421 Hydrocarbon types

A carbon atom can form covalent bonds with other carbon atoms or with other atomic species. Each atomic species has its own preferred number of bonds that it requires to achieve a stable configuration. This preferred number of bonds is called the covalent valence of the element. Carbon atoms seek to form four bonds and hydrogen atoms one bond. Hydrocarbons are molecules that contain only hydrogen and carbon. The simplest hydrocarbon molecule, methane, is formed when four hydrogen atoms attach to a carbon.

Those atomic species that have a covalent valence greater than one may form double or triple bonds. Hydrocarbons with only single bonds are called alkanes, those with one or more double bonds are called alkenes, and those with triple bonds are called alkynes. Aromatic hydrocarbons are another class that involves a special type of bond that is between a single and a double bond.

Methane is part of a class of hydrocarbons that contains no double or triple bonds. These hydrocarbons are called alkanes or paraffins. Alkanes contain the largest possible number of hydrogens bonded to carbons in the molecule and thus are called saturated. Other alkanes can be formed from methane by adding carbons. For example, two carbons can be bonded to each other, with each carbon having three hydrogens attached to it to form C2H6, or ethane, As additional carbons are chained together, many familiar molecules such as propane, butane, and pentane are formed. These molecules are sometimes called straight-chain hydrocarbons because their carbon atoms are arranged in a single interconnected string. They are also called normal paraffins, a term that is sometimes used as a prefix for the molecule name. For example, octane is referred to as normal-octane or n-octane.

Table 4.1 shows some of the physical properties of selected normal paraffins. It also shows their motored octane number (MON) and cetane number. These two properties are measures of the fuel's readiness to auto-ignite. Some relationships between the size of the molecules and their properties are clear from Table 4.1. As the molecules become longer, their boiling points and melting points increase. This is reflected in the normal states where the smaller paraffins are normally gaseous, the molecules in the mid-range are liquid, and the large molecules are solid at room temperature. The tendency toward auto-ignition also shows a clear trend. Fuels with a high resistance to autoignition have high octane numbers and are valuable in spark-ignition engines. Short-chain paraffins have high octane numbers. However, as the chain length increases, the octane number decreases quickly. The cetane number, which is inversely related to the octane number, increases with longer chain lengths, indicating a readiness to auto-ignite.

It is possible to have alkane structures other than straight chains. Two hydrocarbon molecules can have the same number

Table 4.1 Physical properties of some normal paraffin hydrocarbons27 ;

Name Formula M.P., °C B.P.,°C Specific Normal MON Cetane gravity state number



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