Outline for CH 231 at UA-Elem Organic Chem I (1)
Outline for CH 231 at UA-Elem Organic Chem I (1)
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Date Created: 02/06/15
Chapter 4 Outline I 41 Naming Cycloalkanes 0 Cyclic alkanes are named by appending the cyclo pre x to the alkane name 0 Find the parent I Evaluate the largest ring and the longest linear chain separately The one with more carbons will be the parent hydrocarbon I If ring is larger alkylcycloalkane I If chain is longer cycloalkylalkane 0 Number substituents I If only one substituent on ring no number I For more than one substituent number substituents to give the lowest possible substituent numbers I If two numbering systems give the same substituent numbers give lowest number to substituent coming rst alphabetically I 42 Cis Trans Isomerism in Cycloalkanes o A disubstituted cycloalkanes can have two possible isomeric structures I Both substituents can be pointed in the same direction This called cis I The substituents can be pointed in opposite directions This is called trans o Cis and trans isomers are examples of stereoisomers Stereoisomers are the same constitutional isomer but differ in the 3dimensional arrangement of the atoms I 43 Stability of Cycloalkanes o In nature 5 and 6membered rings are very common but 3 and 4membered rings are very rare Why I von Baeyer said this was due to the angle strain associated with smaller rings I Cyclopropane would have 60 bond angles which are signi cantly smaller than the ideal 1095 I Cyclobutane would have 90 bond angles cyclopentene would be 108 and cyclohexane 120 I This analysis predicts a decrease in strain from cyclopropane to cyclopentene but that cyclohexane would have more strain I What is the actual stability of cycloalkanes I Measurements show that cyclopropane 115 kJmol and cyclobutane 110 kJmol have very high strain energy Cyclopentane has a small amount of strain 26 ldmol while cyclohexane has no strain energy I Why are these energies different than what von Baeyer predicted I 44 Conformations of Cycloalkanes o Cyclopropaneil 15 ldmol strain energy 3 atoms can only be in a planar arrangement A Newman projection looking along one of the CC bonds shows that the CH bonds are all eclipsed This induces a large amount of torsion strain in addition to the large angle strain 0 Cyclobutaneil 10 kJmol strain energy If cyclobutane were planar all of the CH bonds would be eclipsed This would result in even more torsion strain than cyclopropane To relieve some of the torsion strain cyclobutane adopts a nonplanar geometry where two of the carbon atoms are folded down slightly This bending slightly decreases the bond angles which increases angle strain The release in torsion strain more than makes up for the increase in angle strain 0 Cyclopentane726 kJmol strain Cyclopentane adopts an envelope conformation where one of the carbon atoms is above the plane of the other 4 This conformation lowers the torsion strain significantly The larger bond angles also means there is little angle strain I 45 Conformation of Cyclohexane o Cyclohexane adopts a chair conformation Four carbons are in the same plane while one is below and one above H H2 H H c H This conformation results in a perfectly staggered conformation H H Ea H H The bond angles are 1095O There is no strain in the structure of cyclohexane 0 Drawing the chair Draw two parallel lines Place one carbon above the 4 carbons and to the right or left Place the other carbon before the original 4 carbons and to the other side When looking at the chair you are looking at the cyclohexane from the side The lower set of bonds are always the front carbons I 46 Axial and Equatorial Bonds in Cyclohexane o In looking at the chair conformation of cyclohexane there are two different kinds of CH bonds The aXial hydrogens point vertically up and down relative to the plane of the carbon atoms The equatorial bonds point along the plane of the of the carbon atoms Each carbon has one axial and one equatorial hydrogen Going around the ring the axial hydrogens alternate from pointing up and down On each carbon the equatorial bonds point in the opposite direction to the axial group Thus if the axial bond is pointed up the equatorial group will point down 0 The chair conformation is not static Chair conformations undergo a process called ring ipping In the ring ip the carbon pointed down rotates up and the carbon pointed up rotates down This process exchanges all of the axial and equatorial positions I 47 Conformations of Monosubstituted Cyclohexanes o If we consider methylcyclohexane we can draw two conformations The methyl group could be in the axial position or in the equatorial position The conformation with methyl equatorial is 76 ldmol more stable than the conformation with the methyl group axial When the methyl group is axial there is strain due to interactions with the 2 ring carbons with axial hydrogens pointing in the same direction This is really a gauche interaction between the axial methyl and the ring carbons see Newman projection Note that the 76 ldmol value is twice the gauche strain of 38 ldmol because the axial group has two gauche interactions with ring carbons When the substituent is equatorial there is no strain The Newman projection shows that the methyl group is anti to the ring carbons Table 41 shows the strain associated with different substituents These values are for l axial interaction so need to be doubled These values are proportional to the size of the ring substituent I 48 Conformations of Disubstituted Cyclohexanes cis l 2dimethylcyclohexane I In the cis conformation both methyl groups would be up or down In the chair the axial and equatorial positions alternate up and down Thus one methyl group will be axial and one equatorial I There will be 76 kJmol strain due to the axial methyl The two methyl groups are also gauche to each other causing another 38 ldmol I The conformation formed after doing the ring ip has the same strain as one methyl is still axial and one equatorial trans l2dimethylcyclohexane I In the trans case one methyl is up and one down Thus they can both be axial or both equatorial I In the diaxial conformation there is 152 ldmol of strain I In the diequatorial conformation there is no axial strain but there is a gauche interaction between the methyl groups 38 kJmol I The transdiequatorial conformation has the lowest strain so the trans isomer is more stable than the cis one I If there are different substituents on the ring the largest substituent should be placed in the equatorial position I 49 Conformation of Polycyclic Molecules O O Decalin is made up of 2 cyclohexane rings fused together cisDecalin I We can consider one ring as the substituents on the other I Like the case above cisdecalin always has one equatorial and one axial substituent transDecalin I In the trans isomer both ring carbons can be equatorial This will be the more stable form Steroids have a complex ring structure with 3 6membered rings and a 5membered ring I Examples of steroids include estrogen testrosterone and cholesterol I Their preferred stereochemistry has all of the rings with transstereochemistry like transdecalin This gives a rigid stable structure
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