Chapter 4

Chapter 4 - CHAPTER 4 Cyclic Alkanes 4-1 Names and Physical...

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HAPTER 4 CHAPTER 4 Cyclic Alkanes
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Names and Physical Properties of Cycloalkanes 4-1 he names of the cycloalkanes follow IUPAC rules The names of the cycloalkanes follow IUPAC rules. The general formula of a cyclealkane is: C n H 2n . To name a cycloalkane, prefix the alkane name with “cyclo”. Numbering a cycloalkane ring is necessary only when there is more than one substituent. In nonsubstituted cycloalkanes, the attached carbon is numbered number 1.
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Use the lowest possible numbering sequence for polysubstituted ycloalkanes. If two identical numbering sequences are possible, cycloalkanes. use alphabetical precedence. Substituted cycloalkanes are sometimes named as cycloalkyl erivatives The smaller unit is generally treated as a substituent derivatives. to the larger unit. •Propylcyclopentane (not cyclopentylpropane) •Cyclohexyloctane (not octylcyclohexane)
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Disubstituted cycloalkanes possess stereoisomers. Disubstituted cycloalkanes having substituents on different carbons possess cis (same side) / trans (opposite side) isomers.
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Cis/trans isomers are examples of stereoisomers. These are non- superimposable molecules having the same molecular formula and connectivity. Conformers are also stereoisomers but can be interconverted by tations about C- bonds. rotations about C C bonds.
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The properties of the cycloalkanes differ from those f their straight chain analogs. of their straight chain analogs. Cycloalkanes have higher boiling points, melting points, and densities than their straight-chain analogs. •Increased London forces are due to more rigid and symmetric cyclic systems. he smaller cycloalkanes with an odd number of carbon atoms The smaller cycloalkanes with an odd number of carbon atoms have lower melting points than expected compared to cycloalkanes with an even number of carbon atoms. This is ttributed to crystal- acking forces between the two types of attributed to crystal packing forces between the two types of cycloalkanes.
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Ring Strain and the Structure of Cycloalkanes 4-2 he heats of combustion of the cycloalkanes reveal The heats of combustion of the cycloalkanes reveal the presence of ring strain. Cyclopropane (60 o ) and cyclobutane (90 o ) possess C-C-C bond angles significantly different from 109.5 o and are strained. This is termed ring strain . yclohexane has essentially no ring strain. Cyc o e a e as esse a y o g s a The ring strain in cycloalkanes can be estimated by comparing the theoretical heats of combustion to the measured heats of ombustion: combustion: Four groupings: Small (3,4) Common (5,6,7) edium (8- 2) Medium (8 12) Large (>12)
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Strain affects the structures and onformational function of the smaller conformational function of the smaller cycloalkanes. Cyclopropane: All of the methylene hydrogens are eclipsed ( eclipsing strain ) here is no available bond rotation to There is no available bond rotation to achieve a staggered conformation.
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Chapter 4 - CHAPTER 4 Cyclic Alkanes 4-1 Names and Physical...

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