Benzene and Its Derivatives Chapter 4

-very "aromatic," hence the name. -benzene the most common. Toxic, industrially important. -hybrid ring structure, since 3 double bonds not always in the same location. so, C-6 H-6 formula. -Nomenclature involves both the use of benzene suffix as well as common names. (see examples below) -important application: Fused Aromatics and Carcinogens, such as Benzo-a-pyrene and smoke

Kekule’s Structure of Benzene

(A)The bonds in C 6 H 6 are something between single and double, which gives it different chemical properties than double- bonded hydrocarbons. (B) The six-sided symbol with a circle represents the benzene ring. Organic compounds based on the benzene ring are called aromatic hydrocarbons because of their aromatic character.

Benzene

Benzene Benzene is one of the most fascinating molecules. The structure of this molecule eluded chemists until 1865 when Friedrich August Kekulé proposed that it consisted of a hexagonal ring with a carbon atom at each vertex. Every student of Organic Chemistry has heard the story of how the structure appeared to Kekulé in a dream in which he saw chains of carbon atoms dancing in circles like a snake chasing its own tail. Alas, benzene is both toxic and carcinogenic. In fact, it might be considered "the mother of all carcinogens," as a large number of carcinogens have structures that include benzene rings. (See the link below for the explanation of this.) I recall my Organic Chemistry professor joking about how he used to "practically bathe in benzene up to the elbows" when he would use it in his research (presumably before it was identified as a carcinogen). He predicted that this would probably lead to his demise. He was right--he died due to leukemia several years ago.

Aromatic Compounds and Benzene Aromatic compounds contain benzene. Benzene, C 6 H 6, is represented as a six carbon ring with 3 double bonds. Two possible can be drawn to show benzene in this form.

Aromatic Compounds and the Structure of Benzene In the early days the word aromatics was used to described many fragrant molecules isolated from natural sources. Today the term aromatic is used to describe benzene like molecules. Benzene is a flat, symmetrical molecule with the molecular formula C 6 H 6. It has alternating three carbon-carbon double and three single bonds.

Benzene’s relatively lack of chemical reactivity is due to its structure. There are two possible structures with alternating double and single bonds.

Experimental evidence suggest that all six carbon- carbon bonds in benzene are identical. The properties, including the above one, of benzene can only be explained by assuming that the actual structure of benzene is an average of the above two possible equivalent structures-known as resonance. Simple aromatic compounds like benzene are non- polar, insoluble in water, volatile, and flammable. Unlike alkenes, several aromatic hydrocarbons are toxic. Benzene itself is implicated as a cancer causing chemical.

Aromatic Compounds in Nature and Health Many aromatic compounds are common in nature and in medicine.

Naming Aromatic Compounds Aromatic compounds are named with benzene as the parent chain. One side group is named in front of the name benzene. - No number is needed for mono-substituted benzene since all the ring positions are identical. methylbenzenechlorobenzene (toluene)

Naming Aromatic Compounds When two groups are attached to benzene, the ring is numbered to give the lower numbers to the side groups. The prefixes ortho (1,2), meta (1,3-) and para (1,4-) are also used.

Some Common Names Some substituted benzene rings also use a common name. Then naming with additional more side groups uses the ortho-, meta-, para- system.

Many substituted aromatic compounds have common names in addition to their systematic names.

Learning Check Select the names for each structure: a. Chlorocyclohexane b. Chlorobenzene c. 1-chlorobenzene a. Meta-xylene b. Meta-dimethylbenzene c. 1,3-dimethylbenzene

Learning Check Write the structural formulas for each of the following: A. 1,3-dichlorobenzene B. Ortho-chlorotoluene

New Attached Groups Phenyl Benzyl Nitro -NO 2 2,4,6-trinitrotoluene Benzyl alcohol 4-phenyl-1-butene Refer to your chart for order of priority!

DDT and Benzene websitewebsite DDT the first of the chlorinated organic insecticides, was originally prepared in 1873, but it was not until 1939 that Paul Muller of Geigy Pharmaceutical in Switzerland discovered the effectiveness of DDT as an insecticide he was awarded the Nobel Prize in medicine and physiology in 1948 for this discovery). The use of DDT increased enormously on a worldwide basis after World War II, primarily because of its effectiveness against the mosquito that spreads malaria and lice that carry typhus. The World Health Organization estimates that during the period of its use approximately 25 million lives were saved. DDT seemed to be the ideal insecticideit is cheap and of relatively low toxicity to mammals (oral LD50 is 300 to 500 mg/kg). However, problems related to extensive use of DDT began to appear in the late 1940s. Many species of insects developed resistance to DDT, and DDT was also discovered to have a high toxicity toward fish. The chemical stability of DDT and its fat solubility compounded the problem. DDT is not metabolized very rapidly by animals; instead, it is deposited and stored in the fatty tissues. The biological half-life of DDT is about eight years; that is, it takes about eight years for an animal to metabolize half of the amount it assimilates. If ingestion continues at a steady rate, DDT builds up within the animal over time. The use of DDT was banned in the United States in 1973, although it is still in use in some other parts of the world. The buildup of DDT in natural waters is a reverisble process: the EPA reported a 90% reduction of DDT in Lake Michigan fish by 1978 as a result of the ban

DDT/Benzene continued… Benzene rings have two important properties: 1. Two or more benzene rings can themselves bind together; and, 2. Chlorine can replace hydrogen on the outside of the ring. These principles explain the formation of the very toxic families of PCBs, furans and dioxins. A pair of benzene rings joined together forms biphenyl: If chlorine is present when benzene is burned (and there is plenty of chlorine in plastics), hydrogen atoms can be released and chlorine atoms can replace them. The result is poly-chlorinated biphenyls, knows as PCBs. Their production was banned in the 1970's. If oxygen (abbreviated "O") forms another link between the two benzene rings the result is furans. If chlorine replaces hydrogen atoms, the furans are also very toxic: Sometimes benzene molecules bind together with two oxygen atoms, with the resulting name of dioxin. Unlike furans, dioxins are symmetrical (the same at the top and bottom).

Estrogen and aromatics

Reproductive Hormones and Benzene-like carcinogens website The importance of other possible factors such as breast-feeding history and exposure to environmental toxins, however, have been more difficult to establish. A review of the literature on these two possible factors, with commonly used organochlorine pesticides as the environmental toxins being examined reveals linkage between the two, with far-reaching implications for women to consider.

Estrogen and Chemicals Too Much Estrogen... Excessive levels of estrogen can also increase the risk of breast cancer. The female hormone signals cells to proliferate, which is necessary and normal during conception and fetal attachment to the womb. At other times, however, estrogen may tell normal cells to grow uncontrollably. Twenty years ago, nutrition educator Carlton Fredericks, Ph.D., noted that some women produced five more estrogen than did other women, increasing their risk of breast cancer. Other women may increase their risk through estrogen-replacement therapy. But several years ago, researchers stumbled across something completely unexpected: many common pesticides and plastics contain chemicals that mimic estrogen in the body. These estrogenic chemicals, called xenoestrogens, end up in the food and can greatly increase the risk of breast cancer. Four years ago, Mary Wolff, Ph.D., of the Mt. Sinai School of Medicine in New York City, found that high levels of estrogen- mimicking pesticides increased breast cancer risk in women by four times.

Estrogen continued… Because hormones are so powerful, only small quantities can do a lot of damage. In one experiment, researchers found that 2 to 5 parts per billion of bisphenol-A (BPA), one type of xenoestrogen, are enough to trigger hormonal changes. Combinations of pesticides magnify the risk, and such combinations are common in foods and the environment. Last year, John McLachan, Ph.D., an expert on pesticides and hormones at Tulane University, New Orleans, reported in the journal Science that combinations of just two estrogen-like pesticides were 1,000 times more potent than they were by themselves. What happens when you mix together dozens of common estrogen-mimicking chemicals? It's anyone's guess - but it doesn't bode well. According to Malins, these xenoestrogens, along with the estrogen a woman normally produces, could add to the free radical burden on breast cancer cells. Additional free radicals, he pointed out, might also be generated when the body breaks down estrogens and xenoestrogens. All these free radicals would greatly increase a person's "oxidative stress" - and the need for compensatory antioxidant vitamins.

The connection between: Hormones in the Human Body and Organic Aromatics

PCBs (Polychlorinated Biphenyls) There are 209 varieties of PCBs, known individually as congeners. A congener may have between 1 and 10 chlorine atoms, which may be located at various positions on the PCB molecule.

PCBs PCBs are mixtures of man-made chemicals with similar chemical structures. PCBs can range from oily liquids to waxy solids. Due to their non- flammability, chemical stability, high boiling point and electrical insulating properties, PCBs were used in hundreds of industrial and commercial applications including electrical, heat transfer, and hydraulic equipment; as plasticizers in paints, plastics and rubber products; in pigments, dyes and carbonless copy paper and many other applications. More than 1.5 billion pounds of PCBs were manufactured in the United States prior to cessation of production in 1977.

Food Dyes Food dye website Food dye website from manufacturer

- contain OH directly attached to aromatic ring -initially used as antispectics -since this is burning to healthy tissue, replacements have been found to be used in mouthwashes, etc.