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# Toluene

Benzene is the simplest aromatic compound and all other aromatic compounds must have at least one benzene ring. There are six carbon atoms in benzene molecule which are $sp^2$ hybridized and bonded with two carbon atoms and one hydrogen atom. All bonded hydrogen atoms are equivalent and can be easily substituted by substituents. Such aromatic compounds with substituents on benzene ring are known as benzene derivatives.

Generally, benzene derivatives are better known by their common names. Hence their nomenclature is less systemic compare to other hydrocarbons like alkanes, alkene and alkynes. Apart from common names, the mono substituted benzene derivatives are names by using the group name as a prefix to ‘benzene’. For example, if there is a methyl group bonded on benzene ring, it named as methyl benzene or toluene as common name.

Some other benzene derivatives with their systematic names as common names are as follow.

 Structure Common name Systematic name Toluene Methylbenzene Aniline Benzenamine Anisole Methoxybenzene Phenol Benzenol Tosic acid p-Toluenesulfonic acid Xylene 1,4-Dimethylbenzene Mesitylene 1,3,5-Trimethyl benzene Durene 1,2,4,5-Tetramethyl benzene

## What is Toluene?

Toluene is a benzene derivative with methyl group attached on benzene ring by the replacement of one of the hydrogen atom by methyl group. It is a colorless liquid which produced by the petrochemical plants or petroleum refineries or as a side product in steel plants from coke oven gas. The chemical formula for toluene is C7H8 or C6H5CH3 with molecular mass 92.13 gm/mol.

Toluene is a typical water insoluble aromatic compound having a typical paint thinner odor. The solubility of toluene is 0.47 gm/L in water at 20-25 °C. The boiling point of toluene is 383.3 k and melting point is 178.2 K. The other physical properties of toluene are as follow.

 Physical properties Toluene Density 0.865g/cm3 Dynamic viscosity 5.6x10-4Pa s Vapor pressure 22mmHg Surface tension 0.02971N/m UV cutoff wavelength 286nm Refractive index 1.496 Critical pressure 4.09MPa Critical temperature 588k

Toluene Structure

Toluene is a methyl derivative of benzene.  All carbon atoms of benzene are sphybridized while the carbon atom of methyl group is sp3 hybridized bonded with three hydrogen atoms and one aromatic carbon atom. The carbon-carbon bond length is almost similar like in benzene; hence an
intermediate value of single and double bond and carbon-hydrogen bond length is 1.1 A$degree$.

The bond energy of carbon-hydrogen bonds is 411 kj//mol. There are two types of carbon-carbon bonds. The bond energy of carbon-carbon double bonds is 602 kj/mol while the bond energy of carbon-carbon single bond is 346 kj/mol. The methyl group on benzene ring in toluene is ortho and para directing and activating group due to hypercojugation and positive inductive effect of methyl group.

Out of ortho and para products, ortho products are formed as major products around 63%, with para 34% and 3% meta products.

Toluene Density

The density of toluene is 866.90 kg/m³ at 298 K temperature. The vapor density of toluene is 3.2 with respect to air. A two-sinker hydrostatic-balance densimeter is used for the determination of density over the temperature range 213 K to 473 K and at pressures up to 35 MPa. The density value of toluene make it valuable in many industrial applications as it is a stable chemical of relatively low toxicity with freezing point of 178 K.

## Halogenation of Toluene

Oxidation of Toluene

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

The copper-based binary metal oxides and iron-copper binary oxide are found to be the best catalyst for the liquid phase oxidation of toluene to benzaldehyde and than benzoic acid. The oxidation of benzaldehyde to benzoic acid can be prevented by the use of pyridine. Another catalyst used for gas phase selective catalytic oxidation of toluene to form Benzaldehyde in the presence of V2O5-Ag2O/-Al2Ocatalyst. The peroxide solution and trimethylacetic acid also act as good oxidising agent for toluene to form benzoic acid.

Partial oxidation of toluene in the presence of manganese dioxide and dilute sulfuric acid at 208 K temperature also forms benzaldehyde. Similarly the oxidation in the presence of chromic acid and anhydride forms benzaldehyde through benzylideneacetate.

CrO3                       H+/H2O
C6H5CH3 C6H5CH(OCO CH3)2 C6H5CHO + 2CH3COOH
(CH3CO)2O

Bromination of Toluene

The introduction of bromine in toluene is known as bromination of toluene. The bromination of toluene can be takes place either on side chain or on aromatic ring. Both bromination reactions proceed with different mechanism. Generally aromatic bromination follows electrophilic substitution mechanism and side chain by free radical mechanism.

Bromination on aromatic ring: toluene has one methyl group on aromatic group on benzene ring which imparts positive inductive effect and hyperconjugation. Because of these two effects, methyl groups increases the electron density on aromatic ring and acts as activating group. Hence toluene is an ortho and para directing for electrophilic substitution reactions like bromination.The bromination takes place in the presence of catalyst like ferric bromide (FeBr3) and follows same steps as for other electrophilic substitution reactions. The steps are as follows;
1. Formation of electrophile: Catalyst like FeBr3 reacts with bromine to forms bromonium ion which further acts as electrophile for reaction.
2. Attack of electrophile on toluene: Bromonium ion attacks on ortho and para position of toluene to forms arenium ion as an intermediate which gets stabilised through the resonance.
3. Lose of proton: Arenium ion reacts with base (FeBr4-) and lose proton to restore the aromaticity of ring with ortho-bromotoluene and para-bromotoluene as major product with little meta-product.

### Bromination on side chain

The bromination at side chain follows free radical mechanism through benzyl radical and form benzyl bromide. The side chain bromination takes place in the presence of bromine water with carbon tetrachloride or in the presence of light.

Another reagent used for side chain bromination is hydrogen bromide with hydrogen peroxide in the presence of incandescent light bulb for ten hours.

N-bromosuccinimide in the presence of carbon tetrachloride (NBS) also acts as good reagent for side chain bromination as NBS is a good brominating and oxidizing agent and used as source for bromine in radical reactions for allylic position.

## Nitration of Toluene

The introduction of nitro group in toluene forms ortho-toluene & para-toluene and reaction is called as nitration of toluene. Reaction follows electrophilic substitution mechanism and mixture of concentrated sulfuric and nitric acids acts as nitrating agent. Here concentrated sulfuric acid acts as catalyst and generates nitronium ion which acts as electrophile. Nitronium ion attacks on  aromatic ring predominately at ortho and para position which further form ortho and para-products.

Due to the presence of methyl group on aromatic ring in toluene, the nitration of toluene is around twenty five times faster than benzene. As the methyl group is an activating and –o , -p-directing group, therefore 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 aromatic ring.
Under normal conditions, toluene forms all three isomers, out of that ortho-derivative forms around 63 % with 34% of para-product and 3% of meta-product.

The high yield of ortho product can be explained by using the resonating structure of arenium ion which forms as an intermediate. During ortho and para-substitution, one of the contributing tertiary carbocation formed as an intermediate which is not formed during meta-substitution.

At high temperature, di- and trisubstituion is also takes place and ultimately form trinitrotoluene as final product which is an explosive.

## Toluene Toxicity

Toluene is a developmental toxicity hazard aromatic compound and show adverse effect on body. Some adverse effects of toluene on various organelles are as follows.
1. SKIN: Toluene is an irritating substance on skin and may cause dermatitis on prolong expose.
2. NERVOUS SYSTEM: Prolong expose can cause damage of central nervous system, tremors, dysfunction, narcosis, CNS depression, ataxia and death are main adverse effects of long expose of toluene.
3. EYES/VISION: Toluene mainly effects on color vision by persistent impairment of color vision. It is a very mild eye irritant as a very short exposure for 3-5m minutes has caused slight eye irritation at 300 ppm.
4. HEARING: The long-term exposure of very high concentrations of toluene can affect the hearing loss.
5. LIVER EFFECTS: The high level of toluene expose can effect the liver and responsible for liver injury. kidney necrosis, swollen liver, necrosis of myocardial fibers are main effects of toluene.
6. INHALATION: The inhalation of high level of toluene can cause fatigue, dizziness, drowsiness, and headache, sleepiness, irritation in nose, throat and respiratory tract.
7. Toluene also affects the pregnant women. Many adverse effects like attention deficits, CNS dysfunction, dysmorphism, limb anomalies, renal tubular acidosis, growth retardation, minor craniofacial and delay in developmental are observes in the infants.

## Toluene Sulfonic Acid

Toluene sulfonic acid is a white solid with molecular formula CH3C6H4SO3H. The systematic name of this compound is 4-methylbenzenesulfonic acid and also known as Tosylic acid, tosic acid, PTSA.
Because 4- CH3C6H4SO2- is also called as tosyl group. It is a strong acid compare to carboxylic acid and also soluble in water and alcohol. Generally it exists in the form of monohydrate (TsOH-HOH) whose molar mass is 190.22 gm/mol. Because of the presence of one water molecule as water of hydration, it physical properties gets change. For example; melting point of anhydrous acid is 311 K while for monohydrate melts at 376 K temperature.

Toluene Sulfonic acid is used in organic synthesis and as catalysts in alkylation, esterification and condensation reactions. The salts of toluene sulfonic acid are called as Sulfonates which are soluble in water and present in organic dyes. Toluene Sulfonic acid is also used in the detergent industry as largest-volume synthetic surfactant due to its low cost, good performance and due to its biodegradable environmental friendliness. Another use of Toluene sulfonic acid is in the production of toluene sulfonamide which acts as the parent material for the production of saccharin and as a raw material of flow-promoting agents for paints, nitrocellulose, hot-melt adhesives, thermosetting resins, coating materials and phenolic resins. It is also used in manufacturing of hydrazine based blowing agents like p,p' Oxybis(benzenesulfonylhydrazide), p-Toluenesulfonylhydrazide, p-Toluenesulfonyl acetone hydrazone.

Toluene sulfonic acids formed as intermediates for the synthesis of isocyanate compounds which are used as water scavengers and catalysts for the production of thermosetting resins. The esters of Toluene sulfonic acid act as alkylating agents in organic synthesis and acid itself is used as a non-oxidizing catalyst in the manufacture of plasticizers and as a curing agent for epoxy-phenolic resins.

Vinyl Toluene

On the basis of the position of vinyl group with respect to methyl group in vinyl toluene can exists in three possible isomers; -o,-p and meta isomer named as 1-ethenyl-2-methylbenzene, 1 ethenyl-3-methylbenzene and 1-ethenyl-4-methylbenzene. Vinyltoluene is produced fromdehydrogenation of ethylbenzene in the presence of zinc oxide as catalyst or by alkylation followed bu dehydrogenation of toluene. All the three isomers are liquid at STP and insoluble in water. Some other properties of vinyl toluene are as follow.

 o-vinyl toluene p-vinyl toluene m-vinyl toluene Boiling point 164.5oC 162oC 158.5oC Melting point -17oC -62oC -95.5oC Density 0.888g/cm2 0.865g/cm3 0.861g/cm3 Vapor pressure 2.62mmHg 3mmHg 3mmHg Surface tension 0.0152N/M 0.02996N/M 0.02907N/M Flash point 39.44oC 36.67oC 38.33oC

• Vinyl toluene is a colorless liquid with a strong, disagreeable odor. It is combustible in nature and commercially available as a mixture of the meta- and para-isomers with rarely, the ortho-isomer. The evaporation rate of vinyl toluene is slower than water.
• They are insoluble in water but soluble in acetone, benzene, alcohol and other organic solvents. They have tendency to get polymerized and explode in container so must be store in the presence of a polymerization inhibitor like 10 to 50 ppm of tert-butyl catechol.
• Generally peroxides, aluminum chloride, ferric chloride, strong acids, or other ionic materials act as polymerization catalyst and promote the polymerization of vinyl toluene. The fire involving vinyl toluene released toxic gases and vapor like carbon monoxide and aromatic hydrocarbons.
• Vinyl toluene is generally absorbed by inhalation or by the absorption through skin and gastrointestinal tract.
• Ingestion of ortho-, meta- and para-vinyl toluenes into male albino Wistar rats formed around 11 urinary metabolites like para-methylphenyl acetaldehyde, para-vinylbenzoic acid, para-methylmandelic acid, para-vinylbenzoyl glycine, vinyltoluene-7,8-oxide, para methylphenylglyoxylic acid, para-methylphenylacetic acid, para-methylbenzoic acid, para-methylphenylacetyl glycine, para-methylphenylethylene glycol, para-methylbenzoyl glycine.
• These metabolites are mainly excreted within 24 hours.The expose of vinyl toluene can causes irritation in eye, skin, and mucous membrane and depression in central nervous system.
• At 200 ppm, the odor of vinyl toluene is tolerable, at 300 ppm it has a strong, objectionable odors while at 400 ppm cause eye, nose, and mucous membrane irritation.
• It shows anesthetic and systemic effects at high concentrations as well as induces sister chromatid exchanges and chromosomal aberrations in human lymphocytes in vitro.

## Toluene Uses

Some major applications of toluene are listed as follows.
1. Used for the improvement of octan rating and in the production of benzene.
2. Used as a solvent in coatings, adhesives, inks, paints, synthetic fragrances, and cleaning agents.
3. Used for the manufacturing of plastic soda bottles, cosmetic nail products, nylon, and polyurethanes and for pharmaceuticals and dyes.
4. Used for the synthesis of organic chemicals like benzoic acid, benzaldehyde, benzyl chloride, saccharin, benzal chloride, benzyl alcohol etc.
5. Used in the manufacturing of trinitrotoluene (TNT).
6. It mainly used for the manufacturing of certain dyes and drugs.
7. Toluene is used as to make spray and wall paints, aviation gasoline, rubber, paint thinner, detergents, fingernail polish, lacquers, spot removers, adhesives and antifreeze.
8. Toluene also acts as an industrial solvent and used in dry cleaning, as a petrol substitute.
9. It is also used in leather tanning processes and in some printing processes.
10. Toluene acts as a precursor for many petrochemicals such as urethane foam, phenol formaldehyde, downstream petrochemicals and pesticides.

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