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Petrochemicals: Builder Molecules Chemists as Molecular Architects • Just as an architect uses knowledge of available construction materials in designing a building, a chemist—a “molecular architect”— uses knowledge of available molecules in designing new molecules • Architects must know about the structures and properties of common building materials. Likewise, chemists must understand the structures and properties of their raw materials, common hydrocarbon ‘builder’ molecules. Today, many common objects and materials created by the chemical industry are unlike anything seen or used by citizens of the 1800s or even the mid-1900s. Early 1900s What similarities and differences do you see? Early 2000s Petrochemicals • The multitude of useful compounds and materials made directly or indirectly from hydrocarbons (oil or natural gas) are called petrochemicals. • Until the early 1800s, all objects and materials used by humans were either created directly from wood or stone or crafted from metals, glass, and clays. • Available fibers included cotton, wool, linen, and silk. • All medicines and food additives came from natural sources. • Celluloid (from wood) and shellac (from animal materials) were the only sources for commercially produced polymers, which are large molecule ‘chains’ typically composed of 500 to 20,000 or more repeating units of simpler molecules known as monomers. Direct and Indirect Petrochemical Use • Many of these differences are due to the use of petrochemicals. • Some petrochemicals, such as detergents, pesticides, pharmaceuticals, and cosmetics, are used directly. • Most petrochemicals, however, serve as raw materials in producing other synthetic substances, particularly a wide range of polymers. • Synthetic polymers include paint components, fabrics, rubber, insulating materials, foams, adhesives, molding, and structural materials. • Worldwide production of petroleum-based polymers is more than four times that of aluminum products. The astonishing fact is that it takes relatively few builder molecules (small-molecule compounds) to make thousands of new substances, including many polymers. • One builder molecule is ethene, C2H4, a hydrocarbon compound commonly called ethylene. The two carbon atoms in an ethene molecule share two pairs of electrons in a double covalent bond. Double Bonds are Highly Reactive • Ethene or ethylene is an alkene, a hydrocarbon with a double covalent bond between the carbon atoms. • Because of the high reactivity of its double bond, ethene is readily transformed into many useful products. • A simple example—the formation of ethanol (ethyl alcohol) from water and ethene—illustrates how ethene reacts: Acid catalyst Ethene Water Ethanol Addition Reaction • In this reaction, the water molecule (H2O) “adds” to the double-bonded carbon atoms by adding an H to one carbon atom and an OH group to the other carbon atom. This type of chemical change is called an addition reaction. Notice that the double bond between the carbon atoms has been broken. Addition Reactions • In these reactions, the second bond between the carbons is broken (requiring energy), and new bonds are formed between the carbon atoms and added atoms (releasing energy). • Addition reactions can ONLY take place at a multiple bond. This enables formation of new bonds without cleaving (chopping up) the original molecule. Long Chain Polymers • Ethene can also undergo an addition reaction with itself. Because the added ethene molecule also contains a double bond, another ethene molecule can be added, and so on. • This creates a long-chain polymer called polyethene, commonly known as polyethylene, a polymer consisting of 500 to 20,000 or more repeating units of ethene (ethylene) monomer. This is the ‘shorthand’ structure of polyethylene. •Polymers formed by reactions such as this are called—sensibly enough—addition polymers. •Polyethylene is commonly used in grocery bags and packaging. The United States produces millions of kilograms of polyethylene annually. More Addition Polymers • We can make a great variety of addition polymers from monomers that closely resemble ethene. • The most common variation is to replace one or more hydrogen atoms in ethene with an atom or atoms of another element. • In the following examples, note the replacements for hydrogen atoms in each monomer and polymer. Orlon fabric, telephone sets, shoe soles, auto parts Acrylonitrile monomer The ring shown here is benzene, which we will learn about shortly. POLYPROPYLENE Bottle caps, buckets, carpets, long johns, ropes, chairs, crates, straws, tents, etc. POLYVINYL CHLORIDE Raincoats, PVC pipe; bottles for oil, shampoo, baby products; plastic wrap; garden hoses POLYSTYRENE Styrofoam food containers, egg cartons, building insulation, coat hangers, CD cases, disposable pens How Are Polymers’ Molecules Arranged? • During your computer assignment, you saw the molecular arrangement of polyethylene. What was it? • What would a collection of these molecules look like? • Sketch this arrangement on your paper, using a pencil or pen line to represent each linear polymer molecule. • Ductility refers to the ability of a material to be drawn out, as into thin strands. For most polymers like the ones you just drew, flexibility and ductility depend on temperature. When the material is warm, the polymer chains can slide past one another easily. The polymer becomes more rigid when it cools. • Such polymers, classified as thermoplastics, are used in many everyday products, such as soft drink bottles, milk bottles, and plastic bags. These include PVC, polyethylene, polystyrene and polyacrylonitrile • The flexibility of a polymer can also be enhanced by adding molecules that act as internal lubricants among the polymer chains. • For example, untreated polyvinyl chloride (PVC) is used in rigid pipes and house siding. • With added lubricant molecules, PVC becomes flexible enough for use in such consumer goods as raincoats and inflatable pool toys. Branched Polymers • The reactions that form polymer chains can also take place perpendicular to the main chain, forming side chains. These polymers are called branched polymers. • Branching changes the properties of a polymer by affecting the ability of chains to slide past one another and by altering intermolecular forces. Draw at least two different models of branched-chain polymers. Try to vary your representations—the forms of branched polymers can differ greatly. Cross Linking • Another way to alter the properties of polymers is through crosslinking. • Polymer rigidity can be increased if the polymer chains are cross-linked so that they can no longer move or slide readily. (Think about the cap on a soda bottle.) • Cross-linking is having links between polymer chains. Draw several linear polymer chains that have been cross-linked. • Chemists can make even more different kinds of polymers by using BOTH branching and cross-linking. Draw several cross-linked, branched polymers. Then describe how cross-linked branched polymers compare to cross-linked linear polymers. Aligned Linear Chains • Chemists can control polymer strength and toughness. To do this, the polymer chains are arranged so that they lie in the same general direction. The aligned chains are stretched until they uncoil. • Polymers that remain uncoiled after this treatment make strong, tough films and fibers. Such materials include polyethene, which is used in everything from garbage bags to artificial ice rinks, and polypropylene, which is used in bottles and some carpeting. Draw several aligned linear polymer chains. Draw several aligned and cross-linked linear polymer chains. Saturated and Unsaturated Hydrocarbons • Each carbon atom in an alkane molecule is bonded to four other atoms. Compounds such as alkanes are called saturated hydrocarbons because each carbon atom forms as many single covalent bonds as it can. • In some hydrocarbon molecules, though, carbon atoms bond to three other atoms, not four. Members of this series of hydrocarbons are called alkenes. You already learned about ethene, C2H4. • The carbon–carbon bonding that characterizes alkenes is a double covalent bond. In a double bond, four electrons (two electron pairs) are shared between the bonding partners. Unsaturated Hydrocarbons • Alkenes, which contain carbon–carbon double bonds, are described as unsaturated hydrocarbons which means that not all carbon atoms are bonded to their full capacity with four other atoms. • Because of their double bonds, alkenes are more chemically reactive—and therefore better builder molecules—than are alkanes. Substituted Alkenes • The substituted alkenes make up another class of builder molecules. In addition to carbon and hydrogen, these molecules contain one or more other atoms, such as oxygen, nitrogen, chlorine, or sulfur. Remember PVC? • Adding atoms of other elements to hydrocarbon structures significantly changes their chemical reactivity. More Builder Molecules Cycloalkanes and Aromatic Compounds Another alkane structure Consider hexane C6H14, a straight chain molecule CH3 ̶ CH2 ̶ CH2 ̶ CH2 ̶ CH2 ̶ CH3 What happens if we remove one hydrogen from each end and bond the end carbons to each other? Cyclohexane! (used for making nylon) Cycloalkanes • Cycloalkanes are saturated hydrocarbons made up of carbon atoms joined in rings • Cyclo- is from the Greek for “circle.” cyclohexane, cyclobutane, cyclopentane What’s this molecule? Aromatic Compounds • Another class of hydrocarbon builder molecules with distinctly different properties are the AROMATIC COMPOUNDS. • The simplest aromatic compound is benzene, C6H6. When it is shown without any letters, each vertex of the six-carbon hexagon represents a carbon atom with its hydrogen atom. How is the structure of benzene different from cyclohexane? Aromatic Compounds Chemists have discovered, however, that benzene doesn’t behave like it has ANY double bonds (its chemical reactivity isn’t strong enough), so this structural representation isn’t correct. Experiments have determined that all carbon – carbon bonds in benzene are identical, with the bonding electrons shared among all 6 atoms. Benzene’s symbol is composed of two parts: the inner ring represents the equal sharing of bonding electrons among the carbon atoms, and the outer hexagon represents the bonding of the 6 carbon atoms to each other. Aromatic Compounds • Although only small amounts of aromatic compounds are found in petroleum, fractionation and cracking produce large quantities which are primarily used as building molecules. • Used in the pigment and dye and pharmaceutical manufacturing industries. Also used in the production of soaps, tanning agents, insecticides, fungicides, plastics, and processing of certain foods. Polycyclic Aromatic Hydrocarbons • Polycyclic aromatic hydrocarbons (PAH) are composed of two or more benzene rings which are fused together when a pair of carbon atoms is shared between them. The resulting structure is a molecule where all carbon and hydrogen atoms lie in one plane (i.e., they’re flat) • These substances are toxic to aquatic organisms or carcinogenic. Builder Molecules Containing Oxygen • Recall ethanol (ethyl alcohol), CH3-CH2-OH, and methanol (methyl alcohol), CH3-OH. • These molecules have an –OH group attached to a carbon atom. This general structure is characteristic of a class of compounds called alcohols. • The –OH group is one type of a functional group, an atom or group of atoms that give characteristic properties to organic compounds. • NOTE: The –OH group is NOT the same as a OH- ion found in ionic compounds. Alcohols • The general formula for an alcohol is R ̶ OH where R is the rest of the molecule (everything except the functional group) and the line represents the covalent bond linking the oxygen of the –OH group with the adjacent carbon atom in the molecule. In ethanol R is CH3CH2– . • In these alcohols, what would R represent? CH3CH2CH2OH 1-Propanol Cyclohexanol More Builder Molecules with Oxygen • Two more classes of oxygen-containing organic compounds are carboxylic acids and esters. • The functional group for each of these classes of compounds contains TWO oxygen atoms, one doubly bonded to the carbon atom and the other singly bonded to the same carbon atom: Notice that esters have TWO ‘rest of molecule’ groups Carboxylic acids Esters Alcohols Carboxylic acids Esters Isopropyl alcohol Ethanoic acid or acetic acid Methyl ethanoate (methyl acetate) Formic acid Carboxylic acids Esters Alcohols Condensation Polymers • So far we have seen how ADDITION POLYMERS are formed from alkene monomers (substitution alkenes like PVC, polyethylene and polystyrene). • Are natural polymers—like proteins, starches, and cellulose—addition polymers? • No, they’re not! And neither are some familiar synthetic polymers like nylon and polyester. Condensation Polymers • These polymers are formed with the loss (removal) of simple molecules, such as water, when monomer units join, instead of the breaking of a double bond. • The term condensation reaction applies to this second type of polymer-making process, and the resulting product is called a condensation polymer. • Here is a simple representation of this process: -R1–O–H + H–O–R2- → -R1–O–R2- + H–OH Monomer 1 Monomer 2 Condensation polymer water Condensation Process • One common condensation polymer is polyethylene terephthalate (PET). • This polymer is often used in large soft-drink containers. It has many other applications—as thin film for videotape and as Dacron for clothing, surgical tubing, and fiberfill. • More than two million kilograms of PET are produced each year in the United States. Can all PET be recycled? • While some PET may be recycled, material containing high quantities of additives, such as dyes or other polymers, is either disposed of in a landfill or is incinerated. • Another option is a DuPont process that breaks down PET into its monomers, which are then repolymerized into high-quality PET products. This technology decreases the need for new petroleum-based builder molecules as well as decreases the quantity of PET discarded in landfills. More uses for condensation reactions • Condensation reactions can be used to make small molecules as well as polymers. In tomorrow’s investigation, you will use condensation reactions to produce several esters. These reactions illustrate how you can chemically combine organic compounds to create new, useful substances. Other Uses for Condensation Rx • Condensation reactions can be used to make small molecules as well as polymers. • In tomorrow’s investigation, you will use condensation reactions to produce several esters. • These reactions illustrate how you can chemically combine organic compounds to create new, useful substances.