"Street Chemistry" For Emergency Responders - Part 4

Two basic groups of chemical compounds are formed from elements: salts and non-salts. Within each group are families of compounds that present particular hazards. By understanding these family relationships, emergency responders can determine the general hazards of materials by identifying the family to which each belongs. This "rule of thumb" information, however, does not eliminate the need to research the chemicals further before mitigation actions are taken. The first basic group of compounds, the salts, were covered in the September 1997 column.

Photo by Robert Burke
Propane is an alkane hydrocarbon that is a flammable gas.

The second basic group of chemical compounds will be referred to here as non-salts, or non-metal compounds. Non-metal compounds are combinations of non-metallic elements. These are the elements to the right and above the dividing line between metals and non-metals on the Periodic Table of the Elements.

Non-metals and their compounds may be solids, liquids and gases. Some may burn; some may be toxic; and they may also be reactive, corrosive and oxidizers. The largest quantities of hazardous materials encountered are made up of non-metal (non-salt) materials. Non-metals combine in a covalent bonding process. When elements bond covalently, the bonding electrons are shared between the elements.

Non-metal compounds are formed from non-metal elements. For example, when the non-metal carbon is combined with the non-metal sulfur, the compound formed is carbon disulfide. It is a poison by skin absorption and is a highly flammable, dangerous fire and explosion risk, has a wide flammable range from 1% to 50%, and can be ignited by friction. Carbon disulfide also has a low ignition temperature and can be ignited by a steam pipe or light bulb. Non-metal compounds can be represented in three ways: chemical name, molecular formula and structural formula. In the above example, carbon disulfide is the chemical name, the molecular formula is CS2 and diagrammed below is the structural formula:

S = C = S

The structural formula illustrates the way bonding takes place between the atoms of elements in a compound. The molecular formula shows the numbers of each of an element atom in a compound. Responders may encounter the molecular formula along with the name; however, it is unlikely they will see a structural formula in the field.

As can be seen with the structural formula of carbon disulfide, there are two bonds between the sulfur and the carbon. This configuration of bonding is referred to as double bonding. Double bonds are very unstable, which is why carbon disulfide is so very flammable.

Carbon dioxide is another common non-metal compound. Carbon dioxide is a colorless, odorless gas with a vapor density of 1.53, heavier than air. It is the 20th most produced industrial chemical, and is used as a fire extinguishing agent, in carbonated beverages, as an aerosol propellant and in many other uses. Carbon dioxide is a liquid cryogenic (very cold) material with a boiling point of -130 degrees Fahrenheit. Carbon dioxide is non-flammable and relatively non-toxic, however, it is a simple asphyxiation hazard (will displace oxygen in the air, causing suffocation). Below is the molecular and structural formula for carbon dioxide:

O = C = O

Carbon dioxide (CO2)

Some non-metal compounds can be separated into families which have similar hazards among individual family members. These compounds are hydrocarbons and hydrocarbon derivatives. The first group we will discuss are the hydrocarbons. These compounds contain only carbon and hydrogen in their formulas. Carbon has the ability to bond with itself to satisfy its need for electrons, this happens frequently in the hydrocarbon families. The carbons form straight chained structures with particular physical and chemical characteristics. Below is an example of a straight chained hydrocarbon compound:


Carbons in a compound can be arranged in configurations other than straight chains. Changing the configuration also changes some of the physical characteristics. These additional configurations will be discussed later. The hydrogens in compounds are not represented in the naming process. Hydrogen fills out the remaining bonds after the carbons connect to each other.

Carbon is in family four on the Periodic Table and has four electrons in its outer shell. These four electrons are available to be shared with electrons from other elements to form an electrically stable compound. Carbon needs four more electrons to be complete. In non-metal compounds electrons are shared. Hydrogen is in family one on the periodic table and has one electron in its outer shell to share. Below are examples of the connections on hydrogen and carbon where other elements can be attached:

Hydrogen has one connection and can have one other element attached.

Carbon has four connections and can have four other elements attached.

When hydrogen shares a bond with one of the electrons of carbon, hydrogen is complete with two electrons. Carbon, however, needs three more, so three additional hydrogens are attached to the carbon. Once this occurs, both the hydrogens and the carbons are electrically satisfied. This process occurs with other elements also when they attach to carbon. This will be discussed further in the column on hydrocarbon derivatives.

The naming of the hydrocarbon and hydrocarbon derivative families in "street chemistry" will focus primarily on the "Trivial Naming System." This naming system uses prefixes indicating the number of carbons in a compound. For example, meth=1 carbon, eth=2 carbons, prop=3 carbons, but=4 carbons, pent=5 carbons, hex=6 carbons, hept=7 carbons, oct=8 carbons, non=9 carbons and dec/dek=10 carbons.

Carbon chains can go on indefinitely. There are prefixes for as many carbons as you might want to connect together. However, the vast majority of important hazardous materials are formed with prefixes with 10 or less carbons. In this column we will focus on those with less than 10 carbons.

The ending of the hydrocarbon name reflects the family and type of bond between the carbons in the compound. Another naming system was developed by the International Union of Pure and Applied Chemistry (IUPAC). Using the IUPAC system becomes necessary when trying to name hydrocarbon and hydrocarbon derivative compounds with more than four or five carbons. The IUPAC system identifies the locations of other elements and branches on the carbon chain in a large chained compound.

Hydrocarbon compounds can be divided into four families: alkanes, alkenes, alkynes and aromatics. With alkanes, alkenes and alkynes the primary hazard is flammability. The vapors of these compounds may be lighter or heavier than air among the gases and heavier than air with the liquids. Most flammable liquids have a specific gravity less than 1 and will float on water. As flammable liquids they may be incompatible with ammonium nitrate, chromic acid, hydrogen peroxide, sodium peroxide, nitric acid and the halogens (F, Cl, Br and I).

The alkanes are all naturally occurring compounds. They have single bonds, are saturated with hydrogen, and are considered to be stable even though many of them are flammable and found primarily as an ingredient in mixtures. The individual family members have names beginning with one of the prefixes that indicates the number of carbons in the compound. For example, a one-carbon compound begins with the prefix "meth." The alkanes are all single bonded, so the ending for a alkane compound is "ane." So, a one-carbon alkane compound would be called methane. Methane is natural gas, it is odorless, colorless, lighter than air, flammable and a simple asphyxiant.

A three-carbon alkane would begin with "prop" and end in "ane," forming the compound propane, which is also a gas. Propane is very flammable, odorless, colorless, heavier than air and a simple asphyxiant. If an explosion occurs in a structure and the damage is at the foundation level, it may be a heavier than air gas such as propane. If the damage is up high such as in an attic, it may be a lighter than air gas like methane. Below are the molecular and structural formulas for methane and propane:



The same procedures apply to other alkane compounds. There are compounds that have two, four, five, six, seven, eight, nine, 10 and more carbons hooked together in a chain. They are also named by first using the prefix for the number of carbons and ending in "ane."

Many alkanes are pure compounds and can be found in transportation and fixed facilities while others are found primarily in mixtures of compounds such as gasoline and diesel fuel. Alkene hydrocarbons are man-made.

Alkenes are unsaturated and have one or more double bonds between carbons in the structures. The double bond(s) make the compound unstable and oxygen in the air can react with the bond to break it. When a bond breaks, heat is generated. If the heat is confined in the material or a combustible carrier, spontaneous combustion can occur. Sponta-neous combustion does not occur with members and mixtures of the alkane family because the alkanes have all single bonds. Therefore, alkanes and mixtures of alkanes such as octane, gasoline, diesel fuel and fuel oil cannot undergo spontaneous combustion.

Another hazard of double-bonded materials is polymerization. This is a spontaneous expansion type of chemical reaction. If it occurs in a container, the container may explode. Once the polymerization reaction starts, it will continue until it is finished regardless of what responders do.

Photo by Robert Burke
Acetylene is so unstable that it must be dissolved in acetone in a special tank.

Materials capable of undergoing polymerization are monomers. They must have a chemical added to them in transportation and storage to prevent the polymerization from occurring until they are ready for it in a chemical process. This chemical is called an inhibitor. In a chemical process vessel the inhibitor is removed and a catalyst is added which controls the rate of the chemical reaction.

Because the primary characteristic of an alkene is the double bond that occurs between two carbons, there are no single carbon alkenes. The first possible alkene is two carbons, for which the prefix "eth" is used. The ending is "ene" and the compound is called ethene. Alkene compounds may also have an alternative name which uses a "yl" after the prefix. So, ethene may also be called ethylene. Ethylene is the fourth most produced industrial chemical with 46.97 billion pounds annually. It is a colorless gas with a sweet odor and taste. Ethylene is highly flammable with explosive limits in air of 3-36%. In addition to being flammable it is also a simple asphyxiant gas. Ethylene is used in the production of other chemicals such as polyethylene, polypropylene, ethylene oxide, ethylene glycol's, ethyl alcohol and others.

Alkenes with four carbons begin with the prefix "but." It is possible for alkene compounds to have more than one double bond. A four-carbon compound with one double bond would be butene. A compound with two double bonds has the prefix "but" and the ending "ene." However, there must be something in the name to indicate that there are two double bonds. The prefix "di," meaning two, is inserted in front of "ene" indicating two double bonds (plus an "a" to make the name flow). The name of the compound is butadiene.

Butadiene, the 36th most produced industrial chemical, is a colorless gas with a mild aromatic odor. It is highly flammable with explosive limits of 2-11% in air, and a polymer that must be inhibited during transportation and storage. Butadiene is used in to produce plastics. Below are the molecular and structural formulas for ethylene and butadiene:



There are other compounds with one, two or even three double bonds. The rules for naming remain the same. Use a prefix for the number of carbons, end in "ene" and use a prefix to indicate the number of double bonds. Di is two, tri is three and tetra could be used if there were four.

Alkynes are hydrocarbon compounds that have one or more triple bonds in the structure. They are also unsaturated and very reactive. While it is possible to create different alkyne compound on paper using the rules for the family, there is really only one commercially valuable compound that responders are likely to encounter.

As with alkenes, there are no one-carbon alkynes. The first compound has two carbons with the prefix "eth." The ending is "yne" and the name of the compound would be ethyne. Ethyne is the chemical name of the compound but it is commonly known by its trade name acetylene.

Acetylene has two carbons and two hydrogens with a single triple bond between the carbons. Ethyne is a highly flammable, colorless gas with an ethereal odor. It has wide explosive limits from 2.5% to 80%. It can form explosive compounds with silver, mercury and copper.

Acetylene is so unstable that it is dissolved in acetone inside of its specially designed container. Acetylene is not shipped in tank-car or tank-truck quantities. Calcium carbide, a salt, is used to generate acetylene by reacting with water. When water contacts calcium carbide, acetylene gas is generated. Calcium carbide is shipped in specially designed closed containers that are placed on flat bed rail and highway vehicles. The molecular and structural formulas are shown below for ethyne (acetylene):


Alkanes and alkenes are sometimes found with different structural formulas than the usual straight chains. These are referred to as isomers. An isomer is a compound that has the same molecular formula as the straight chained version, but a different structural formula. This concept is also referred to as branching.

Branching a hydrocarbon compound has the effect of lowering the boiling point of that material. For example, butane has a boiling point of around 31 degrees F. Butane can be used as a heating fuel like propane. However, with the boiling point of 31 degrees F, the areas in the country where butane could be used in the winter would be limited by ambient temperatures. If, however, butane is branched to form isobutane; this process lowers the boiling point of butane to around 10 degrees F. Thus, this makes butane more useful as a heating fuel. Below is the molecular and structural formulas for butane and isobutane:



The most commonly encountered hazardous materials are the hydrocarbon fuels (such as gasoline, diesel fuel and fuel oil), which are made up of various pure hydrocarbon compounds mixed together forming different characteristics than the individual compounds. Propane, butane and natural gas are also fuels but are not mixtures; they are pure compounds. These gases may be mixed together to form liquefied petroleum gases. They are transported frequently, stored in large quantities and can present problems to responders in accidents.

Another configuration of a straight-chained hydrocarbon is the "ringed" compound. All of the carbons in the compound have been hooked together end to end. Ringed compounds are referred to as cyclic and when named the prefix "cyclo" is used in front of the hydrocarbon name. This configuration can be very unstable in the case of three or four carbons, or seven or eight carbons. However, five or six carbons in a ring form a stable compound.

Arranging carbons in a circle tends to raise the boiling point of the material. For example, hexane is a six-carbon straight-chained compound with a boiling point of around 155 degrees F. Cyclohexane on the other hand is a ringed compound and has a boiling point of around 178 degrees F. Below are the structural and molecular formula for hexane and cyclohexane:



The last of the hydrocarbon compounds are the aromatics, sometimes referred to as the "BTX" Fraction. Aromatics are very popular industrial solvents. Unlike the alkanes, alkenes and alkynes, the naming conventions and endings do not follow any specific rules. It is important that the responder just become aware of the names of these compounds and their characteristics. Aromatics are known by their characteristic ring formation called the Benzene Ring. They are ringed compounds with some variances. Bonding associated with the aromatics is called the resonant bond. The concept of resonant bonding is too complicated to discuss here, so just understand that the Benzene Ring has six carbons and six hydrogens. The benzene ring is sometimes shown without the carbons and hydrogens. It is understood that they are still in the structure. Below is the structural formulas and molecular formula for benzene:


The aromatics are unsaturated in appearance but in reality act like a saturated compound. The primary hazards of the aromatics are flammability and toxicity, and they are also carcinogenic. While flammable, the aromatics burn incompletely and are very smoky when they burn. The parent member of this family is benzene.

Benzene is very common and is the 16th most produced industrial chemical with over 15 billion pounds produced annually. Benzene is a colorless to light yellow liquid with an aromatic odor. Its boiling point is around 175 degrees F; its explosive limits are 1.5% to 8% in air and has a Threshold Limit Value (TLV) of 10 ppm. Not long ago benzene's TLV was thought to be 1,000 parts per million (ppm) until workers who were exposed to benzene daily in the workplace developed illnesses associated with the exposures. This resulted in the lowering of benzenes TLV to 10 ppm.

The second member of the aromatic hydrocarbons is toluene. It also has a benzene ring but a "methyl radical" is added to distinguish between benzene and toluene. Toluene is the 28th most produced industrial chemical with over 6.5 billion pounds produced annually. It is flammable and toxic with explosive limits of 1.2% to 7% in air and a TLV of 100 parts per million. Toluene's boiling point is 230 degrees F and it is a colorless liquid with a benzene like odor. Toluene can also be drawn in two forms like benzene. It can also have two molecular formulas, while the numbers of carbons and hydrogens in each are the same. The molecular and structural formula for toluene is shown below:


The last aromatic hydrocarbon discussed in this column is xylene. Also known as dimethylbenzene, xylene is the 23rd most produced industrial chemical at 9.37 billion pounds annually. It is a clear liquid with a TLV of 100 ppm. Xylene can be found in several different forms, known as meta-xylene, para-xylene and ortho-xylene. The names come from the location of the methyl radicals on the benzene ring.

Locating the methyl radicals in different spots on the benzene ring can change the characteristics of the compound. Shown below are the various configurations of xylene:



Robert Burke, a Firehouse® contributing editor, is a fire protection/hazardous materials specialist for the University of Maryland and has served on state and county hazmat response teams. Burke is a veteran of over 16 years in career and fire volunteer departments, serving as assistant chief and deputy state fire marshal. Burke holds an associate's degree in fire protection technology and a bachelor's degree in fire science, and is pursuing a master's degree in public administration. He is an adjunct instructor at the National Fire Academy and the Maryland Fire and Rescue Institute and author of the book Hazardous Materials Chemistry For Emergency Responders. Burke can be reached on the Internet at robert.burke@worldnet.att.net