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Chapter 1 Alkanes 1

Chapter 1 Alkanes 1

Chapter 1 Alkanes

Chapter 1 Organic 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 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 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:

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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

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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 “” from a Pt mineral source: O

+ - Heat HHC NH4 OCN NN Ammonium H H 7 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” — 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, , 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

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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 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 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

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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

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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 , 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

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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 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

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Isomers Examples: Isomers • Isomers — compounds having identical molecular • Draw all possible structures having the 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 of acetone? physical and chemical properties of a class of Acetone organic compounds. – The simplest of the functional groups are the O OH , 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. • Molecules with the same functional group tend to 27 share similar chemical and physical properties. 28

Table 1.2 Classes and functional groups of organic compounds Table 1.2 Classes and functional groups of organic compounds Example of expanded Example of condensed Example of expanded Example of condensed Functional Group Class Functional Group structural IUPAC / Common name Class structural formula structural formula IUPAC / Common name O H O O H H Aldehyde C HCCH H CH3CH ethanal (acetaldehyde) Alkane None H C C H CH3CH3 ethane H H H O H O H O H H Ketone CCC HHCCC CH3CCH3 2-propanone (acetone) Alkene CC CC H2CCH2 ethene (ethylene) H H H H

Alkyne CCH CCH HC CH ethyne (acetylene) O H O O

Carboxylic acid C O H H CCO H CH3COH ethanoic acid (acetic acid) HH C C C C H

Aromatic C C HHC C benzene O H O H O methyl ethanoate CC CC CH COCH Ester C O C HCCCO H 3 3 (methyl acetate) HH H H H H H H H Alcohol C O HH COH C CH3CH2OH ethyl alcohol AmineN H HNH C CH3NH2 methylamine H H H H H methoxymethane O H O O CH OCH Ether COCH CCH O 3 3 (dimethyl ether) Amide C N H CCN H CH3CNH2 ethanamide (acetamide) H H 29 30 H H

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A Moderately Complex Organic Molecule

H H H HHH H H H C H C C C H H C C C H H H C H H H C H C H H C H C C H H H H H C H H C H H H H C CC C H C C C H H H H H C C C H HO C C H H H H H

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Expanded Structural Formulas Condensed Structural Formulas •In expanded structural formulas (Lewis formulas, •In condensed structural formulas, only specific Lewis structures), all atoms and bonds are shown: bonds are shown; this is useful in reducing the number of C—H bonds that must be drawn.

H H H H H H3CCH3 HCCH HCCC H CH3 CH3 CH2 CH CH3 H H H CH3CH3

H H H H CH CH OH CH3 CH2 OCH2 CH3 HCCO H HCOCH 3 2 CH CH OH CH3CH2OCH2CH3 H H H H 3 2

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Line Drawings Drawing Organic Molecules •In line drawings (line-angle formulas, skeletal Expanded structural formula Condensed structural formulas structures, stick figures), bonds are represented by () lines; everywhere two lines meet or a line begins or CH3 CH2 CH2 CH3 ends is a C atom. H’s on C’s are not shown (except H H H H for emphasis); H’s on other atoms must be shown. CH3CH2CH2CH3 HCCCCH

CH3(CH2)2CH3 H H H H

= CH3 Line drawing

OH O = CH

= C

35 = CH2 36

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Drawing Organic Molecules Drawing Organic Molecules

H H H HHH Cholesterol H H H C CH3 H C C C H CHCH2CH2CH2CH(CH3)2 H C C C H CH3 H H C H2 H H H C C H H CH C H C H2C C H C C H H H H H H CH3 CH2 C H H C H H 2 H H C CH CH C CC C C H H2C C CH C C C H2 H H H H H CH C CH C C C H 2 HO C C HO C C H2 H H H H H H Expanded Structural Formula Condensed Structure 37 38

Drawing Organic Molecules Examples: Drawing Organic Molecules • Draw acceptable condensed structures and line drawings associated with the following expanded structural formulas. H H3CCH3 H H H C H C H H H H C C H H H H H C CCCCH C C H C H H H H H H HO H O H HCC C H Line Drawing H H 39 40

Examples: Drawing Organic Molecules Examples: Drawing Organic Molecules • Draw an acceptable expanded structure and line • Draw acceptable expanded structures, condensed drawing for the molecule CH3CH2CH2OH. structures, and line drawings for the following molecules:

, CH3CH(OH)CH3

– acetic acid, CH3COOH – acetaldehyde, CH CHO • Draw an acceptable expanded structure and line 3 drawing for the molecule (CH3)3CCH2CH(CH3)CH3. – acetone, CH3COCH3

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Hydrocarbons • Hydrocarbons — compounds that contain only carbon and hydrogen. • Saturated Hydrocarbons — contain only carbon- carbon single bonds. H H

HCC H

H H Alkanes • Unsaturated Hydrocarbons — contain carbon- carbon double or triple bonds. H

H H H C H C C C C HCHC Alkynes C C H H H C H Alkenes 43 H 44 Aromatics

Alkanes Some Common Alkanes

• Alkanes are saturated hydrocarbons — each carbon • Methane, CH4 holds the maximum number of hydrogen atoms). – major component of natural gas (~85%), which is – Alkanes contain only carbon-carbon single bonds. produced by bacterial decomposition of organisms in the absence of oxygen (marsh gas, – General formula: CnH2n+2 (no rings). cow flatulence). • Most chemical reactions require a functional group – burns cleanly, so is useful for cooking. “handle” to proceed. Since alkanes don’t really have functional groups, they aren’t very useful in many – odorless — ethanethiol is added to make natural biologically important processes. gas leaks detectable.

– Since alkanes undergo combustion easily, they • Ethane, CH3CH3 (C2H6) — a minor component of are a good source of energy (e.g., gasoline). natural gas (~10%).

– Alkanes also provide the raw materials for the • Propane, CH3CH2CH3 (C3H8) — used as an production of many other more complex industrial fuel, and in home heating and cooking. substances (plastics, etc.).

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Some Common Alkanes Conformations of Alkanes

, CH3CH2CH2CH3 (C4H10) • Conformation — the different arrangements of atoms in space achieved by rotation about single – cigarette lighters bonds. – Butane is an unbranched (normal) alkane. • Structures which are related to each other by rotation There is also a branched alkane with the formula around a single bond are the same molecule. C4H10, having a three-carbon chain with a one- carbon group connected to the middle. CH3CH2CH2CH3 – We must give the other isomer a different name: CH3 CH3 CH3 CH3 CH3CH(CH3)CH3 [or CH3CH(CH3)2] is named HH HH HH HH CH3 H H isobutane (or 2-methylpropane). H H

HH HCH3 CH3 H – Butane and isobutane are structural isomers of CH each other. 3 CH CH H CH3 3 3 CH3 H

CH3CH2CH2CH3 CH3CHCH3 H H H Butane Isobutane 47 HH H CH3 48

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Examples: Conformations and Isomers Examples: Conformations and Isomers • Which of the following groups represent structural • Which of the following groups represent structural isomers, and which are simply the same compound? isomers, and which are simply the same compound?

CH3 CH2 CH3 CH3 CH3 CH3 CH CH3 CH3 CH CH2 CH3 CH3 CH2 CH2 CH3 CH3 CH2 CH3 CH2 CH2 CH3 CH CH3 CH CH3 CH2 CH3 CH3 CH3

CH3 CH3 CH3 CH3

CH2 CH2 CH2 CH2 CH2 CH2 CH3 CH3

CH3 CH3 CH3 CCH3 CH3 CH2 CH CH3 CH3 CH CH2 CH3

CH3 CH3

CH2 CH3 CH3 CH CH3 CH2 CH2 CH3 CH3

CH CH CH CH CH CH CH 3 2 CH3 CH2 CH2 3 2 2 3 CH3 CH2 CH CH2 CH3 49 50

Alkane Nomenclature Alkane Nomenclature • Straight-chain alkanes are named by combining a • When alkanes are branched, things get more prefix which indicates the number of carbon atoms complex. Remember there are two isomers of C4H10: in the chain, and a suffix indicating the functional CH group of the molecule. 3 CH3CH2CH2CH3 CH3CHCH3 No. of C’s Prefix Functional Butane Isobutane 1meth- Group Suffix 2eth- Alkane -ane • There are three isomers of C5H12: 3 prop- Alkene -ene CH3 CH3 4 but- Alkyne -yne CH3CH2CH2CH2CH3 CH3CHCH2CH3 CH3 C CH3

5pent- Pentane Isopentane CH3 6hex- Neopentane

7hept- • There are 75 isomers of C10H22! 8oct- • We need a way to name molecules that doesn’t 9 non- require memorizing a huge number of prefixes. 10 dec- 51 52

IUPAC System of IUPAC Nomenclature of Alkanes • The system of nomenclature used to name organic • Step 1. Identify and name the longest continuous compounds was developed by the International chain of C atoms (#C + -ane for alkanes). If there is Union of Pure and Applied Chemistry (IUPAC). more than one way to get the same # of C’s in the longest chain, use the one that gives more . –A root identifies the longest continuous chain of carbon atoms. CH3 CH3 CH3 CH CH CH CH CH CH CH CH C CH –A suffix identifies the main functional group in 3 2 2 3 3 2 3 the molecule. CH3 – A set of prefixes identifies the numbers and

positions of the substituents (groups which are CH3 CH2 CH3 CH3 CH CH3 attached to the longest chain). ( groups are substituents which contain a carbon chain.) CH3 CH CH2 CH CH3 CH3 CH2 CH CH2 CH3 PrefixRoot Ending CH3 CH2 CH3 number and longest carbon functional identity of chain class CH3 CH2 CH CH2 CH CH2 CH3 attached groups 53 54

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IUPAC Nomenclature of Alkanes IUPAC Nomenclature of Alkanes • Step 2. Number the atoms in the longest chain. • Step 3. Name the alkyl groups (#C + -yl) and other substituents connected to the longest chain. – Number consecutively from the end that will give In front of each alkyl group name, put the number of the lower number to any C to which a group is the carbon the group is attached to, separated from attached. the name by a dash (e.g., 2-methyl). – If two or more alkyl groups are attached to the • Step 4. If there is more than one of a particular longest chain, use the numbering path that gives , combine them into a single word using the lowest number for the first point of difference. the appropriate counting prefix (di-, tri-, tetra-, – If two different alkyl groups are attached at the etc.). Include all of the carbon numbers which the same distance from either end of the chain, the groups are attached to, separated by commas (e.g., one that comes first in alphabetical order has the 2,2,3-trimethyl). highest priority. No. of Groups Prefix No. of Groups Prefix 1— 6hexa- 2di-7hepta- 3tri-8octa- 4 tetra- 9 nona- 55 5 penta- 10 deca- 56

IUPAC Nomenclature of Alkanes Examples: Alkane Nomenclature • Step 5. Arrange the alkyl groups in front of the • Draw structural formulas and give the correct names parent name in alphabetical order (ignoring for all of the possible structural isomers of butane counting prefixes, sec- and tert-; iso- is used in (C4H10). alphabetizing). Separate numbers from each other by commas, and numbers from words by dashes. CH3 CH3 CH3

CH3 CH CH2 CH2 CH3 CH3 CH CH2 C CH3

CH3

CH3 CH2 CH3 CH3 CH CH3

CH3 CH CH2 CH CH3 CH3 CH2 CH CH2 CH3

CH3 CH2 CH3

CH3 CH2 CH CH2 CH CH2 CH3 57 58

Examples: Alkane Nomenclature Examples: Alkane Nomenclature • Draw structural formulas and give the correct names • Draw structural formulas and give the correct names for all of the possible structural isomers of pentane for all of the possible structural isomers of hexane (C5H12). (C6H14).

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Examples: Alkane Nomenclature Examples: Alkane Nomenclature • Provide acceptable IUPAC names for the following • Provide acceptable IUPAC names for the following molecules: molecules:

CH2 CH3 CH2 CH3 CH3 CH2 CH2 CH CH3 CH3 CH CH3 CH3 CH2 CH CH2 CH3 CH3 CH2 CCH3 CH 2 CH3 CH CH3 CH2 CH2 CH3

CH3

CH3 CH2 CH2 CH2 CH CH2 CH2 CH2 CH3

CH3 CH3 CH3 CH3 CH2 CH2 CH3

CH3 CH CH2 CH CH3 CH CH CH CH CH 3 2 3 CH2 CH3 CH3

CH2 CH2

CH2 CH CH2 CH2

61 CH3 CH CH3 62

Common Substituents Examples: Alkane Nomenclature • Provide acceptable IUPAC names for the following Common Alkyl Groups molecules:

methyl CH3 butyl CH2CH2CH2CH3 CH3 Br Cl ethyl CH CH CH 2 3 3 CH3 CH2 CCH2 CH CH3 CH3 CH CH CH3 sec-butyl CH CH CH propyl CH2CH2CH3 2 3 CH3 CH2 CH3

CH3 CH3

isopropyl CH CH isobutyl CH2 CH CH3 3 CH3 CH3 CH CH3 CH3 CH3CCH3 CH CHCH CH CH CH CH tert-butyl C CH 3 2 2 2 3 3 CH3CH2CH2CHCHCH2CH2CH3 Common Nonalkyl Groups F CH3 NO fluoro F iodo I 2 chloro Cl nitro NO2

bromo Br amino NH2 63 64

Examples: Alkane Nomenclature Examples: Alkane Nomenclature • Provide acceptable IUPAC names for the following • Draw condensed structural formulas or line drawings molecules: for each of the following compounds: – hexane

– 3-ethylpentane

– 2,2-dimethylbutane

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Examples: Alkane Nomenclature Examples: Alkane Nomenclature • Draw condensed structural formulas or line drawings • The following names have been assigned incorrectly. for each of the following compounds: Draw the structure corresponding to the name, and assign the correct IUPAC name. – 3-ethyl-2-methylhexane –3-sec-butylpentane

– 4-isopropyloctane – 2-ethyl-2,6-dimethylhexane

–6-sec-butyl-7-ethyl-2,2,5,8-tetramethylnonane

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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) H H ~ 60° Note that these C molecules are not structural CH3CH2CH3 H CCH isomers of each other! H H Acyclic Cyclic Propane Cyclopropane

cyclobutane cyclopentane 69 cyclooctane 70

Cycloalkane Nomenclature Examples: Cycloalkane Nomenclature • When naming cycloalkanes, the ring is taken to be • Provide acceptable IUPAC names for the following the longest chain; the prefix cyclo- is added to the molecules: normal root + -suffix. CH3 • When mono-substituted cycloalkanes are named, it is not necessary to specify the position number, since all positions in the ring are equivalent. CH3 • 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 CH3 Cl the lowest possible number to the next attached group. CH3 CH3

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Examples: Cycloalkane Nomenclature The Shape of Cycloalkanes • Provide acceptable IUPAC names for the following • Cyclopropane has bond angles of 60°, which is bent molecules: far away from the “normal” 109.5° bond angles of straight-chain alkanes. It is a flat molecule. CH3 CH3 H H CH3 ~ 60° CH3 H HH CH2CH3 H • Cyclobutane has bond angles of about 90°; it is also CH CH CH less stable than a “normal” alkane. It is mostly flat, Cl 3 3 but there is some slight puckering of the ring. H Cl HH H CH3 ~ 90° Cl CH2 CH CH3 H HH 73 H 74

The Shape of Cycloalkanes Stereoisomers of Cycloalkanes • Cyclopentane has bond angles of about 108°; it • The molecules below are different molecules forms a mostly flat but slightly puckered ring. because there is no free rotation around carbon- Cyclopentane rings are very common in nature. carbon bonds in cycloalkanes.

H H H CH3 H H H ~ 108° H CH3 CH3 CH3 H H H cis-1,2-dimethylcyclopentane trans-1,2-dimethylcyclopentane • If cyclohexane were flat, the bond angles would be about 120°; but this molecule can adopt a “chair” or “boat” conformation in which the bond angles are • These molecules are stereoisomers — compounds 109.5°. Cyclohexane rings are extremely common. with the same molecular and structural formula but different spatial arrangements of atoms. • Stereoisomers in which the spatial arrangement is maintain by rings (or double bonds) are called "chair" "boat" geometric isomers or cis-trans isomers. conformation conformation 75 76

Examples: Stereoisomers Examples: Stereoisomers • State whether each possible pairing of the molecules • Provide acceptable IUPAC names for the following below are structural isomers, geometric isomers, or molecules: the same molecule. Br Br

CH3 CH3 CH3

CH3 Br Br

CH3 CH3 CH3 CH3

CH3 CH3 CH3 CH3

CH3 CH3 CH3 CH3

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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.

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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. 81 82

Alkane Reactions Alkane Reactions • Alkanes are the least reactive of all organic • In the absence of enough oxygen for complete compounds. They do not usually react with strong conversion to carbon dioxide, some common waste acids or bases, or with most oxidizing or reducing products are generated in the incomplete burning of agents. alkanes:

• They do, however, burn very easily in combustion CH4(g) + 2O2(g)  CO2(g) + 2H2O(g) reactions, releasing a great deal of energy: 3 CH4(g) + /2 O2(g)  CO(g) + 2H2O(g) CH (g) + O (g)  C(s) + 2H O(g) CH4(g) + 2O2(g)  CO2(g) + 2H2O(g) + 212.9 kcal 4 2 2 – CO, carbon monoxide, is poisonous, colorless, C3H8(g) + 5O2(g)  3CO2(g) + 4H2O(g) + 488.8 kcal and odorless. In the exhaust train of most cars, a catalytic converter converts CO to CO2.

2C8H18(g) + 25O2(g)  16CO2(g) + 18H2O(g) + – Solid elemental carbon produces engine deposits; 2448 kcal but this reaction is done to produce lampblack, which is used in some ink pigments.

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Alkyl Halides Examples: Nomenclature of Alkyl Halides • Alkyl halides, or haloalkanes, are alkanes in which • Provide acceptable IUPAC names for the following one or more hydrogen atoms are replaced by molecules: halogen atoms (F, Cl, Br, or I). Cl Br Cl CH2 CH2 CH3 • Most alkyl halides are not very water-soluble. Alkyl fluorides and chlorides have densities that are higher CH3 CH CH2 CH CH3 CH3 CH CH2 CH Br than those of alkanes, but still less than that of water. Alkyl bromides and iodides are generally more dense than water. Compounds containing more than one halogen are often more dense than water. Cl F H Cl • Alkyl halides are named as alkanes with halo- substituents (fluoro-, bromo-, chloro-, and iodo-). Cl C CH3 F C C F • A number of simple alkyl halides are better known Cl F H by their common names; for instance, CHCl3, Cl trichloromethane, is almost always referred to as Cl “chloroform.” 85 86

Some Common Alkyl Halides Some Common Alkyl Halides

Cl Cl Cl

H C H H C Cl Cl C Cl Cl H F

Cl Cl Cl ClC C H Br C Cl Dichloromethane Trichloromethane (chloroform) Tetrachloromethane (methylene chloride) A colorless liquid (bp 60°C); a (carbon tetrachloride) Cl H F A colorless , mildly toxic very commonly used organic Formerly a common organic liquid (bp 41°C) more dense solvent. Chloroform vapor is a solvent, and was widely used 1,1,1-Trichloroethane Bromochlorodifluoromethane (Halon 1211) than water. It is used as a anesthetic: James Young for dry cleaning and spot Formerly a very commonly used An example of a halon, a haloalkane that has paint remover and degreaser. Simpson was the first to use removal; it has been shown to organic solvent; heavily used in bromine atoms in addition to chlorine and fluorine It is also used to decaffeinate chloroform as an anesthetic be toxic and carcinogenic, dry cleaning, but it has been atoms. Halons are very stable, and are useful in coffee beans; since it has during childbirth in 1846 and contributes to ozone replaced by other solvents (such fire extinguishers, since they do not damage such a low boiling point, the (presumably, not on himself!), depletion, so it has been as tetrachloroethylene). electronic equipment. Their use has largely been residual solvent can be and it was widely used in surgery replaced by other solvents. phased out under the Montreal Protocols, but they removed from the beans at in the 19th and early 20th are still used in fire suppression systems aboard fairly low temperatures. centuries. However, since some aircraft, since no completely satisfactory and chloroform is carcinogenic, and safe alternatives have been discovered. toxic to the liver, it is not widely used for this purpose anymore.

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Chlorofluorocarbons (CFCs) Petroleum Cl Cl • Petroleum is a mixture of hydrocarbons formed F C F F C F over millions of years, primarily from the decay of microscopic ocean-dwelling plants and animals. Cl H The resulting crude oil collects in underground Dichlorodifluoromethane (Freon-12) Chlorodifluoromethane (Freon-22) pockets in sedimentary rock. An example of the chlorofluorocarbons (CFCs, or An example of a hydrochlorofluorocarbon freons), developed in the 1920s; they are relatively (HCFC), developed as alternatives to the nontoxic, very unreactive, and boil at low temperatures, and CFCs. The HCFCs are not fully halogenated, • Petroleum is separated into different fractions by were thus ideal for use as refrigerants; they were also and are less stable than the CFCs, and degrade fractional distillation. widely used as aerosol propellants and as foaming agents. before they reach the upper atmosphere. Unfortunately, they persist in the environment for a long time (up to a century), and make their way into the upper F H • Most petroleum products are burned as fuel, but atmosphere, where they are split by high energy light from about 2% is used to synthesize other organic the Sun, releasing chlorine atoms. These Cl atoms destroy FFC C ozone in the stratospheric ozone layer that shields us from compounds. (That’s still a lot!) much of the Sun's UV radiation. (F. Sherwood Rowland, F H Mario J. Molina, Paul Crutzen, Nobel Prize in Chemistry, • Over half of all synthetic industrial organics, 1995) In 1987, a treaty called the Montreal Protocol on 1,1,1,2-Tetrafluoroethane (Freon-134a) including dyes, drugs, plastics, fibers, detergents, Substances that Deplete the Ozone Layer was signed, which A hydrofluorocarbon (HFC), another cut back on the production and use of CFCs; in 1990, in group of CFC-alternatives that are not insecticides, etc., are made from petroleum sources response to the alarming increase in the size of the "ozone damaging to the ozone layer. Freon-134a hole" over the South Pole, the agreement was extended to is now widely used in the air conditioning become a ban on the use of CFCs starting in 2000. systems of automobiles in place of Freon-12. 89 90

15 Chapter 1 Alkanes

Petroleum Fractions Molecular- Carbon, in fact, is a singular element: it is the Fraction Boiling Range size range Typical uses only element that can bind itself in long stable Gas -164-30°C C -C Heating, cooking chains without a great expense of energy, and 1 4 for life on earth (the only one we know so far) Gasoline 30-200°C C -C Motor fuel precisely long chains are required. Therefore 5 12 carbon is the key element of living substance: Fuel for stoves; diesel Kerosene 175-275°C C12-C16 but its promotion, its entry into the living and jet engines world, is not easy and must follow an obligatory, Heating oil Up to 375°C C15-C18 Furnace oil intricate path . . . If the elaboration of carbon were not a common daily occurrence, on the Lubricating 350°C-up C -C Lubrication, mineral oil oils 16 20 scale of billions of tons a week, wherever the green of a leaf appears, it would by full right Greases Semisolid C -up Lubrication, petroleum 18 jelly deserve to be called a miracle. Paraffin (wax) Melts 52-57°C C20-up Candles, toiletries Primo Levi, “Carbon” in The Periodic Table (1975) Pitch and tar Residue in High Roofing, asphalt paving boiler 91 92

The End

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