CHEMICAL NAME = biphenyl CAS NUMBER = 92–52–4 MOLECULAR FORMULA = C12H10 MOLAR MASS = 154.2 g/mol COMPOSITION = C(93.5%) H(6.5%) MELTING POINT = 68.9°C
BOILING POINT = 254°C DENSITY = 1.04 g/cm3
Biphenyl, also called diphenyl, consists of two benzene rings joined by a single bond. It exists as colorless to yellowish crystals, has a distinctive odor, and occurs naturally in oil, natural gas, and coal tar. Biphenyl is used as an antifungal agent to preserve citrus fruit, in citrus wrappers to retard mold growth, in heat transfer fl uids, in dye carriers for textiles and copying paper, as a solvent in pharmaceutical production, in optical brighteners, and as an intermediate for the production of a wide range of organic compounds. Th ere are hundreds of biphenyl derivatives.
Biphenyl was once used extensively for the production of polychlorinated biphenyls (PCBs) before their production was banned in the United States in 1979. PCBs are formed by direct substitution of hydrogen atoms in biphenyl with chlorine using chlorine gas under pres-sure with a ferric chloride (FeCl3) catalyst. Th ere are 209 possible PCB compounds referred to as congeners. PCBs were discovered in 1865 as a by-product of coal tar and fi rst synthesized in 1881. Commercial production of PCBs, originally called chlorinated diphenyls, began in 1929 by the Swann Chemical Company located in Anniston, Alabama. Swann was taken over by Monsanto in 1935.
PCBs are synthetic chemicals that exist as oils or waxy substances; they do not occur naturally. Th ey were once used in many products including hydraulic fl uids, pigments, inks, plasticizers, lubricants, and heat transfer fl uids, but their primary use was as a dielectric fl uid in electrical equipment. Because of their high thermal stability, chemical stability, and electri-cal insulating properties, PCB fl uids were used extensively in transformers, fl uorescent light
44 | Th e 100 Most Important Chemical Compounds
ballasts, capacitors, and other electrical devices. PCBs were produced as mixtures of diff erent congeners, which could contain between 1 and 10 chlorine atoms substituted for the hydro-gen atoms in biphenyl. Individual PCB conhydro-geners are named using a numbering system where one of the carbon atoms at the single bond in biphenyl is given the number 1, and then other carbon atoms in that ring are sequentially numbered 2–6 (Figure 14.1). Th e same procedure is used for the other ring, using 1' for the bonded carbon and 2'-6' for the remaining carbons in the ring. Th e numbers are used to identify the position of the chlorine atoms in the particular congener with the unprimed ring chosen to give the lowest numbered carbon. For example, the following PCB is named 2,3',4'-trichlorobiphenyl.
Figure 14.1 Numbering system used for PCBs.
Th e primary producer of PCBs in North America was Monsanto, which manufactured PCBs under the trade name Arochlor. Arochlor congeners were numbered with a 4-digit num-ber. Th e fi rst two numbers indicated the number of carbon atoms in the biphenyl, and the last two numbers gave the mass percentage of chlorine in the PCB mixture (Arochlor 1016 was an exception since it had 12 carbons). Diff erent PCB mixtures were used for various applications that expanded over the years from its original applications in electrical devices. Properties of diff erent PCB mixtures varied according to the percentage of chorine in the mixture. As chlo-rine content increased, so did the PCBs boiling point, persistence, and lipophilicity (ability to dissolve in fat). Water solubility decreased as chlorine content increased.
Production of PCBs increased between 1930 and 1970, with a large increase in production after World War II. It is estimated that 650,000 tons of PCBs were produced in the United States during this 40-year period, with a peak of 42,000 tons in 1970. Global production was approximately twice that of the United States during this same period. Another 400,000 tons have been produced outside the United States after their production was banned in this country in 1979. Health problems started to appear in PCB plant workers soon after their commercialization. Th e fi rst problem observed among PCB workers was chloracne, a condi-tion resulting in skin lesions associated with overexposure to chlorine compounds.
As PCB production increased, more concerns were raised about the health and environmental eff ects of PCBs, which entered the environment through leakage, production processes, and improper disposal. Th e persistence and lipophilicity of PCBs resulted in its biomagnifi cation in the environment (see DDT). Problems associated with PCB contamination in wildlife include deformities, tumors, disruption in the endocrine and reproduction systems, and death. Human exposure to PCBs occurs through environmental and occupational routes. Th e primary exposure
Biphenyl and PCBs | 45
of humans to PCBs is through ingestion of food, especially fi sh, meat, and dairy products. Acute human health eff ects of PCBs include skin, eye, and throat irritation; breathing diffi culties;
nausea; loss of weight; and stomach pain. Th ere is evidence associating long-term increased PCB exposure in occupational settings to an increased incidence of liver and kidney cancer.
Because of public concerns, in October 1976, the U.S. Congress mandated that PCBs be regulated. PCB production in the United States stopped in 1977. Th e fi rst regulations were put into place in 1978 and dealt with labeling and disposing of PCB materials. In 1979, the manufacture, processing, distribution, and use of PCBs at a concentration of 50 ppm were regulated. Th e 50 ppm criterion was challenged by environmental groups. Court action resulted in PCBs below 50 ppm being regulated by rules established in the early 1980s, although exemptions were granted depending on the application and form in which the PCBs were used. Although production ceased in the United States, other countries continued to produce PCBs. Russia did not stop production until 1995, and other countries continued to produce PCBs through the early 2000s. Th e Stockholm Convention on Persistent Organic Pollutants fi nalized in 2001 prohibits new PCB production after 2005 and calls for eliminat-ing electrical equipment that contains high concentrations of PCBs by 2025.
PCBs will continue to remain in the environment for many years. Because they are still present in buildings, paints, soils, and throughout the environment, PCB remediation and disposal must be considered during remodeling, demolition, or decommissioning activities.
Many methods are used to destroy PCBs. Chemical dechlorination is used when PCB concen-trations are lower than 12,000 ppm. Chemical dechlorination separates the chlorine molecule from the PCBs, typically using sodium reagents to form inorganic salts. Th e bioremediation of soils uses select bacteria to break down the chlorinated hydrocarbons in a soil. Th e degra-dation reaction is slow and can take several weeks to years. Incineration is the most common method used to destroy PCBs and is required with higher concentrations. Temperatures in the range of 900°C to 1,200°C are used to volatilize and combust (in the presence of oxygen) the PCBs, although the temperature required to degrade PCBs depends on their concentration and residence time in the incinerator. Cement kilns are also used for this purpose. Incinerators are required to destroy 99.9999% of the PCBs. One concern with PCB incineration is the for-mation of dioxin and dioxin-like compounds such as dibenzo furans, polychlorinated diben-zofurans (PCDFs), and polychlorinated dibenzo-p-dioxins (PCDDs). Dioxin and dioxin-like compounds persist in the environment for decades. Th ey can cause cancer and are toxic to the fetal endocrine system.
Th e problem of PCB pollution continues into the 21st century. Several legal cases have dominated the media in recent years, such as General Electric’s responsibility to remediate the Hudson River and Monsanto’s and Solutia’s $600 million dollar settlement in 2003 with Alabama over claims concerning Monsanto’s Anniston, Alabama plant.
15. Butane
CHEMICAL NAME = butane MOLECULAR FORMULA = C4H10 MOLAR MASS = 58.1 g/mol
COMPOSITION = C(82.66%) H(17.34%)
MELTING POINT = −138.3°C for n-butane, −159°C for isobutane
BOILING POINT = −0.50°C for n-butane, −11.6°C for isobutane DENSITY = 2.6 g/L (vapor density = 2.05, air = 1)
Butane is a fl ammable, colorless gas that follows propane in the alkane series. Butane is also called n-butane, with the “n” designating it as normal butane, the straight chain isomer.
Butane’s other isomer is isobutane. Th e chemical name of isobutane is 2-methylpropane.
Isomers are diff erent compounds that have the same molecular formula. Normal butane and isobutane are two diff erent compounds, and the name butane is used collectively to denote both n-butane and isobutane; the names n-butane and isobutane are used to distinguish prop-erties and chemical characteristics unique to each compound. Butane derives its root word but from four-carbon butyric acid, CH3CH2CH2COOH. Butyric acid comes from butterfat and the Latin word butyrum means butter (see Butyric Acid). Butane, along with propane, is a major component of liquefi ed petroleum gas (LPG, see Propane). It exists as a liquid under moderate pressure or below 0°C at atmospheric pressure, which makes it ideal for storage and transportation in liquid form. Butane is the common fuel used in cigarette lighters and also as an aerosol propellant, a calibration gas, a refrigerant, a fuel additive, and a chemical feedstock in the petrochemical industry.
Butane is extracted from natural gas and is also obtained during petroleum refi ning.
Butane can be obtained from natural gas by compression, adsorption, or absorption. All three processes were used in the early days of the LPG industry, but compression and adsorption were generally phased out during the 20th century. Most butane now is obtained from absorption and separation from oil. Very little butane is obtained from distillation.
Gas stream from cracking units in the refi ning process contain appreciable amounts of
Butane | 47
butane, which is separated from the gas mixture by oil absorption, distillation, and various other separation processes. Th e supply of n-butane is adequate to meet demands for this compound, but generally the demand for isobutane exceeds supply. Normal butane can be converted to isobutane through a process called isomerization. In isomerization, straight chain isomers are converted to branched isomers. Isomerization of n-butane to isobutane takes place through a process in which n-butane is fed to a reactor at approximately 300°C and 15 atmospheres pressure. Hydrogen is added to prevent the formation of olefi ns (alkenes) and aluminum, platinum, and hydrochloric acids catalysts are used. Th e product from the reactor contains a mixture of butanes, and isobutane is separated from n-butane in a fractionator.
Isomerization of n-butane to isobutane is important in the oil industry because it provides feedstock for alkylation. Alkylation is a process in which an alkyl group is transferred between molecules. With respect to the refi ning of gasoline, alkylation refers to the combination of iso-butane with alkenes such as propylene and butylenes to produce branch-chained alkanes. Th e products of alkylation, called alkylates, are valued because they represent high-octane blend-ing stock that boosts the octane ratblend-ing of gasoline. Straight-chain alkanes have low octane and produce knocking (premature combustion) in internal combustion engines. Th rough alkylation reactions, straight-chain alkanes are converted to branched alkanes, and branching increases the octane rating. Isooctane, 2,2,4-trimethylpentane, with an octane rating of 100, is produced through the alkylation reaction of isobutane and isobutylene:
Butane undergoes typical alkane reactions (see Methane, Ethane, Propane). Pure butane produces a relatively cool fl ame, and its 0°C boiling point means that it does not vaporize well below this temperature. Butane’s use as a camp stove fuel during the summer or in southern locales does not present vaporization problems, but in winter or geographic areas that expe-rience cold temperatures, butane is an ineffi cient fuel. To overcome vaporization problems, butane-propane mixtures with 20–40% propane are used. An advantage of using blended fuel over a propane fuel is fuel canister weight. Propane has a lower boiling and higher vapor pressure than butane at the same temperature. Pure propane fuel requires a heavier walled container adding weight to stoves used in backpacking situations. Blending of fuels is also used by oil refi neries to account for seasonal diff erences. Butane is added to gasoline in winter to improve performance in cold temperatures.
Butane is used in the petrochemical industry to produce a variety of other compounds.
Oxygenated products of n-butane include acetic acid (CH3COOH), methanol (CH3OH),
48 | Th e 100 Most Important Chemical Compounds
ethanol (C2H5OH), propionic acid (CH3CH2COOH), butyric acid (CH3(CH2)2COOH), acetone, and methyl ethyl ketone. Dehydrogenation of butanes produces butylenes (C4H8) and butadiene.
16. Butene
CHEMICAL NAMES = see structure diagram CAS NUMBER = 106–98–9 (1-butene)
107–01–7 (2-butene) MOLECULAR FORMULA = C4H8 MOLAR MASS = 56.1 g/mol
COMPOSITION = C(85.6%) H(14.4%) MELTING POINT BOILING POINT 1-BUTENE −185°C −6.5°C CIS-2-BUTENE −139°C 3.7°C TRANS-2-BUTENE −106°C 0.9°C METHYLPROPENE −141°C −6.9°C DENSITY = 2.5 g/L (vapor density = 1.9, air =
1.0)
Butenes or butylenes are hydrocarbon alkenes that exist as four diff erent isomers. Each isomer is a fl ammable gas at normal room temperature and one atmosphere pressure, but their boiling points indicate that butenes can be condensed at low ambient temperatures and/or increase pressure similar to propane and butane. Th e “2” designation in the names indicates the posi-tion of the double bond. Th e cis and trans labels indicate geometric isomerism. Geometric isomers are molecules that have similar atoms and bonds but diff erent spatial arrangement of atoms. Th e structures indicate that three of the butenes are normal butenes, n-butenes, but that methylpropene is branched. Methylpropene is also called isobutene or isobutylene.
Isobutenes are more reactive than n-butenes, and reaction mechanisms involving isobutenes diff er from those of normal butenes.
Most butenes are produced in the cracking process in refi neries along with other C-4 frac-tions such as the butanes. Butenes are separated from other compounds and each other by several methods. Isobutene is separated from normal butanes by absorption in a sulfuric acid solution. Normal butenes can be separated from butanes by fractionation. Th e close boiling points of butanes and butenes make straight fractional distillation an inadequate separation
50 | Th e 100 Most Important Chemical Compounds
method, but extractive distillation can be used. Extractive distillation is a vapor-liquid process in which a solvent is used to promote a chemical separation. Th e less volatile compound has greater solubility in the extractive solvent, increasing separation effi ciency. Th e less volatile compound and the solvent are obtained from the bottom of a distillation column, whereas the more volatile compound is recovered from the top of the column. Th e less volatile compound is separated and recovered from the solvent, the latter being recycled. Butenes can also be prepared from the dehydrogenation (elimination of hydrogen) of butane.
Butenes are used extensively in gasoline production to produce high-octane gasoline compounds. In alkylation reactions, butenes combine with isobutane to produce branched gasoline-range compounds (see Butane). Isooctane can be produced by dimerization of iso-butene in the presence of sulfuric acid. Dimerization is the combination of a molecule with itself to produce a molecule called a dimer. Th e dimer has exactly twice the number of atoms in the original molecule. Th erefore the dimerization of isobutene produces two dimers with the formula C8H16:
Th e two trimethylpentenes produced in the dimerization are also called iso-octenes. Th ese can be hydrogenated in the presence of a metal catalyst to produce isooctane (2,2,4-trimethylpen-tane), which has an octane number of 100.
Butene | 51
Another large use of normal butenes in the petrochemical industry is in the production of 1,3-butadiene (CH2 = CH = CH = CH2). In the process, a mixture of n-butenes, air, and steam is passed over a catalyst at a temperature of 500°C to 600°C. Butadiene is used exten-sively to produce synthetic rubbers (see Isoprene) in polymerization reactions. Th e greatest use of butadiene is for styrene-butadiene rubber, which contains about a 3:1 ratio of butadiene to styrene. Butadiene is also used as a chemical intermediate to produce other synthetic organics such as chloroprene, for adhesives, resins, and a variety of polymers.
Primary alcohol and aldehydes are produced from butene through the Oxo process. Th e Oxo process involves the addition of carbon monoxide and hydrogen to an alkene under elevated temperature and pressure in the presence of a catalyst.
Chloroprene
Isobutylene is more reactive than n-butene and has several industrial uses. It undergoes dimer-ization and trimerdimer-ization reactions when heated in the presence of sulfuric acid. Isobutylene dimer and trimers are use for alkylation. Polymerization of isobutene produces polyisobutenes.
Polyisobutenes tend to be soft and tacky, and do not set completely when used. Th is makes polyisobutenes ideal for caulking, sealing, adhesive, and lubricant applications. Butyl rubber is a co-polymer of isobutylene and isoprene containing 98% isobutene and 2% isoprene.
Butene is used in the plastics industry to make both homopolymers and copolymers.
Polybutylene (1-polybutene), polymerized from 1-butene, is a plastic with high tensile strength and other mechanical properties that makes it a tough, strong plastic. High-density polyethylenes and linear low-density polyethylenes are produced through co-polymerization by incorporating butene as a comonomer with ethene. Similarly, butene is used with propene to produce diff erent types of polypropylenes.
Another use of 1-butene is in the production of solvents containing four carbons such as secondary butyl alcohol and methyl ethyl ketone (MEK). Secondary butyl alcohol is produced by reacting 1-butene with sulfuric acid and then hydrolysis: