What is Toluene?

by Lucy Bell-Young

Toluene is mainly used as a chemical solvent, especially in paint thinners. As a solvent, it has many important industrial applications, such as being a feedstock of various products like rubber and printing ink. 

Toluene is a colourless liquid that is highly flammable and has dissociative anaesthetic effects. Because of this property, some people use it as a recreational inhalant. However, toluene is toxic when inhaled at high concentrations, causing severe neurological problems. 

What is the Chemical Name for Toluene?Chemical structure and information on toluene

Systematic and other names

While toluene is the preferred chemical name for this substance, its systematic IUPAC (International Union of Pure & Applied Chemistry) name is methylbenzene. The systematic name is more descriptive because it mentions the two molecular groups that comprise the compound. It is also known by other names, like phenyl methane, toluol, and anisen.

Toluene is a colourless aromatic hydrocarbon, which means that it has a benzene ring constituent. Chemically, it’s a mono-substituted benzene derivative with a methyl group (CH₃) attached to a phenyl group. It has the chemical formula C7H8.

What is the Density of Toluene?

Toluene has a density of 0.87 g/mL at 20°C. In comparison, water has a density of 9.97 g/mL. Toluene’s relative low density compared to water is one reason why it will not mix with water; the other reason being that it’s nonpolar.

This may seem ironic considering that a molecule of toluene is heavier than a water molecule. While toluene has a molar mass of 92.141 g·mol−1, water has a molar mass of 18.01528 g·mol−1

The molar mass of any compound can be estimated by adding together the atomic masses of the elemental composition of a molecule. For example, water has two hydrogen atoms with a total mass of 1.008 each and one oxygen atom with an atomic weight of 15.999. Doing simple arithmetic calculations will give us the mass of a molecule of water:

1.008 (2)Hydrogen + 15.999 (1)Oxygen = 18.015

Although density is directly proportional to mass, the space or volume occupied by the molecules of toluene is proportionally larger than the volume occupied by the molecules of water. Therefore, the former has lower density compared to the latter. Put simply, this means that toluene floats on water.

What is the Chemical Structure of Toluene?

Toluene is an aromatic hydrocarbon that has a benzene ring (called a phenyl group when it’s a substituent) with a methyl group attached to it. It’s the simplest aromatic hydrocarbon next to benzene. The molecular structure of this compound is illustrated below:

Chemical structure of toluene

The systematic nomenclature of aromatic hydrocarbons is based on the attachments and the position of the attachments. You can include the prefixes ortho, meta, and para based on the positions of the attachments. For example:

Aromatic hydrocarbon compounds with single substituents can be named by using phenyl as the prefix, followed by the name of the attached hydrocarbon. For example, toluene is also called phenyl methane. Similarly, an aromatic hydrocarbon with an ethylene attachment can be called phenylethyne, with the molecular structure illustrated as:


Phenylethyne molecular structure

On the other hand, when it comes to multiple substituents on a benzene ring, the substituents are given the lowest possible numbers. You can use the common nomenclature when there are only two attachments to a benzene ring: 

  • ortho (o-) (1-2 placement)
  • meta (m-) (1-3 placement)
  • para (p-) (1-4 placement)

For example, ortho xylene, also written as o-Xylene, has the following molecular structure:


o-Xylene molecular structure

What Are the Hazards of Toluene?

The toxicity of toluene isn’t as high as the toxicity of benzene, which is a carcinogen. Although there’s no evidence that indicates that toluene is carcinogenic, exposure to this hydrocarbon still has a lot of major hazards associated with it, including:

  • Flammability
  • Aspiration hazard
  • Skin irritation
  • Neurological hazard when inhaled
  • Threat to the reproductive system
  • Potential congenital defects for unborn babies
  • Environmental hazard against marine organisms

The global production of toluene is from oil refineries and as a byproduct of ethylene crackers. It’s also used as the basis for the production of other aromatic hydrocarbons. As an additive to gasoline, and as the primary ingredient in paints, lacquers, and various types of resins, we can be exposed to toluene in a number of ways. 

Low to moderate inhalation of toluene can result in the following symptoms:

  • Tiredness or fatigue
  • Confusion
  • Weakness
  • Feeling of drunkenness, like a loss of motor coordination
  • Memory loss
  • Nausea and vomiting
  • Loss of appetite
  • Hearing loss
  • Colour vision loss
Lab technician wearing white mask PPE
Toluene is hazardous if inhaled, so always ensure you wear the proper PPE when handling this substance

Exposure to high concentrations of this hydrocarbon can result in drowsiness, unconsciousness or even death. People exposed to high concentrations of toluene must be properly ventilated and taken to hospital for immediate medical treatment.

In terms of its environmental impact, bioremediation can be used to degrade toluene. Some types of fungi can even biodegrade this hydrocarbon, preventing further ecological damage. These fungi include:

  • Cladophialophora
  • Exophiala
  • Leptodontium
  • Pseudeurotium zonatum
  • Cladosporium sphaerospermum


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