Organic Chemistry Jonh Mc Murry4

Organic Chemistry Jonh Mc Murry4 - 105 CHAPTER 3 Organic...

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Unformatted text preview: 105 CHAPTER 3 Organic Compounds: Alkanes and Their Stereochemistry 3.53 We’ll look in the next Chapter at cydoulkanes—saturaled cyclic hydrocarbons— and we’ll see that the molecules generally adopt puckered, nonplanar confor- mations. Cyclohexane, for instance, has a puckered shape like a lounge chair rather than a flat shape. Why? Nonplanar cyclohexane Planar cyclohexane 3.54 We’ll see in the next chapter that there are two isomeric substances both named l,2-dimethylcyclol‘lexane. Explain. H CH3 1,2-Dimethylcyclohexane CH3 H I Assignable in OWL limit Dilfigfl‘i Thomson Throughout this chapter, sign in at www.thomsonedu.com for online self-study and interactive tutorials based on your level of understanding. Online homework for this chapter may be assigned in Organic OWL. ../ 4r Organic Compounds: Cycloalkanes and Their Stereochemistry We’ve discussed only open-chain compounds up to this point, but most organic compounds contain rings of carbon atoms. Chrysanthemic acid, for instance, whose esters occur naturally as the active inSecticidal constituents of Chrysan- themum flowers, contains a three-membered (cyclopropane) ring. H3C\ CH3 /' Chrysanthemic acid c02H Prostaglandins, potent hormones that control an extraordinary variety of physiological functions in humans, contain a five-membered (cyclopentane) ring. Prostaglandin E1 \ \ ‘ / CH3 Ho'xx O H (km r7,”\/\\/\/COZH ,‘/ R I ,1. H HO H Steroids, such as cortisone, contain four rings joined together—3 six- membered (cyclohcxane) and 1 five-membered. We’ll discuss steroids and their properties in more detail in Sections 27.6 and 27.7. CH20H \ 0 CH3 f:: \\L/_,.\,;._OH CH3} H I \ Cortisone //’-"~\ 7//Jr‘\\ //"“‘- .J J a H A\ 107 108 CHAPTER 4 Organic Compounds: Cycloalkanes and Their Stereochemistry 4.1 Rulel Rule 2 WHY THIS CHAPTER? We’ll see numerous instances in future chapters where the chemistry of a given functional group is strongly affected by being in a ring rather than an open chain. Because cyclic moleCLrles are so commonly encountered in all classes of biomolecules, including proteins, lipids, carbohydrates, and nucleic acids, it’s important that the effects of their cyclic structures be understood. Naming Cyclnalkanes Saturated cyclic hydrocarbons are called cycloalkanes, or alicyclic compounds (aliphatic cyclic). Because cycloalkanes consist of rings of *CHZe units. they have the general formula (CHEM. or C,.,H2,,, and can be represented by polygons in skeletal drawings. “' . .i v‘t F; l\/I Cyclohexane O Cyclopentane D Cyclobutane A Dyclopropane Substituted cycloalkanes are named by rules similar to those we saw in the previous chapter for open-chain alkanes (Section 3.4). For most compounds, there are only two steps. Find the patent. Count the number of carbon atoms in the ring and the number in the largest substituent chain. it the number of carbon atoms in the ring is equal to or greater than the number in the substituent, the compound is named as an alkyl- substituted cycloalkane. If the number of carbon atoms in the largest sub- stituent is greater than the number in the ring, the compound is named as a cycloalkyl-substituted alkane. For example: ,-r-"" \- jzkcng, [>7crr2cn9cngcrr3 ‘7 "'J/ g_,_/ k if, I 3 carbons 4 carbons Methylcyclopentane 'I-Cyclopropylbutane Number the substituents, and write the name. For an alkyl- or halo-substituted cycloalkane, choose a point of attachment as carbon 1 and number the substituents on the ring so that the second substituent 4‘1 Naming Cycloalkanes 109 has as low a number as possible If ambiguity still exists, number so that the third or fourth substituent has as low a number as possible, until a point of dif— ferenc‘e is found. CH3 {Tr-i .- | Ji 2? 1' j” {.5 1V” NOT '5 , \CHfl J 4 = :1 . ‘l,3-Din1ethylcvclohexane ": ,fiflin'st-i-t=i\,ilcyclohexane Lower Higher H3CWCH2CH3 l ,. / l 2 5 \ /\CH3 [1 3 1-EthyI-2,6-dimethylcycloheptane _ i “Is-"l: \,7:\¢:/CH2L‘HJ Higher / l I, f u i NOT '-‘ " /\ "LL. \_ 7/) L I I. ’) T HfiWCHfiI—b 27Ethyl-ig'rdimeiiwlcycloheptane u . r J I - f “\-t. l | \4 CH3 Lower Lower ‘5 5 3-Ethyl-1A-dirnethylcycloheptane _ Higher (a) When two or more different alkyl groups that could potentially receive the same numbers are present, number them by alphabetical priority. I‘.l~1;_ CH3 2 ‘ ll q 7/CHIZC‘H3 ZK‘ CH20H3 K: K 1 k' L: NOT J a '4 3 1-Etlin-2-i‘1=é’:iiulcyclopentane Z-Ethyl-‘l-methylcyc|opentane (b) If halogens are present, treat them just like alkyl groups. CH». CH /, _ / 3 ,‘a 1 q NOT Br Br 1—Bromo-2-metiwécyclobutane 2-Bromo—1-methvlcyclobutane 110 CHAPTER 4 Organic Compounds: Cycloalkanes and Their Stereochernistry Some additional examples follow: A CH3 I I i l l t ‘ '1' CHCl-lgCl-l3 «I \Ji / i b . CH 3!: i ii ti. H1 .::=-:-_:, l-Bron1o-37e‘tliyiw3 si:.’:._::\;.'- {1'-Wiethyipropyiicyciobufane l-thoro-37etiwiflZ-mei'iwi- cyclohexane or sec-britylcvclobutane cyclopentane U Problem 4.1 Give lUPAC names for the following cycloalkanes: laij: lb) CH2CHch-I3 (cl / | l Q EKV \ CH3 CH3 (d) CH2CH3 (El/\[CHg (fl Br \/ CHiCH3)2 CH3 Br \C(CH3l3 Problem 4.2 Draw structures corresponding to the following lUPAC names: (a) 1,1-Dimethylcyclooctzine (b) 3-C)-'Clobutylhexane (c) 1,2—DichIorocyclopentane (d) 1,3—Dibrom0-5-methylcyclohexane Problem 4.3 Name the following cycloalltane: 4.2 Bis—Trans lsomerism in Cycloalkanes in many respects, the chemistry of cycloalkanes is like that of open-Chain alkanes: both are nonpolar and fairly inert. There are, however, some important differences. One difference is that Cycloalkanes are less flexible than open-chain alkanes. In contrast with the relatively free rotation around single bonds in open- Chain alkzmes (Sections 3.6 and 3.7), there is much less freedom in cycloalkancs. ...
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This note was uploaded on 08/03/2011 for the course ECOM 101 taught by Professor Ata during the Spring '11 term at Acadia.

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Organic Chemistry Jonh Mc Murry4 - 105 CHAPTER 3 Organic...

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