Hydrocarbon molecules are organic compounds consisting of carbon and hydrogen atoms. In petroleum crude oil, for example, these molecules may include from one to 60 or more carbon atoms. The properties of hydrocarbons depend on the number and arrangement of the carbon and hydrogen atoms in the molecules. The simplest hydrocarbon molecule is methane, with one carbon atom linked to four hydrogen atoms. All other variations of hydrocarbons evolve from this molecule.
Hydrocarbons containing up to four carbon atoms are usually gases at normal ambient temperatures and atmospheric pressure, those with five to 19 carbon atoms are usually liquids, and those with 20 or more are typically solids or semi-solids. The petroleum refining process uses chemicals, catalysts, heat, and pressure to separate and combine the basic types of hydrocarbon molecules naturally found in crude oil into groups of similar molecules. The refining process also rearranges their structures and bonding patterns into different hydrocarbon molecules and compounds.
Naturally occurring hydrocarbon compounds
The paraffinic series of hydrocarbon compounds have the general formula CnH2n+2 and can be either straight chains (normal) or branched chains (isomers) of carbon atoms (Figure 1). Paraffinic hydrocarbons are very commonly referred to as alkanes.
The lighter, straight-chain paraffin molecules are found in natural gas as well as in petroleum refinery byproduct gases and low boiling point liquids. Examples of straight-chain molecules are methane, ethane, propane, and butane (gases containing from one to four carbon atoms), and pentane and hexane (liquids with five to six carbon atoms).
The branched-chain (isomer) paraffins are usually found in heavier fractions of crude oil and have higher octane numbers than normal paraffins. These compounds are saturated hydrocarbons, with all carbon bonds satisfied, that ;is, the hydrocarbon chain carries the full complement of hydrogen atoms.
Aromatics are unsaturated ring-type (cyclic) compounds which react readily because they have carbon atoms that are deficient in hydrogen. All aromatics have at least one benzene ring (a single-ring compound characterized by three double bonds alternating with three single bonds between six carbon atoms) as part of their molecular structure (Figure 2).
Naphthalenes are fused double-ring aromatic compounds. The most complex aromatics are the polycyclic aromatic hydrocarbons (PAH), also referred to as polynuclear hydrocarbons, with three or more fused aromatic rings and they typically are found in the heavier fractions of petroleum crude oil.
Naphthenes are saturated hydrocarbon compounds having the general formula of CnH2n, arranged in the form of closed rings (cyclic) and found in all fractions of petroleum crude oil except the very lightest.
Naphthenes are also commonly referred to as cycloparaffins or cycloalkanes. Some examples of typical naphthenes are depicted in the adjacent Figure 3.
Single-ring naphthenes (monocycloparaffins) with five and six carbon atoms predominate, with two-ring naphthenes (dicycloparaffins) found in the heavier ends of the naphtha fraction of petroleum crude oil.
Alkenes are mono-olefins with the general formula CnH2n and contain only one carbon-carbon double bond in the chain (Figure 4). The simplest alkene is ethylene, with two carbon atoms joined by a double bond and four hydrogen atoms. Olefins are usually formed by thermal and catalytic cracking and rarely occur naturally in unprocessed crude oil.
Dienes and alkynes
Dienes, also known as diolefins, have two carbon-carbon double bonds (Figure 5). The alkynes, another class of unsaturated hydrocarbons, have a carbon-carbon triple bond within the molecule. Both these series of hydrocarbons have the general formula CnH2n-2. Diolefins such as 1,2-butadiene and 1,3-butadiene, and alkynes such as acetylene, occur in C5 and lighter fractions from cracking. The olefins, diolefins, and alkynes are said to be unsaturated because they contain less than the amount of hydrogen necessary to saturate all the valences of the carbon atoms. These compounds are more reactive than paraffins or naphthenes and readily combine with other elements such as hydrogen, chlorine, and bromine.
- David R. Lide, ed. 2005. Physical Constants of Organic Compounds, in CRC Handbook of Chemistry and Physics
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