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Copy of Organic Chemistry Jonh Mc Murry3

Copy of Organic Chemistry Jonh Mc Murry3 - 100 CHAPTER 3...

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Unformatted text preview: 100 CHAPTER 3 Organic Compounds: Alkanes and Their Stereochemistry aliphatic,79 alkane, 79 alkyl group, 83 anti conformation, 95 bra netted-chain alkane, BO corttormation,93 conformers, 33 constitutional isomers, 81 eclipsed conformation, 514 functional group,73 gauche conformation, 95 lubricating oils and waxes and leaves a tarry residue ofasphalt. 'l'he distillation of crude oil is only the first step in gasoline production, however. Straight-run gasoline turns out to be a poor fuel in automobiles because of engine knock, an uncontrolled combustion that can occur in a hot engine. The octmie number of a fuel is the measure by which its antiknock proper- ties are judged. it was recognized long ago that straight—chain hydrocarbons are far more prone to induce engine knock than are highly branched com- pounds. Heptane, a particularly bad fuel, is assigned a base value of 0 octane number, and 2,2,4-‘triir1e'rhyipentane, commonly known as isooctane, has a rating of 100. CH3 CH3 CH30HZCH2CH2CH2CH2CH3 CH3ClICI-l2éHCH3 lag Heptane 2,2,4-Trimethylpentane (octane number = 0) (octane number =100l Because straight-run gasoline burns so poorly in engines, petroleum chemists have devised numerous methods for producing higher-quality fuels. One of these methods, catalytic cracking, involves taking the high-boiling kerosene cut (C1 14314) and "cracking" it into smaller branched molecules suit- able for use in gasoline. Another process, called re muting, is used to convert Cur-C8 alkanes to aromatic compounds such as benzene and toluene, which have substantially higher octane numbers than alkanes. The final product that goes in your tank has an approximate composition of 15% C4~C8 straight- chain alkanes, 25% to 40% C4—C7“, branched-chain alkanes, 10% cyclic alkanes, 10% straight—chain and cyclic alltenes, and 25% arenes (aromatics). SUMMARV AND KEY WORDS A functional group is a group of atoms within a larger molecule that has a characteristic chemical reactivity. Because functional groups behave in approx- imately the same way in all molecules where they occur, the chemical reactions of an organic molecule are largely determined by its functional groups. Alkanes are a class of saturated hydrocarbons with the general formula CHHZl‘l-I-Z‘ They contain no functional groups, are relatively inert, and can be either straight-chain (normal) or branched. Alkanes are named by a series of lUPAC rules of nomenclature. Compounds that have the same chemical for- mula but different structures are called isomers. More specifically, compounds such as butane and isobutane, which differ in their connections between atoms, are called constitutional isomers. Carbon—carbon single bonds in alkanes are formed by 0' overlap of carbon sir" hybrid orbitals. Rotation is possible around or bonds because of their cylindrical hydrocarbon, 79 isomers, 81 Newman projection, 93 B group, 84 saturated, 79 staggered conformation, 94 stereochemistry, 33 sterio strain, 96 straight-chain alkane, 80 substituent, 86 torsional strain, 94 Efx EL ctses Exercises 101 symmetry, and alkanes therefore exist in a large number of rapidly intercom-err- ing conformations. Newman projections make it possible to visualize the spa- tial consequences of bond rotation by sighting directly along a carbon~carbon bond axis. Not all alkane conformations are equally stable. The staggered con- formation of ethane is 12 kJ/mol (2.9 kcal/mol) more stable than the eclipsed conformation because of torsional strain. In general, any alkane is most stable when all its bonds are staggered. Thomson Sign in at www.thomsonedu.com to assess your knowledge of this chapter's topics by taking a pretest. The pre—test will link you to interactive organic chemistry resources based on your score in each concept area. Online homework for this chapter may be assigned in Organic OWL. E indicates problems assignable in Organic OWL. VISUALIZING CHEMISTRY (Problems 3.1—3.18 appear within the chapter.) 3.19 [den tify the functional groups in the following substances, and convert each drawing into a molecular formula (red : 0, blue = N): (a) . (b) Phenylalanine Lidocaine lg Assignable in OWL 102 CHAPTER 3 Organic Compounds: Alkanes and Their Stereochemistry 3.20 I Give IUPAC names for the following alkanes, and convert each drawing into a skeletal structure: (a) j‘ M lb) (cl - : ‘ (d) ADDFHONALPROBLEMS 3.21 l Locate and identity the functional groups in the following molecules. In these representations, each intersection of lines and the end of each line rep— resents a carbon atom with the appropriate number (it hydrogens attached. la) / /CH20H (hi 0 let I? N 0 \ / \C¢ NHCH3 I t \ CH3 (CH C”) (e) if) ) CH3CHCOH AA \ $H ‘\\ a 2 O / 32?. Draw structures that meet the following descriptions (there are many possi- biiities): (a) Three isomers with the formula Cal-1m (b) Two isomers with the formula Call-1303 3.23 Draw structures of the nine isomers of Cytllé. I Assignable in OWL Exercises 103 3.24 at In each of the following sets, which structures represent the same com- pound and which represent different compounds? (a) Bit (EH3 [I3H3 CH3CHCIHCH3 CH3CHC|HCH3 CH3$HCHCH3 CH3 Br Br (bi OH HL) / / l l. \ \Oi-l HO \ (Ci CH3 CH2CH3 CH3 CH3 | | l | CH3CH2[IZHCH2CHCH3 HOCHZeHcHZCHCH3 CH3CH2CHCH2CHCHZOH CH20H CH3 3.25 There are seven constitutional isomers with the formula C4HIOO. Draw as many as you can. 3.26 ml Propose structures that meet the following descriptions: (a) A ketone with five carbons (b) A four-carbon amide (c) A five-carbon ester (d) An aromatic aldehyde (e) A keto ester (f) An amino alcohol 3.27 In Propose structures for the following: (a) A ketone. C4HSO (b) A nitrite, C5Hgl\‘ (c) A dialdehyde, C_1l-l..,02 (d) A bromoalkene, C6H] I Br (e) An alkane, COHH (t) A cyclic saturated hydrocarbon, (361-112 (g) A diene [diall<ene), CSHS (h) A l<eto alkene, 651-180 3.28 Draw as many compounds as you can that fit the following descriptions: (a) Alcohols with formula Cal-1300 (b) Amines with formula C5H13N (c7) Ketones with formula Csi—imO (d) Aldehydes with formula CsH-IOO (e) Esters with formula (2411802 (t) Ethers with formula C4l-IIUO 3.29 E Draw compounds that contain the following: (a) A primary alcohol (l3) A tertiary nitrile (c) A secondary thiol (cl) Both primary and secondary alcohols ((-3) An isopropyl group (f) A quaternary carbon 3.30 Draw and name all monobromo derivatives otpentane, CSHI ]B[. 3.31 Draw and name all monochloro derivatives of 2,5»dimethylhexane, CSHUCI. 3.32 Predict the hybridization of the carbon atom in each of the following func- tional groups: (a) Ketone (b) Nitrile (c) Carboxylic acid 3.33 a: Draw the structures of the following molecules: (at) Biaceh'l, C411602, a substance with the aroma ofbutter; it contains no rings or carborvcarbon multiple bonds. (1)) Ethylenimine, Czl-JSN, a substance used in the synthesis of melamine poly- mers; it contains no multiple bonds. (c) Glycerol, (7 :H803, a substance isolated from fat and used in cosmetics; it has an -OH group on each carbon. it's”. Assignable in OWL 104 CHAPTER 3 Organic Compounds: Alkanes and Their Stereochemistry 3.34 I! Draw structures for the following: (a) Z-lx’iethylheptane (b) 4—Ethyl-2,2-dimethy|hexane (c) 4~Ethyl-3,4-dimethyloctane (d) 2,4,4-"l‘rimethylheptane (e) 3/3-Diethyl-2,S-dimethylnonane (f) 4-1sopropyl-LS-methyilieptane 3.35 Draw a compound that: (21) Has only primary and tertiary carbons (1)) Has no secondary or tertiary carbons (c) Has four secondary carbons '40 it.) ‘07 Draw a compound that: (21) Has nine primary hydt‘ogens (1)) Has only primary hytlrogeng 3.37 For each of the following compounds, draw an isomer that has the same func- tional groups. Each intersection of lines represents a carbon atom with the appropriate number of hydrogens attached. (a) CH (bl OCH l 3 /\ 3 CH3CHCH2CHZBI’ (d) /\ OH (e)CH3CH2CHO if) / /CH2C02H ‘ |/\‘ 3.38 a Dive [UPAC names for the following compounds: [cl CH3CH2CH2CE N (3) CH3 (b) CH3 [0) H3C CH3 CH3C|HCH2CH2CH3 CH3CH2C|CH3 CH3CHfl3CH2CH2CH3 dug cHg idi cHzcng C|H3 (e) EH3 (’ZHZCH3 (r) H3(|I cH3 CH3CH2CHCH2CHZCHCH3 CH3CH2CH2CHCH20CH3 CH3C—CCH2CH2CH3 32H. Hal: at. 3.39 Name the five isomers ol‘ Chi-1H. 3.4D Explain why each of the following names is incorrect: (a) 2,2-DimCthylAoeethylheptane (b) 4-Ethyl-5,S-dimethylpentane (c) 3-Ethyl-4,+dimethylhcxane (d) 5,5,6-Trimethyloctane (e) 2-[50propyl-4—methyliieptane 3.41 Propose structures and give IUPAC names for the following: (a) A diethyldin'tethy‘lhexane (b) A (fa-methylbu'tyl)-substititted alkane 3.42 til Consider 2-methylbutane iisopentane). Sighting along the C2—C3 bond: (21) Draw a Newman projection of the most stable conformation. (h) Draw a Newman projection of the least stable conformation. (c) Since a CH3H>CH3 eclipsing interaction costs 11 kJ/mol (2.5 kcai/moi) and a CI~13HCH3 gauche interaction costs 3.8 kJ/mol (0.9 kcal/moli, make a quantitative plot of energy versus rotation about the C243 bond. 3.43 B What are the relative energies of the three possible staggered conformations around the C27C3 bond in 2,3-dimetl‘iylbutane? [See Problem 3.42.) Assignable in OWL 3.44 3.45 3.46 3.47 3.48 3.49 3.50 3.51 3.52 Exercises 105 Construct a qualitative potential-energy diagram for rotation about the C—C bond of 1,2-dibrom oethane. Which conforniatiOn would you expect to be more stable? Label the anti and gauche conformations of 1,2-dibromoethane. Which conformation of 1,2-dibromoethane (Problem 3.44) would you expect to have the larger dipole moment? The observed dipole moment of 1,2-dibromoethane is u : 1.0 D. What does this tell you about the actual structure of the molecule? ta The barrier to rotation about the C—C bond in bromoethane is 15 kJ/mol (3.6 kcal/rnoli. (a) What energy value can you assign to an l-lvlsr eclipsing interaction? (b) Construct a quantitative diagram of potential energy versus bond rotation for bl‘Omoctliane. Draw the most stable conformation ofpentane, using wedges and dashes to rep- resentbonds coming out of the paper and going behind the paper, respectively. Draw the most stable conformation of 1,4—dichlorobutane, using wedges and clashes to represent bonds coming out of the paper and going behind the paper, respectively. Malic acid, (T411605. has been isolated from apples. Because this compound reacts with 2 molar equivalents of base, it is a dicarboxylic acid. (a) Draw at least five possible structures. (b) If malic acid is a secondary alcohol, what is its structure? ta“: Formaldehyde, ]-13C=O, is known to all biologists because of its usefulness as a tissue preservative. When pure, formaldehyde trimerizr's to give trioxane, Cal-1603, which, surprisingly enough, has no carbonyl groups. Only one monobromo derivative (C3HsBrO3) of trioxane is possible. Propose a structure for trioxane. lncreased subsfitution around a bond leads to increased strain. Take the four substituted butanes listed below, for example. For each compound, Sight along the C27C3 bond and draw Newman projections of the most stable and least stable conformations. Use the data in Table 3.5 to assign strain energy values to each conformation. Which of the eight conformations is most strained? Which is least strained? (a) 2-lv1ethylbutane (b) 2,2-Di.methylbutane (C) 3,3—DiITletl'iy‘lbutane (d) 2,2,3vTrimetl‘iylbutane The cholesterol-lowering agents called statins, such as simvastatin (Zocorl and pravastatin (Pravachol), are among the most widely prescribed drugs in the world. Identify the functional groups in both, and tell how the two Sub- stances differ. O // HO 0 r‘“ R HO OH 0 OH 0 / o ‘l , o o / / | /CI-I3 \/CH3 H3C / / HO / / Simvastatin Pravastatin (Zocorl (Pravacholl t3 Assignable in OWL ...
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