presence of a methyl group on the ring of toluene, the nitration of toluene is around twenty-five times faster than benzene. As the methyl group is an activating towards –ortho and -para directing group, hence the nitration of toluene gives poly substituted nitro-products. 

    However, the use of low temperature can prevent the substitution of more than one nitro group on the aromatic ring.
    Note

    Under normal conditions, toluene gives all three isomers, out of which ortho-derivative forms around 63 % and 34% of para-product and 3% of meta-product is formed.



    High yield of ortho product can be explained by the resonating structure of arenium ion which forms as an intermediate
    .

      Uses and application of toluene: - 

  • • It is mostly used as a solvent in industries and laboratories.

  • • Moreover, it is used as a precursor of the reaction of substitution and oxidation to perform ready-to-use products or another precursor.

  • • It is also added to gasoline to improve the octane ratings, and it is an essential combustible in aviation. Toluene is widely used to produce paints, coats, and resins.

  • • Toluene is used as a precursor to benzene via hydrodealkylation:

  • C6H5CH3 + H2 → C6H6 + CH4


  • • The second-ranked application involves its disproportionation to a mixture of xylene and benzene.

  • • Trinitrotoluene explosive synthesis requires toluene. Toluene is nitrated to dinitrotoluene, and then nitrated to trinitrotoluene.

  • • Toluene is needed for dinitrotoluene, which is required for toluene diisocyanate, which is used to produce polyurethane foams.

  • • Toluene is a universal solvent, e.g. for paints, paint thinners, rubber, disinfectants, silicone sealants, many chemical reactants, lacquers, adhesives (glues), leather tanners, and printing ink.

  • Toxicology and metabolism of toluene
  • • Toluene is an irritator to eyes. inhaling in large quantities can be toxic to the eyes. It is a potential carcinogen agent.

  • • Inhalation of it in low to moderate levels can cause hearing loss, tiredness, nausea, weakness, loss of appetite, drunken-type actions, confusion, memory loss, and, and color vision loss.

  • • When the exposure is inhibited Some of these symptoms generally disappear. Inhaling high levels of toluene in a short time can cause nausea, light-headedness, unconsciousness, or sleepiness, and even death.

  • • When compared to benzene, Toluene is much less toxic, thus it is used largely. It replaces the latter as an aromatic solvent in chemical preparation.

  • • According to the US Environmental Protection Agency (EPA), toluene has a carcinogenic potential and it cannot be evaluated due to insufficient information.

  • • Similar to many other solvents such as some alkylbenzenes and, 1,1,1-trichloroethane toluene has been shown to act as a GABAA receptor positive allosteric modulator and non-competitive NMDA receptor antagonist

  • • Additionally, toluene has been shown to display antidepressant-like effects in rodents in the forced swim test (FST) and the tail suspension test (TST), likely due to its NMDA antagonist properties.

  • • Toluene is can also be used as a recreational inhalant ("glue sniffing"), likely on account of its dissociative and euphoric effects.

  • • Toluene inhibits excitatory ion channels including the nicotinic acetylcholine receptors (nAChRs) and N-methyl-D-aspartate (NMDA) glutamate. It potentiates the function of inhibitory ion channels such as the glycine, gamma-aminobutyric acid receptor type A, and serotonin receptors. Also, toluene disrupts voltage-gated calcium channels and ATP-gated ion channels.