Chapter 1 Alkanes 1

Chapter 1 Alkanes Chapter 1 Organic chemistry nowadays Organic Compounds: almost drives me mad. To Alkanes me it appears like a primeval tropical forest full of the Chapter Objectives: most remarkable things, a • Learn the differences between organic and inorganic compounds. • Learn how to identify isomers of organic compounds. dreadful endless jungle into • Learn how to write condensed, expanded, and line structures for organic compounds. which one does not dare • Learn how to recognize the alkane functional group in organic compounds. • Learn the IUPAC system for naming alkanes and cycloalkanes. enter, for there seems to • Learn the important physical and chemical properties of the alkanes. be no way out. Mr. Kevin A. Boudreaux Friedrich Wöhler Angelo State University CHEM 2353 Fundamentals of Organic Chemistry Organic and Biochemistry for Today (Seager & Slabaugh) www.angelo.edu/faculty/kboudrea 2 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: 3 4 What is Organic Chemistry? The Periodic Table • Organic chemistry is concerned with the study of • There are 92 naturally occurring elements, and many the structure and properties of compounds artificial ones, in the (in)famous Periodic Table: containing carbon. I A VIII A – All organic compounds contain carbon atoms. 1 H II A III A IV A V A VI A VII A He 2 – Inorganic compounds contain no carbons. Most Li Be B C N O F Ne 3 III B inorganic compounds are ionic compounds. Na Mg III B IV B V B VI B VII B I B II B Al Si P S Cl Ar • Some carbon compounds are not considered to be 4 K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr organic (mostly for historical reasons), such as CO, 5 Rb Sr Y Zr NbMoTcRuRhPdAgCd In Sn Sb Te I Xe CO2, diamond, graphite, and salts of carbon- 6 Cs Ba La Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn containing polyatomic ions (e.g., CO 2-, CN-). 3 7 Fr Ra Ac Rf Db Sg Bh Hs Mt Ds Rg Cn Fl Lv • Inorganic chemistry is the study of the other elements and non-carbon containing compounds. Lanthanides Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Actinides 5 Th Pa U Np Pu AmCm Bk Cf Es Fm Md No Lr 6 1 Chapter 1 Alkanes The Periodic Table of Organic Chemistry Origins of Organic Chemistry • Organic chemists look at the Periodic Table a little • Organic literally means “derived from living differently: organisms” — organic chemistry was originally the study of compounds extracted from living organisms and their natural products. • It was believed that only living organisms possessed H the “vital force” necessary to create organic B NOF compounds (“vitalism”). Mg Al PSCl • This concept started to change in 1828 after Cr Mn Fe Co Ni Cu Br Friedrich Wöhler showed that it was possible to Pd C I make urea, a known “organic compound” from a Pt mineral source: O + - Heat HHC NH4 OCN NN Ammonium H H 7 Cyanate Urea 8 Origins of Organic Chemistry What’s So Great About Carbon? • What this and later experiments showed was that • Carbons atoms can be linked by strong, stable “organic” molecules — even those made by living covalent bonds. organisms — can be handled and synthesized just like minerals and metals • What was special about these molecules was that C they contained the element carbon. neutral carbon, C H H C C H C H H C H H carbon cation, C4+ H 9 carbide anion, C4- 10 What’s So Great About Carbon? What’s So Great About Carbon? • Carbon atoms can form stable bonds to many other • Complex organic compounds can perform a number elements (H, F, Cl, Br, I, O, N, S, P, etc.). Most of useful biological functions (vitamins, organic compounds contain a few hydrogens, and carbohydrates, lipids, proteins, enzymes, ATP, DNA, sometimes oxygen, nitrogen, sulfur, phosphorus, etc. RNA are all organic compounds) which are studied in biochemistry. • Carbon atoms can form complex structures, such as long chains, branched chains, rings, chiral • Complex organic compounds are present in the foods compounds (having a particular “handedness”), we eat (carbohydrates, proteins, fats, etc.) complex 3D shapes, etc. •Most medicines, whether they come from a chemical • Because of this variety in bonding and complexity, plant or a green plant, are organic compounds. carbon atoms can form a tremendous variety of compounds. More than 16,000,000 organic •Most fuels are organic compounds (wood, coal, compounds are known, as opposed to about 600,000 natural gas, gasoline, kerosene, diesel fuel, oil, and inorganic compounds. other petroleum-based products). • Complex organic compounds are also useful in technology (paints, plastics, rubber, textiles, etc.). 11 12 2 Chapter 1 Alkanes Organic vs. Inorganic Compounds Organic vs. Inorganic Compounds • Organic compounds are held together by covalent bonds, while inorganic compounds are held together Table 1.1 Properties of typical organic by ionic bonds. and inorganic compounds. Property Organic Inorganic H H H Bonding within molecules Covalent Often ionic methane H C H H C H H C H Forces between molecules Generally weak Quite strong H H H Gases, liquids, or low Usually high melting- Normal physical state melting-point solids point solids Flammability Often flammable Usually Na+ Cl– Na+ Cl– nonflammable Cl– Na+ Cl– Na+ Solubility in water Often low Often high sodium chloride Na+ Cl– Na+ Cl– Conductivity of aqueous solutions Nonconductor Conductor Cl– Na+ Cl– Na+ 13 14 Atomic Orbitals on Carbon • A carbon atom does not form ions easily, since it has four valence electrons (1s22s22p2). It satisfies the octet rule in compounds by sharing electrons. 2p s orbital 2s p orbital 1s Energy • These are the orbitals that exist on atomic carbon (not connected to anything). 15 16 Hybrid Orbitals Hybrid Orbitals • When carbon atoms form bonds with each other, we describe the resulting bonds using hybrid orbitals, which are formed by mixing (hybridizing) the 2p carbon’s atomic orbitals. (Linus Pauling, 1950s) sp3 • When carbon atoms bond to 4 other atoms, the 2s 2s orbital and all three 2p orbitals in the valence shell hybridization combine to produce four sp3 hybrid orbitals: +++ +++ 1s 1s Energy 3 3 • All four sp orbitals are at the same energy level, 2s 2p sp with one electron in each hybrid orbital. 1 atomic 3 atomic 4 hybrid orbital orbitals orbitals 17 18 3 Chapter 1 Alkanes The Shape of an sp3 Carbon 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. sp3 109.5° C C sp3 sp3 sp3 19 20 Bonding in Ethane Bonding in Ethane (CH3CH3) • Bonds arise from the overlap of orbitals on adjacent atoms. H H – End-on-end overlap of sp3 orbitals produces a - H C C H bond (sigma bond). H H – All single bonds are -bonds. – Free rotation is possible around -bonds. H H 3 H • Each carbon in the ethane molecule, CH3CH3, is sp - hybridized and tetrahedral in shape. Free rotation is C C possible around the C—C bond. (See next slide) H H H 21 22 Carbon Chains Multiple Bonds • Each carbon atom can form four bonds, either to • Carbon atoms form four bonds to other things, but other carbon atoms, or to different atoms (such as H, sometimes those bonds are multiple bonds (double O, N, S, P, etc.) Three more sites or triple bonds): to make bonds C C CC = CC CCCC CCCC C single bond double bond results from the sharing results from the sharing C of two electrons of four electrons CCCC CCC C CCCC triple bond results from the sharing C of six electrons CCCC etc. 23 24 4 Chapter 1 Alkanes Isomers Examples: Isomers • Isomers — compounds having identical molecular • Draw all possible structures having the formulas formulas, but different arrangements of atoms. C4H10, C5H12, and C6H14. • Structural Isomers — the atoms in each molecule are connected in a different order. H H H H C2H6O C7H16 9 isomers H C C OHH CCH O C8H18 13 isomers C9H20 35 isomers H H H H C10H22 75 isomers Ethyl Alcohol Dimethyl Ether C20H42 366,319 isomers Colorless liquid Colorless gas mp -117°C mp -139°C C30H62 4,111,846,763 isomers! bp 78.5°C bp -25°C C40H82 62,481,801,147,341 isomers! density 0.789 g/mL (20°C) density 0.00195 g/mL (20°C) Intoxicant Refrigerant 25 26 Examples: Isomers Functional Groups O • Organic molecules are often organized by structures • Which of the following called functional groups, which are characteristic molecules is a structural CH3 C CH3 arrangement of atoms which define many of the isomer of acetone? physical and chemical properties of a class of Acetone organic compounds. – The simplest of the functional groups are the O OH hydrocarbons, which include the alkanes, alkenes, alkynes, and aromatic hydrocarbons. H CCHCH 2 H2CCHCH2 – Many functional groups contain oxygen atoms, such as alcohols, ethers, aldehydes, ketones, O O carboxylic acids, and esters. – Some other functional groups contain nitrogen H3CCH2 C OH H3CCH2 C H atoms, such as the amines and amides.

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