Unsaturated Hydrocarbons - K. A. Boudreaux, Angelo State University, Lecture notes of Chemistry

Download Unsaturated Hydrocarbons - K. A. Boudreaux, Angelo State University and more Lecture notes Chemistry in PDF only on Docsity! Chapter 2 Alkenes 1 Chapter Objectives: • Learn to recognize the difference between saturated and unsaturated hydrocarbons. • Learn to recognize the alkene, alkyne, and aromatic functional groups. • Learn the IUPAC system for naming alkenes, alkynes, and aromatic rings. • Learn the important physical properties of the unsaturated hydrocarbons. • Learn the major chemical reaction of alkenes, and learn how to predict the products of halogenation, hydrogenation, addition of acids, and hydration reactions. • Learn the important characteristics of addition polymers. Chapter 2 Unsaturated Hydrocarbons 2 Unsaturated Hydrocarbons • Saturated Hydrocarbons — contain only carbon- carbon single bonds. • Unsaturated Hydrocarbons — contain carbon- carbon double or triple bonds (more hydrogens can be added). C H H H C H H H Alkanes C H H C H H CH C H C C C C C C H H H H H H Alkenes Alkynes Aromatics Mr. Kevin A. Boudreaux Angelo State University CHEM 2353 Fundamentals of Organic Chemistry Organic and Biochemistry for Today (Seager & Slabaugh) Chapter 2 Alkenes 2 3 Alkenes 4 Alkenes • Alkenes contain carbon-carbon double bonds. – General formula: CnH2n (for one double bond) – Suffix = -ene • In the carbon-carbon double bond, two pairs of electrons are being shared, leaving the carbon free to bond to two other things. CH2 CH2 ethylene, C2H4 CH2 CH propylene, C3H6 CH3 CH2 CH CH2 CH3 CH3 CH CH CH3 CH2 C CH3 CH3 C4H8 Chapter 2 Alkenes 5 9 IUPAC Nomenclature of Alkenes • If there is more than one double bond: – a counting prefix (di-, tri-, tetra-, etc.) is placed immediately in front of the suffix -ene to indicate the number of double bonds (diene, triene, tetraene, etc.). – Usually, an “a” is placed before the counting prefix to make pronunciation easier (e.g., butadiene). – The starting position of each double bond is indicated by the lower number, separated by commas (e.g., 1,3-butadiene). • For cycloalkenes, the ring is named as cyclo- + #C + -ene; one of the carbons of the double bond must be numbered “1.” 10 Examples: Nomenclature of Alkenes • Provide acceptable IUPAC names for the following molecules: CH2 CH CH CH2 CH2 C C CH CH3 CH3 CH CH2 CH3 CH C CH2 Chapter 2 Alkenes 6 11 Examples: Nomenclature of Alkenes • Provide acceptable IUPAC names for the following molecules: CH2CH3 CH3 CH3 12 Examples: Nomenclature of Alkenes • Draw structural formulas for the following molecules: – 2-methyl-2-butene – 4-methyl-1-pentene – 2-methyl-3-pentene (what’s wrong with this name?) Chapter 2 Alkenes 7 13 Examples: Nomenclature of Alkenes • Draw structural formulas for the following molecules: – 4-ethyl-3-hexene (what’s wrong with this name?) – 2,3,4-trimethyl-1,3-pentadiene – 1,6-dimethylcyclohexene 14 Examples: Nomenclature of Alkenes • Draw structural formulas for the following molecules: – 1,1-dimethyl-2-cyclohexene (what’s wrong with this name?) – 5-ethyl-1,3-cyclopentadiene Chapter 2 Alkenes 10 19 Sigma and Pi Bonds • When two sp2-hybridized carbons are next to each other, two kinds of orbital overlap take place: – end-on-end overlap of the sp2 orbitals to make a -bond (sigma bond). – side-to-side overlap of the unhybridized p orbitals to make a -bond (pi bond). 20 Bonding in Ethylene • Because of the -bond, free rotation is not possible around carbon-carbon double bonds. H H CC H H -bond -bond sp2 sp2 p p Chapter 2 Alkenes 11 21 Bonding in Ethylene 22 The Shape of the Ethylene Molecule • Since each carbon in the double bond is trigonal planar in shape, the entire ethylene molecule is a flat molecule, with the atoms separated by bond angles of 120°. C C H H H H 120° 120° 120° C C H H H H Chapter 2 Alkenes 12 23 Geometric Isomers in Alkenes • Because free rotation is not possible around double bonds, there are two different forms of 2-butene, which are geometric isomers (or cis/trans isomers) of each other: C C H CH3 CH3 H C C H CH3 H CH3 C C H CH3 CH3 H C C H CH3 H CH3 H H H H same molecule different molecules Geometric Isomers in Alkenes • The geometric isomers have to have different names, since they are different molecules: – the prefix cis- is used when the arms of the longest chain are on the same side of the double bond – the prefix trans- is used when they are on opposite sides of the double bond. • Geometric isomers have overall different molecular shapes, and may have drastically different chemical and physical properties. 24 C C H CH3 CH3 H C C H CH3 H CH3 cis-2-butene trans-2-butene Chapter 2 Alkenes 15 29 Some Important Alkenes 30 Some Important Alkenes C C H H H H Ethylene (ethene) Used in the manufacture of the plastic polyethylene. The release of ethylene stimulates the beginning of the ripening process in many plants; some plants can be picked while unripe (when they are less fragile), and exposed to ethylene gas to cause ripening once they reach their destination C C H H H CH3 Propylene (propene) Produced in the cracking of petroleum; used in the manufacture of the plastic polypropylene. C C H H H Cl Vinyl chloride (chloroethene) A carcinogenic gas manufactured from ethylene; used in the manufacture of the plastic polyvinyl chloride (PVC). Chapter 2 Alkenes 16 31 Some Important Alkenes C C Cl Cl Cl Cl Tetrachloroethylene Better known as perchloroethylene (“Perc”); a non- flammable organic solvent, widely used in dry cleaning; it is also used as an industrial solvent, degreaser, and paint remover. C C CH3(CH2)11CH2 H H CH2(CH2)6CH3 Muscalure (cis-9-tricosene) The sex pheromone of the female common housefly (Musca domestica). 32 Some Important Alkenes HO OH Lycopene A red pigment found in tomatoes, watermelon, guava, papaya, pink grapefruit, apricots, and rosehips; lycopene is a good antioxidant, and is more readily absorbed from cooked tomatoes and tomato paste, especially if the foods contain fat. Zeaxanthin A yellow pigment found in corn, egg yolk, orange juice, mangoes; also contributes to the yellowish color of animal fats. It is also found in the macula region of the retina (macula lutea, “yellow spot”), where it filters out some blue and UV light, acting like internal sunglasses; macular degeneration is the most common cause of blindness in the elderly. Chapter 2 Alkenes 17 33 HO OH O O O O Astaxanthin A pink pigment found in salmon, trout, red seabream and the carapaces of lobster and shrimp. In live shellfish, the astaxanthin is wrapped in a protein which gives it a blackish color; when the shellfish are boiled, the protein uncoils, liberating the pink astaxanthin. Canthaxanthin A pink pigment found in the feathers of American flamingos. It is obtained from shrimp in their diet; flamingos in captivity lose their pink color if they are not supplied with shrimp. Some Important Alkenes 34 Terpenes and Essential Oils Terpenes are a diverse group of molecules which are biologically synthesized from isoprene units. They are found in many plants, and often have distinctive flavors and aromas. They are often components of essential oils, so named because they have a characteristic “essence” or fragrance. Many of these molecules are components of common foods and perfumes. (Lycopene and its related compounds are also terpenes.) Isoprene P OH O O P OH O OH O Isopentenyl pyrophosphate precursor for the natural synthesis of terpene hydrocarbons H (R)-(+)-Limonene citrus fruits Camphene turpentine, cypress oil, oil of citronella -Pinene turpentine Chapter 2 Alkenes 20 39 Physical and Chemical Properties of Alkenes 40 Physical Properties of Alkenes • Many of the physical properties of alkenes are similar to those of alkanes. – Alkenes are nonpolar compounds. • insoluble in water. • soluble in nonpolar solvents. – They are less dense than water. • Range of physical states: –  4 C's — gases – 5 - 17 C's — liquids –  18 C's — solids • The chemical properties of alkenes, however, are completely different from those of alkanes. Chapter 2 Alkenes 21 41 Reactions of Alkenes — Addition Reactions • Most reactions of alkenes can be classified as addition reactions, in which both parts of a reactant are added to the carbon-carbon double bond: C C RR R R A B R C C R RR A B General Reaction: + 42 Halogenation of Alkenes • In a halogenation reaction, an alkene reacts with molecular bromine (Br2) or chlorine (Cl2) to form an alkyl halide (or haloalkane). CH3 CH2 CH CH2 Br Br CH3 CH2 CH CH2 Br Br + C C RR R R X X R C C R RR X X + Halogenation: X = Cl or Br a haloalkane or alkyl halide Chapter 2 Alkenes 22 43 Hydrogenation of Alkenes • In a hydrogenation reaction, a hydrogen atom is added to each carbon of the double bond, converting the alkene into an alkane. • The reaction only takes place in the presence of a metal catalyst (usually Pt). • This same process is used to produce hydrogenated vegetable oils such as margarine and shortening. CH3 CH CH CH3 H H CH3 CH2 CH2 CH3+ Pt C C RR R R H H R C C R RR H H + Hydrogenation: Pt 44 Addition of Acids • Acids (HF, HCl, HBr, and HI) can add to a double bond to produce an alkyl halide. CH3 CH CH CH3 H Cl CH3 CH CH CH3 H Cl + C C RR R R H X R C C R RR H X + Addition of acids across a double bond: HF, HCl, HBr, HI; not H2SO4 Chapter 2 Alkenes 25 49 Examples: Reactions of Alkenes • Complete the following reactions: CH3 C CH CH3 H H+ Pt CH3 H OH+ CH3 H2SO4 50 Examples: Reactions of Alkenes • Complete the following reactions: CHCCH3 CH3 CH3 H Cl+ CH3 CH CH CH3 Br Br+ Chapter 2 Alkenes 26 51 Examples: Reactions of Alkenes • Complete the following reactions: CH3 CH CH2 H H+ CH3 CH C CH3 H2O+ H2SO4 CH3 52 Examples: Reactions of Alkenes • Complete the following reactions: H F+ CH2 H Br+ CH3 CH3 Chapter 2 Alkenes 27 53 Addition Polymers 54 Addition Polymers • Polymers are long, chain-like organic molecules built up from simpler subunits called monomers. • In addition polymers, every atom in the reacting molecules becomes incorporated into the resulting polymer molecule. In most addition polymers, the monomer contains a double bond. n A = —A— monomer polymer —A—A—A—A—A—A— n CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 ethylene CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 polyethylene Chapter 2 Alkenes 30 59 Some Common Addition Polymers C C H H H CN Acrylonitrile Polymer = Polyacrylonitrile This polymer forms fibers which can be used in carpets and fabrics. It is known by the trade names Orlon, Acrilan, and Creslan. C C H H H CO2CH3 Vinyl acetate Polymer = Polyvinyl acetate (PVA) This polymer is used in adhesives, latex paints, wood glue / carpenter’s glue, safety glass, textile coatings, etc. Chloroprene Polymer = Polychloroprene CH2 C Cl CH CH2 When cross-linked with ZnO, this polymer forms a material called neoprene rubber, which is very resistant to oil and gasoline. CH CH2 CH2 CH CH CH2 Styrene + butadiene Polymer = Styrene butadiene rubber (SBR) These monomers form a copolymer which, when cross-linked with peroxides, forms a very tough, heat-stable rubber; used in automobile tires. 60 Rubber Polyisoprene (“natural rubber”) A polymer found in the rubber tree, Heva brasiliensis; formed by polymerizing isoprene. The name “rubber” was given to this compound by Joseph Priestly for its ability to rub out pencil marks. It is tough and brittle when cold, but softens and becomes sticky at higher temperatures. The rubber can be further hardened by the vulcanization process (Charles Goodyear, 1839), which involves heating the rubber with sulfur, which links separate polyisoprene molecules through dissulfide bridges (—S—S—) through the reactive double bonds. Depending on the catalyst used, a rubber which is either all cis-double bonds or all trans-double bonds (gutta percha) can be formed. Copolymerization of isoprene with isobutylene results in butyl rubber, which is less permeable to gases, and is used in bicycle tires. Isoprene Polyisoprene S S S S Chapter 2 Alkenes 31 61 Alkynes 62 Alkynes • Alkynes contain carbon-carbon triple bonds. – General formula: CnH2n-2 (for one triple bond) • The nomenclature system for alkynes is identical to that of alkenes, except the suffix -yne is used to indicate a the triple bond. Alkynes do not have cis- and trans- isomers. CH C H Acetylene (ethyne) C C HCH3CH2 C C CHCH2CH2CH2CH3CH3CH2 CH2CH2CH3 Chapter 2 Alkenes 32 63 Hybridization of Alkynes • When a carbon is connect to two other things (that is, one of the bonds is a triple bond), the molecule is modeled by combining the 2s and one of the 2p orbitals to produce two sp orbitals. • Since only one of the 2p orbitals was hybridized, there are two leftover p orbitals in an sp-hybridized carbon atom. 2s 2p sp hybrid orbitals leftover p orbitals 64 Hybrid Orbitals • Both sp orbitals are at the same energy level, with one electron in each hybrid orbital, and one in each of the unhybridized 2p orbitals. 2s 2p Energy 1s sp 1s hybridization 2p Chapter 2 Alkenes 35 69 The Shape of the Acetylene Molecule • Since each carbon in the double bond is linear in space, the entire acetylene molecule is a linear molecule. C C HH 180° 180° 70 Reactions of Alkynes • Anything an alkene does, an alkyne does twice: CH3 CH2 C C Br Br+H CH3 CH2 C C H Br Br Br Br CH3 CH2 C C H Br+H CH3 CH2 C CH3 Br Br Chapter 2 Alkenes 36 71 Aromatic Compounds 72 Aromatic Compounds • Aromatic compounds are those that contain the benzene ring (C6H6) or its structural relatives. • Aliphatic compounds do not contain benzene rings. C C C C C C H H H HH H Benzene or Chapter 2 Alkenes 37 73 The Structure of Benzene • The name “aromatic” originally referred to fragrant oils having similar chemical properties (such as oil of wintergreen, vanilla extract, etc.), including a very low carbon-to-hydrogen ratio. • The molecular structure of benzene was a puzzle for a long time after its discovery; although the formula indicates the presence of double bonds, benzene does not undergo the typical alkene reactions. • The ring structure of benzene was proposed by August Kekulé in 1865; he suggested that the double bonds switch positions to give two equivalent structures: C C C C C C H H H HH H C C C C C C H H H H H H 74 The Resonance Structures of Benzene • In reality, the double bonds and single bonds in benzene do not change position. The two Kekulé structures for benzene are resonance structures, meaning that each individual structure is fictitious. • The benzene ring is sometimes represented as a hexagon with a circle inside, which emphasizes that all of the positions on the benzene ring are equivalent. = Kekulé structures Chapter 2 Alkenes 40 79 Nomenclature of Benzene Derivatives • Rule 2. A number of benzene derivatives are known by their common (trivial) names rather than by their formal IUPAC names. toluene phenol aniline benzoic acid CH3 OH NH2 CO2H 80 Nomenclature of Benzene Derivatives • Rule 3. Compounds formed by replacing a hydrogen of benzene with a more complex hydrocarbon group can be named by naming the benzene ring as the substituent, called the phenyl group. C6H5— = phenyl group= CH2CH2CH3 CH3CHCH3 CH3CHCH2CH3 1-phenylpropane (propylbenzene) 2-phenylpropane (isopropylbenzene) 2-phenylbutane (sec-butylbenzene) Chapter 2 Alkenes 41 81 Nomenclature of Benzene Derivatives • Rule 4. When two groups are attached to a benzene ring, three isomeric structures are possible: • In naming these compounds, either the ortho / meta / para prefixes may be used, or position numbers (begin numbering at the group which comes first in alphabetical order) X Y X X Y Y ortho meta para 82 Nomenclature of Benzene Derivatives • Rule 4. examples CH3 CH3 CH3 CH3 CH3 CH3ortho-dimethylbenzene 1,2-dimethylbenzene ortho-methyltoluene 2-methyltoluene meta-dimethylbenzene 1,3-dimethylbenzene meta-methyltoluene 3-methyltoluene para-dimethylbenzene 1,4-dimethylbenzene para-methyltoluene 4-methyltoluene Chapter 2 Alkenes 42 83 Nomenclature of Benzene Derivatives • Rule 5. When two or more groups are attached to the benzene ring, their positions can be indicated by numbering the carbon atoms of the ring to obtain the lowest possible numbers for the attachment positions. Groups are arranged in alphabetical order. meta-bromochlorobenzene 1-bromo-3-chlorobenzene 1,2,4-trichlorobenzene 3,5-dichlorobenzoic acid Br Cl Cl Cl Cl CO2H Cl Cl 84 Examples: Nomenclature of Aromatic Rings • Provide IUPAC names for the following molecules: CH3CHCH2CH2CH3CH3 CH2CH3 CH3CCH2CHCH3 CH3 CH3 CO2H NH2 Chapter 2 Alkenes 45 89 Physical Properties of Aromatic Compounds • Just like alkanes and alkenes, aromatic compounds are nonpolar, and therefore insoluble in water (unless other substituents, such as OH groups, are present). They are also usually less dense than water. • Many aromatic compounds are obtained from petroleum and coal tar. • Benzene and toluene are commonly used as solvents, and are the starting materials for the synthesis of other useful organic compounds. • Some foods contain aromatic compounds, which can be synthesized by some plants. Some aromatic amino acids and vitamins are listed as essential, because we lack the ability to synthesize them, and must obtain them from our diet. 90 Chemical Properties of Aromatic Compounds • Aromatic compounds are chemically stable (unlike alkenes). They do NOT undergo any of the reaction of alkenes which we have discussed. • The major reaction of interest is a substitution reaction in which a hydrogen is replaced by some other group: – This is an industrially important reaction, because there are mechanisms for converting the nitro group to many other possible functional groups. H NO2 a nitration reaction HNO3 H2SO4 Chapter 2 Alkenes 46 91 The Big Bang CH3 HNO3 H2SO4 3x CH3 NO2 NO2 O2N toluene 2,4,6-trinitrotoluene (TNT) A powerful explosive; a sharp pressure wave from a detonator causes the molecule to rearrange into carbon dioxide, water vapor, nitrogen (N2), and other gases, which expand rapidly and destructively 92 The Big Bang HNO3 H2SO4 3x Nitroglycerin An oily, colorless, extremely unstable liquid; it is usually mixed with an absorbent material, such as clay, to make dynamite (Alfred Nobel, 1866) CH CH2 OH OH CH2 OH CH CH2 O O CH2 O NO2 NO2 NO2 glycerol / glycerin Chapter 2 Alkenes 47 93 Some Important Aromatic Compounds 94 Some Important Aromatic Compounds CH CH2 Styrene Used to make polystyrene and styrofoam CH3O HO O Zingerone A vanilloid; the pungent, hot component of ginger H O Hyacinthin Floral scent found in hyacinth HO CH3O CHO Vanillin Essential component of oil of vanilla; parent compound of the vanilloids Benzaldehyde Odor of almonds and cherries; also found in peach and apricot pits O H Chapter 2 Alkenes 50 99 Poisons Dichlorodiphenyltrichloroethane (DDT) 1,1,1-trichloro-2,2-bis-(p-chlorophenyl)ethane A very powerful insecticide discovered in 1939; it is toxic to insects, but not to mammals. DDT was widely used to kill mosquitoes that spread malaria, and was also effective against the insects that spread sleeping sickness and typhus. Unfortunately, DDT persists in the environment for a long time, and its accumulation in wildlife lead to decreases in the populations of several bird species. In 1972, DDT was banned by the Environmental Protection Agency. Cl Cl Cl Cl Cl Chlordane Used as a pesticide on some crops, and was also used to kill termites. Because of concerns about its toxicity, it was banned by the EPA in 1988. Cl Cl Cl Cl Cl Cl H H Cl Cl H H 100 Fullerenes Buckminsterfullerene (C60) A soccer-ball shaped molecule consisting of 60 carbon atoms, discovered in the 1980s by H. W. Kroto (Univ. of Sussex), R. E. Smalley, and R. F. Curl (Rice University) (Nobel Prize in Chemistry, 1996). They named the structure after the architect R. Buckminster Fuller, because the shape reminded them of his geodesic dome designs. Similar spherical-shaped carbon-only molecules, such as C70, are often referred to as fullerenes or “buckyballs.” All of the carbon atoms in in these molecules are sp2 hybridized, and the entire molecule is somewhat aromatic. The fullerenes are considered another allotrope (stable structural form) of carbon, in addition to graphite and diamond. Nanotubes are cylindrical versions of the fullerenes; they look something like a chain link fence rolled into a cylinder, with a dome-shaped cap on the end (half of a buckyball). Nanotubes (also known as "buckytubes") are extremely strong, as well as being very lightweight (since they are made of nothing but carbon atoms). These materials are being tested for potential use in many materials; some nanotubes also conduct electricity, leading to some potential applications in circuit design and electronics. Chapter 2 Alkenes 51 101 Polycyclic Aromatic Hydrocarbons (PAH) • Polycyclic aromatic hydrocarbons consist of two or more benzene rings fused together; they produced when organic compounds are heated to high temperatures, and are present in tobacco smoke, car exhaust, and sometimes in heavily browned foods. Naphthalene Benzo[a]pyrene Dibenz[a,h]anthracene Anthracene 102 Aromatic Rings That Don’t Look Like Benzene N H pyrrole H N N N H N porphyrin Chapter 2 Alkenes 52 103 N N N N O OCH3 O O O Mg Why Grass is Green and Blood is Red Why grass is green, or why our blood is red Are mysteries which none have reach’d unto. John Donne, “Of the Progress of the Soul” Why grass is green. Chlorophyll a Why blood is red. Heme N N N N Fe2+ O OH O HO 104 Chlorophyll Photosynthetic Reaction Center of a cyanobacterium