Our present world could not exist without hydrocarbons. Nearly everything we touch today is either comprised of or coated with hydrocarbon products. In the practice of clinical toxicology, initial efforts usually entail precisely determining the specific xenobiotics that might be involved in a specific exposure, followed by defining the type and extent of the exposure. In this regard, hydrocarbon exposures are always clinically challenging. Despite significant chemical diversity, most classification schemes group hydrocarbons by specific uses or applications, rather than by chemical structure or physiologic properties. Most hydrocarbons in everyday use, such as gasoline, charcoal starter, and lamp oil, are actually mixtures of chemicals obtained from a common distillation fraction. The chemical diversity within a mixture makes it challenging to try to assess individual contribution to toxicity. As a result, generalities are often needed to describe the behavior of these complex mixtures. This chapter focuses primarily on toxicity of hydrocarbons present in such mixtures. Individual hydrocarbons are addressed only when they are commonly available in purified form, or when individual xenobiotics present unique toxicologic concerns.
Organic chemistry originated in the industrial revolution and evolved largely because of advances in coal tar technology. Coal liberates hydrocarbons in the coking process, when bituminous (soft) coal is heated to remove coal gas. The gas is then separated into a variety of natural gases. The viscous residue from the heating process forms coal tar, which can be further distilled into kerosene and other hydrocarbon mixtures.
Over the years, crude oil has replaced coal tar as the most common source for distillation of organic compounds. Crude oil distillation involves heating the oil to fixed temperatures in large-scale processors that separate hydrocarbons into fractions by vapor (boiling) point. Because of the relationship between boiling point and molecular weight, distillation roughly divides hydrocarbons into like-sized molecules. The most volatile fractions come off early, as gases. These are used primarily as heating fuels. The least volatile fractions (larger than about 10 carbons) are used chiefly for lubricants or as paraffins, petroleum jelly, or asphalt. The remaining volatile distillation fractions (C5 to C10) are the ones most commonly used in combustion fuels and as solvents. Petroleum distillates are also used as chemical feedstocks and as precursors or intermediates for production. The many contemporary applications of petroleum distillates in consumer and household products include paints and thinners, furniture polish, lamp oils, and lubricants (Table 106–1).
Table 106–1. Classification and Viscosity of Common Hydrocarbons |Favorite Table|Download (.pdf)
Table 106–1. Classification and Viscosity of Common Hydrocarbons
|Compound||Common Uses||Viscosity (SUS)a|
|Gasoline||Motor vehicle fuel||30|
|Naphtha||Charcoal lighter fluid||29|
|Mineral spirits||Paint and varnish thinner||30–35|
|Mineral seal oil||Furniture polish||30–35|
|Heavy fuel oil||Heating oil||>450|
|Benzene||Solvent, reagent, gasoline additive||31|
|Toluene||Solvent, spray paint solvent||28|
|Xylene||Solvent, paint thinner, ...|