Organic Compounds-Alkenes - K. A. Boudreaux, Angelo State University, Lecture notes of Chemistry

Download Organic Compounds-Alkenes - K. A. Boudreaux, Angelo State University and more Lecture notes Chemistry in PDF only on Docsity! Chapter 1 Alkanes 1 Chapter Objectives: • Learn the differences between organic and inorganic compounds. • Learn how to identify isomers of organic compounds. • Learn how to write condensed, expanded, and line structures for organic compounds. • Learn how to recognize the alkane functional group in organic compounds. • Learn the IUPAC system for naming alkanes and cycloalkanes. • Learn the important physical and chemical properties of the alkanes. Chapter 1 Organic Compounds: Alkanes 2 Organic chemistry nowadays almost drives me mad. To me it appears like a primeval tropical forest full of the most remarkable things, a dreadful endless jungle into which one does not dare enter, for there seems to be no way out. Friedrich Wöhler Mr. Kevin A. Boudreaux Angelo State University CHEM 2353 Fundamentals of Organic Chemistry Organic and Biochemistry for Today (Seager & Slabaugh) Chapter 1 Alkanes 2 3 4 What Do We Mean By “Organic”? • In everyday usage, the word organic can be found in several different contexts: – chemicals extracted from plants and animals were originally called “organic” because they came from living organisms. – organic fertilizers are obtained from living organisms. – organic foods are foods grown without the use of pesticides or synthetic fertilizers. • In chemistry, the words “organic” and “organic chemistry” are defined a little more precisely: Chapter 1 Alkanes 5 9 Origins of Organic Chemistry • What this and later experiments showed was that “organic” molecules — even those made by living organisms — can be handled and synthesized just like minerals and metals • What was special about these molecules was that they contained the element carbon. 10 What’s So Great About Carbon? • Carbons atoms can be linked by strong, stable covalent bonds. C neutral carbon, C C carbon cation, C4+ C carbide anion, C4- CH H H H C H H HH Chapter 1 Alkanes 6 11 What’s So Great About Carbon? • Carbon atoms can form stable bonds to many other elements (H, F, Cl, Br, I, O, N, S, P, etc.). Most organic compounds contain a few hydrogens, and sometimes oxygen, nitrogen, sulfur, phosphorus, etc. • Carbon atoms can form complex structures, such as long chains, branched chains, rings, chiral compounds (having a particular “handedness”), complex 3D shapes, etc. • Because of this variety in bonding and complexity, carbon atoms can form a tremendous variety of compounds. More than 16,000,000 organic compounds are known, as opposed to about 600,000 inorganic compounds. 12 What’s So Great About Carbon? • Complex organic compounds can perform a number of useful biological functions (vitamins, carbohydrates, lipids, proteins, enzymes, ATP, DNA, RNA are all organic compounds) which are studied in biochemistry. • Complex organic compounds are present in the foods we eat (carbohydrates, proteins, fats, etc.) • Most medicines, whether they come from a chemical plant or a green plant, are organic compounds. • Most fuels are organic compounds (wood, coal, natural gas, gasoline, kerosene, diesel fuel, oil, and other petroleum-based products). • Complex organic compounds are also useful in technology (paints, plastics, rubber, textiles, etc.). Chapter 1 Alkanes 7 13 Organic vs. Inorganic Compounds • Organic compounds are held together by covalent bonds, while inorganic compounds are held together by ionic bonds. C H H HHmethane sodium chloride Na+ Cl– Na+ Cl– Na+ Cl– C H H HH Na+Cl– Na+Na+ Cl– C H H HH Cl– Na+Cl– Cl– Na+ 14 Organic vs. Inorganic Compounds Property Organic Inorganic Bonding within molecules Covalent Often ionic Forces between molecules Generally weak Quite strong Normal physical state Gases, liquids, or low melting-point solids Usually high melting- point solids Flammability Often flammable Usually nonflammable Solubility in water Often low Often high Conductivity of aqueous solutions Nonconductor Conductor Table 1.1 Properties of typical organic and inorganic compounds. Chapter 1 Alkanes 10 19 The Shape of an sp3 Carbon • In order to get as far away from each other as possible (thus minimizing electron-electron repulsions), the sp3 orbitals are arranged in the shape of a tetrahedron around the central carbon atom, with bond angles of 109.5º. CC 109.5° sp3 sp3 sp3 sp3 20 The Shape of an sp3 Carbon Chapter 1 Alkanes 11 21 Bonding in Ethane • Bonds arise from the overlap of orbitals on adjacent atoms. – End-on-end overlap of sp3 orbitals produces a - bond (sigma bond). – All single bonds are -bonds. – Free rotation is possible around -bonds. • Each carbon in the ethane molecule, CH3CH3, is sp3- hybridized and tetrahedral in shape. Free rotation is possible around the C—C bond. (See next slide) 22 Bonding in Ethane (CH3CH3) CH C H H H H H C H C H H H H H Chapter 1 Alkanes 12 23 Carbon Chains • Each carbon atom can form four bonds, either to other carbon atoms, or to different atoms (such as H, O, N, S, P, etc.) = C C Three more sites to make bonds C C C C C C C C C C C C C C C C C C C etc. 24 Multiple Bonds • Carbon atoms form four bonds to other things, but sometimes those bonds are multiple bonds (double or triple bonds): C C C C triple bond results from the sharing of six electrons C C C C C C C C double bond results from the sharing of four electrons single bond results from the sharing of two electrons Chapter 1 Alkanes 15 29 Table 1.2 Classes and functional groups of organic compounds Class Functional Group Example of expanded structural formula Example of condensed structural formula IUPAC / Common name Alkane None H C H C H H H H CH3CH3 ethane Alkene ethene (ethylene)C C C H H C H H H2C CH2 Alkyne ethyne (acetylene)C C C C HC CHHH Aromatic C C C CC C C C C CC C H H H H H H benzene Alcohol C O H CC O H H HH H H CH3CH2OH ethyl alcohol Ether C O C OC C H H H H CH3OCH3 methoxymethane (dimethyl ether) H H 30 N H HAmine CH N H H H H CH3NH2 methylamine Aldehyde Ketone Carboxylic acid Ester Amide C O H C C O H C C O C C O O C O O C O N H H H C C O C H H H H H H C C C O OH H H H C C O O H H H C H H C C O N H H H H CH3CH O CH3CCH3 O CH3COH O CH3COCH3 O CH3CNH2 O ethanal (acetaldehyde) 2-propanone (acetone) ethanoic acid (acetic acid) methyl ethanoate (methyl acetate) ethanamide (acetamide) H H H Table 1.2 Classes and functional groups of organic compounds Class Functional Group Example of expanded structural formula Example of condensed structural formula IUPAC / Common name Chapter 1 Alkanes 16 31 32 A Moderately Complex Organic Molecule C C C C C C C C C C C C C C C C C C C HO C C C C C C C C H H H H H H H H H H H H H H H H H H HH HH H H H H H H H H H H H HH H H H H H H H H H H Chapter 1 Alkanes 17 33 Expanded Structural Formulas • In expanded structural formulas (Lewis formulas, Lewis structures), all atoms and bonds are shown: H C O H H C H H H H C C H H O H H H H C C H C H H H H H C C H H H H H 34 Condensed Structural Formulas • In condensed structural formulas, only specific bonds are shown; this is useful in reducing the number of C—H bonds that must be drawn. H3C CH3 CH3 CH3 CH3CH3 CH2 CH CH3 CH3 CH2 CH3CH2OH OH CH3 CH2 CH3CH2OCH2CH3 O CH2 CH3 Chapter 1 Alkanes 20 39 Line Drawing Drawing Organic Molecules HO 40 Examples: Drawing Organic Molecules • Draw acceptable condensed structures and line drawings associated with the following expanded structural formulas. C H H H C C C C H HHH H C H H HH C C C C C C H3C CH3 H H H H H H H H H H CH C H H O C H H H Chapter 1 Alkanes 21 41 Examples: Drawing Organic Molecules • Draw an acceptable expanded structure and line drawing for the molecule CH3CH2CH2OH. • Draw an acceptable expanded structure and line drawing for the molecule (CH3)3CCH2CH(CH3)CH3. 42 Examples: Drawing Organic Molecules • Draw acceptable expanded structures, condensed structures, and line drawings for the following molecules: – isopropyl alcohol, CH3CH(OH)CH3 – acetic acid, CH3COOH – acetaldehyde, CH3CHO – acetone, CH3COCH3 Chapter 1 Alkanes 22 43 44 Hydrocarbons • Hydrocarbons — compounds that contain only carbon and hydrogen. • Saturated Hydrocarbons — contain only carbon- carbon single bonds. • Unsaturated Hydrocarbons — contain carbon- carbon double or triple bonds. 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 Chapter 1 Alkanes 25 49 Examples: Conformations and Isomers • Which of the following groups represent structural isomers, and which are simply the same compound? CH3 CH2 CH2 CH3 CH3 CH2 CH2 CH3 CH3 CH2 CH2 CH3 CH3 CH2 CH2 CH3 CH CH3 CH3CH3 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH3 CH3 CH3 CH3 CH2 CH2 CH3 CH3 50 Examples: Conformations and Isomers • Which of the following groups represent structural isomers, and which are simply the same compound? CH3 CH CH3 CH3 CH3 CH CH3 CH3 CH3 CH2 CH CH3 CH3 CH3 CH CH3 CH2 CH3CH3 C CH3 CH3 CH3 CH3 CH CH3 CH2 CH3 CH3 CH CH3 CH2 CH3 CH3 CH CH2 CH3 CH2 CH3 CH3 CH2 CH CH3 CH2 CH3 Chapter 1 Alkanes 26 51 Alkane Nomenclature • Straight-chain alkanes are named by combining a prefix which indicates the number of carbon atoms in the chain, and a suffix indicating the functional group of the molecule. No. of C’s Prefix 1 meth- 2 eth- 3 prop- 4 but- 5 pent- 6 hex- 7 hept- 8 oct- 9 non- 10 dec- Functional Group Suffix Alkane -ane Alkene -ene Alkyne -yne 52 Alkane Nomenclature • When alkanes are branched, things get more complex. Remember there are two isomers of C4H10: • There are three isomers of C5H12: • There are 75 isomers of C10H22! • We need a way to name molecules that doesn’t require memorizing a huge number of prefixes. Butane Isobutane CH3CH2CH2CH3 CH3CHCH3 CH3 Pentane Isopentane CH3CH2CH2CH2CH3 CH3CHCH2CH3 CH3 C CH3 CH3 CH3CH3 Neopentane Chapter 1 Alkanes 27 53 IUPAC System of Chemical Nomenclature • The system of nomenclature used to name organic compounds was developed by the International Union of Pure and Applied Chemistry (IUPAC). – A root identifies the longest continuous chain of carbon atoms. – A suffix identifies the main functional group in the molecule. – A set of prefixes identifies the numbers and positions of the substituents (groups which are attached to the longest chain). (Alkyl groups are substituents which contain a carbon chain.) Prefix Ending longest carbon chain functional class Root number and identity of attached groups 54 IUPAC Nomenclature of Alkanes • Step 1. Identify and name the longest continuous chain of C atoms (#C + -ane for alkanes). If there is more than one way to get the same # of C’s in the longest chain, use the one that gives more substituents. CH CH3 CH2CH3CH CH2CH3 CH2 CH3 CH3 CH CH2CH3 CH CH3 CH3 CH CH2CH2 CH CH2 CH2 CH3 CH3 CH2 CH3CH3 CH3 CH CH2CH2 CH3 CH CH3 CH3 C CH3 CH3 CH3 CH3 Chapter 1 Alkanes 30 59 Examples: Alkane Nomenclature • Draw structural formulas and give the correct names for all of the possible structural isomers of pentane (C5H12). 60 Examples: Alkane Nomenclature • Draw structural formulas and give the correct names for all of the possible structural isomers of hexane (C6H14). Chapter 1 Alkanes 31 61 Examples: Alkane Nomenclature • Provide acceptable IUPAC names for the following molecules: CHCH3 CH CH3 CH3 CH3 CH2CH3 CH2 CH CH3 CH2 CH3 CHCH3 CH2 CH CH3 CH3 CH3 CH2CH3 CH CH CH3 CH3CH3 62 Examples: Alkane Nomenclature • Provide acceptable IUPAC names for the following molecules: CH2CH3 CH CH2 CH3 CH3CH2 CH2CH3 C CH3 CH2 CH3 CH2 CH2 CH3 CH2 CH2 CH2 CH CH2 CH2 CH2 CH2 CH2 CH3 CH3 CH3 CH2 CH2 CH CH2 CH2 CH2 CHCH3 CH3 CH3CH2 CH3 Chapter 1 Alkanes 32 63 Common Substituents CH3 CH2CH3 CH2CH2CH3 CH CH3 CH3 methyl ethyl propyl isopropyl CH2CH2CH2CH3 CH CH3 CH2 CH3 CH2 CH CH3 CH3 C CH3 CH3 CH3 butyl sec-butyl isobutyl tert-butyl Ffluoro Clchloro Brbromo Iiodo NO2nitro NH2amino Common Alkyl Groups Common Nonalkyl Groups 64 Examples: Alkane Nomenclature • Provide acceptable IUPAC names for the following molecules: CH3CH2CH2CHCHCH2CH2CH3 CH3CCH3 CH3 NO2 CH2CH3 C CH2 CH3 CH3 CH CH3 Br CH3 CH CH CH3 CH2 CH3 Cl CH CHCH3 CH3 CH2 CH2CH CH2 CH3CH3 F Chapter 1 Alkanes 35 69 70 Cycloalkanes • Alkanes may also possess cyclic structures in addition to the straight- and branched-chain acyclic molecules we have already seen. • General formula: CnH2n (for one ring) C C C HH H HH H Acyclic Cyclic Propane Cyclopropane ~ 60° CH3CH2CH3 cyclobutane cyclopentane cyclohexane cyclooctane Note that these molecules are not structural isomers of each other! Chapter 1 Alkanes 36 71 Cycloalkane Nomenclature • When naming cycloalkanes, the ring is taken to be the longest chain; the prefix cyclo- is added to the normal root + -suffix. • When mono-substituted cycloalkanes are named, it is not necessary to specify the position number, since all positions in the ring are equivalent. • When more than one substituent is located on a ring, the numbering begins at the carbon to which the group is attached which comes first in alphabetical order, and then proceeds in a direction which gives the lowest possible number to the next attached group. 72 Examples: Cycloalkane Nomenclature • Provide acceptable IUPAC names for the following molecules: CH3 CH3 CH3 Cl CH3 CH3 Chapter 1 Alkanes 37 73 Examples: Cycloalkane Nomenclature • Provide acceptable IUPAC names for the following molecules: CH2 CH CH3 CH3 CHCH3 CH3 CH3 CH3 CH3 CH2CH3 CH3 Cl Cl Cl 74 The Shape of Cycloalkanes • Cyclopropane has bond angles of 60°, which is bent far away from the “normal” 109.5° bond angles of straight-chain alkanes. It is a flat molecule. • Cyclobutane has bond angles of about 90°; it is also less stable than a “normal” alkane. It is mostly flat, but there is some slight puckering of the ring. H H H H H H H H ~ 90° H H H H H H ~ 60° Chapter 1 Alkanes 40 79 80 Physical Properties of Alkanes • Since alkanes are composed of relatively nonpolar C—C bonds and C—H bonds, alkanes are nonpolar molecules. • Because they have only weak attractions for each other, they tend to have lower melting points and boiling points than other organic compounds of comparable molecular weights. • The straight chain alkanes make up a homologous series in which each members differs from a previous member by having one additional CH2 group. In a homologous series, the physical properties are closely related and vary in a systematic way. Chapter 1 Alkanes 41 81 82 Physical Properties of Alkanes • The general rule when judging solubility is “like dissolves like” — polar substances mixes with polar substances, nonpolar with nonpolar, but not polar with nonpolar. • Alkanes (nonpolar) are insoluble in water (polar), and since they are less dense than water, they float (e.g., oil slicks). • Alkanes and other substances that do not dissolve in water are often referred to as being hydrophobic (“water fearing”). • Liquid alkanes of high molecular weight serve as emollients (skin softeners) to replace oils washed away by bathing or swimming. – Vaseline is a semisolid mixture of alkanes. Chapter 1 Alkanes 42 83 Alkane Reactions • Alkanes are the least reactive of all organic compounds. They do not usually react with strong acids or bases, or with most oxidizing or reducing agents. • They do, however, burn very easily in combustion reactions, releasing a great deal of energy: CH4(g) + 2O2(g)  CO2(g) + 2H2O(g) + 212.9 kcal C3H8(g) + 5O2(g)  3CO2(g) + 4H2O(g) + 488.8 kcal 2C8H18(g) + 25O2(g)  16CO2(g) + 18H2O(g) + 2448 kcal 84 Alkane Reactions • In the absence of enough oxygen for complete conversion to carbon dioxide, some common waste products are generated in the incomplete burning of alkanes: CH4(g) + 2O2(g)  CO2(g) + 2H2O(g) CH4(g) + 3/2 O2(g)  CO(g) + 2H2O(g) CH4(g) + O2(g)  C(s) + 2H2O(g) – CO, carbon monoxide, is poisonous, colorless, and odorless. In the exhaust train of most cars, a catalytic converter converts CO to CO2. – Solid elemental carbon produces engine deposits; but this reaction is done to produce lampblack, which is used in some ink pigments. Chapter 1 Alkanes 45 89 Chlorofluorocarbons (CFCs) Dichlorodifluoromethane (Freon-12) An example of the chlorofluorocarbons (CFCs, or freons), developed in the 1920s; they are relatively nontoxic, very unreactive, and boil at low temperatures, and were thus ideal for use as refrigerants; they were also widely used as aerosol propellants and as foaming agents. Unfortunately, they persist in the environment for a long time (up to a century), and make their way into the upper atmosphere, where they are split by high energy light from the Sun, releasing chlorine atoms. These Cl atoms destroy ozone in the stratospheric ozone layer that shields us from much of the Sun's UV radiation. (F. Sherwood Rowland, Mario J. Molina, Paul Crutzen, Nobel Prize in Chemistry, 1995) In 1987, a treaty called the Montreal Protocol on Substances that Deplete the Ozone Layer was signed, which cut back on the production and use of CFCs; in 1990, in response to the alarming increase in the size of the "ozone hole" over the South Pole, the agreement was extended to become a ban on the use of CFCs starting in 2000. C Cl Cl FF Chlorodifluoromethane (Freon-22) An example of a hydrochlorofluorocarbon (HCFC), developed as alternatives to the CFCs. The HCFCs are not fully halogenated, and are less stable than the CFCs, and degrade before they reach the upper atmosphere. C H Cl FF 1,1,1,2-Tetrafluoroethane (Freon-134a) A hydrofluorocarbon (HFC), another group of CFC-alternatives that are not damaging to the ozone layer. Freon-134a is now widely used in the air conditioning systems of automobiles in place of Freon-12. C F F CF F H H 90 Petroleum • Petroleum is a mixture of hydrocarbons formed over millions of years, primarily from the decay of microscopic ocean-dwelling plants and animals. The resulting crude oil collects in underground pockets in sedimentary rock. • Petroleum is separated into different fractions by fractional distillation. • Most petroleum products are burned as fuel, but about 2% is used to synthesize other organic compounds. (That’s still a lot!) • Over half of all synthetic industrial organics, including dyes, drugs, plastics, fibers, detergents, insecticides, etc., are made from petroleum sources Chapter 1 Alkanes 46 91 Fraction Boiling Range Molecular- size range Typical uses Gas -164-30°C C1-C4 Heating, cooking Gasoline 30-200°C C5-C12 Motor fuel Kerosene 175-275°C C12-C16 Fuel for stoves; diesel and jet engines Heating oil Up to 375°C C15-C18 Furnace oil Lubricating oils 350°C-up C16-C20 Lubrication, mineral oil Greases Semisolid C18-up Lubrication, petroleum jelly Paraffin (wax) Melts 52-57°C C20-up Candles, toiletries Pitch and tar Residue in boiler High Roofing, asphalt paving Petroleum Fractions 92 Carbon, in fact, is a singular element: it is the only element that can bind itself in long stable chains without a great expense of energy, and for life on earth (the only one we know so far) precisely long chains are required. Therefore carbon is the key element of living substance: but its promotion, its entry into the living world, is not easy and must follow an obligatory, intricate path . . . If the elaboration of carbon were not a common daily occurrence, on the scale of billions of tons a week, wherever the green of a leaf appears, it would by full right deserve to be called a miracle. Primo Levi, “Carbon” in The Periodic Table (1975)