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Sect 4.6: Monosubstituted cyclohexane rings

Sect 4.6: Monosubstituted cyclohexane rings. Methylcyclohexane conformations. Axial methyl. Equitorial methyl. Energy difference between an axial and an equitorial methyl group. E N E R G Y. 1.7 kcal/mol. 0 kcal/mol. 1,3-Diaxial interactions on the top of the ring.

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Sect 4.6: Monosubstituted cyclohexane rings

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  1. Sect 4.6:Monosubstituted cyclohexane rings

  2. Methylcyclohexane conformations Axial methyl Equitorial methyl

  3. Energy difference between an axial and an equitorial methyl group ENERGY 1.7 kcal/mol 0 kcal/mol

  4. 1,3-Diaxial interactions on the top of the ring 1,3-diaxial interactions STERIC REPULSION RAISES THE ENERGY OF THE AXIAL CONFORMATION

  5. 1,3-Diaxial interactions 1,3-diaxial interactions CH3 H H H H CH H H H H 1,3-Diaxial interactions: Newman projection view

  6. Monosubstituted cyclohexanes: gauche steric interactions GAUCHE STERIC INTERACTIONS gauche steric nteractions 60o CH3 (like gauche butane) CH3 CH2 CH3 Axial CH2 180o CH3 CH2 Equitorial No gauche steric problem when the group is equitorial CH2

  7. General rule Large groups will generally prefer to occupy an equatorial position where there is an absence of 1,3-diaxial (steric) interactions axial conformation equatorial conformation Keep in mind, however, that the axial conformation will also be present, but in smaller amount.

  8. Table 4.5: Conformational energy differences for substituents attached to a cyclohexane ring Equitorial preferred DGofor group in the axial position Group X kcal/mol kJ/mol Group kcal/mol kJ/mol CH3- 1.7 7.1 Cl- 0.4 1.7 Br- CH3CH2- 1.8 7.5 0.5 2.1 CH3-CH- 8.8 0.7 2.1 2.9 HO- CH3 C6H5- 3.1 13 CH3 CH3-C-O- 0.7 2.9 >5 >21 CH3-C- O CH3

  9. H H H C H C C H H C H H H t -BUTYLCYCLOHEXANE Too big a group to go into the axial position - must go equatorial. Basically “locks” the ring in a chair with the tert -butyl group in the equatorial position. The axial value for this group in Table 4-5 ( >5 Kcal/mole) indicates a minimum value because there is so little axial that it is difficult to measure any real value.

  10. tert-Butylcyclohexane with the group axial HUGE steric strain

  11. Molecules viewed with Chime • Click on START, Click on PROGRAMS • Click on Netscape Communicator (4.7), then launch Netscape Navigator • Using Google, type in the address for the Dept. of Chemistry, WWU: http://www.chem.wwu • Select, course materials, select “WWU virtual molecular model set” • You may need the free program, Chime, to run this program. • Note: Internet Explorer and Netscape 7.1 won’t work!

  12. Sect 4.7: cis and trans isomerization in cycloalkanes

  13. cis-trans isomerism • Different spatial arrangements • The arrangements cannot be converted into one another by rotation • cisBoth substituents on same side of plane • transSubstituents on opposite sides of plane

  14. cis and trans isomers applies to substituents on a ring or (later) double bond Cl Cl Cl Cl cis trans both substituents are on the same side of the ring the substituents are on opposite sides of the ring These two compounds are geometric isomers

  15. Naming cis /trans isomers place designation in front of name Cl Cl cis-1,2-dichlorocyclopropane notice italics Cl trans-1,2-dichlorocyclopropane Cl

  16. cis cis no cis or trans trans trans How many different dimethylcyclobutanes are there? Constitutional isomers 1,1- 1,2- 1,3- cis /trans isomers (geometric) no cis/trans here

  17. Planar ring approximation Notice that it is OK to use planar rings when figuring out cis / trans isomers. Use planar structures on tests! You only need to use puckered rings when you are dealing with conformations.

  18. Sect 4.8: disubstituted cyclohexanes: cis/trans isomerismUse chair structures!!

  19. trans-1,2-dimethylcyclohexane has two possible conformers trans-1,2-dimethylcyclohexane CHAIR-1 CHAIR-2 Methyl above Methyl below e,e a,a Which conformer is more stable? The trans e,e one!

  20. Calculating the energy difference using values from Table 4.5 trans-(a,a)-1,2-dimethylcyclohexane 3.4 kcal/mol higher (two axial methyls) 2 x 1.7 kcal trans-(e,e)-1,2-dimethylcyclohexane Reference = 0 kcal/mol D G = (2)(1.7) – 0 = 3.4 kcal/mol o Group kcal/mol kJ/mol Group kcal/mol kJ/mol CH - 1.7 7.1 Cl- 0.4 1.7 3 Br- CH CH - 1.8 7.5 0.5 2.1 3 2 CH -CH- 8.8 0.7 2.1 2.9 HO- 3 CH C H - 3.1 13 3 6 5 CH 3 CH -C-O- 0.7 2.9 3 >5 >21 CH -C- 3 O CH 3

  21. 1,3-Diaxial interactions (steric) on top and bottom of ring No diaxial interactions lots of room Two axial-axial problems @ 1.7 kcal/mol each Equatorial groups are assumed to be 0 kcal/mol

  22. What about cis-1,2-dimethylcyclohexane? Class exercise!!

  23. What about cis / trans isomers in disubstituted rings other than 1,2-dimethylcyclohexane? 1,1-dimethylcyclohexane: no cis/ trans isomers 1,3-dimethylcyclohexane: 4 chair structures 1,4-dimethylcyclohexane: 4 chair structures

  24. DG = (1.7 - 0.7) = 1.0 kcal/mol Which conformer has the higher energy? Both are trans! This one! CH3 CH3 axial = 1.7 kcal/mol equatorial = 0 kcal/mol OH OH equatorial = 0 kcal/mol axial = 0.7 kcal/mol 1.7 kcal/mol 0.7 kcal/mol

  25. Guideline In substituted cyclohexane rings, the best (lowest energy) conformation will have the largest groups in equatorial positions whenever possible.

  26. Sect 4.9: decalinskip this section, winter 07

  27. cis and trans ring fusions trans-ring fusion cis-ring fusion bonds are cis bonds are trans cis-decalin: less stable trans-decalin

  28. other representations trans cis Drawing Conventions top solid wedge = towards you dashed wedge = away from you bottom A dot implies the hydrogen is towards you (on top).

  29. Sect 4.10: read this section; no lectures Skip this section, winter 07

  30. Sect 4.11: cis/trans isomerism in alkenes

  31. Alkene geometry: planar p bond sp2 p bond sp2 s bond planar s bond END VIEW SIDE VIEW

  32. ROTATION BREAKS THE p BOND Unlike s bonds, p bonds do not rotate. NO! It requires about 50-60 kcal/mole ( ~ 240 kJ/mole ) to break the p bond - this does not happen at reasonable temperatures.

  33. cis /trans isomers (geometric isomers) Because there is no rotation about a carbon-carbon bond, isomers are possible. trans cis substituents on the same side of main chain substituents on opposite sides of main chain

  34. Compare cis / trans isomers in ring compounds to alkenes cis trans cis / trans isomers are also called geometric isomers

  35. Two identical substituents If an alkene has two identical substituents on one of the double bond carbons, cis / trans isomers are not possible. all of these compounds are identical no cis / trans isomers

  36. Some other compounds with no cis / trans isomers no cis / trans isomers

  37. Naming cis / trans isomers of alkenes main chain stays on same side of double bond = cis main chain crosses to other side of double bond = trans cis-3-hexene trans-3-hexene notice that these prefixes are in italics

  38. Rings with double bonds trans double bonds are not possible until the ring has at least eight carbon atoms if C<8 then the chain is too short to join together trans cis cis C = 5 C = 8 smallest ring that can have a trans double bond cis C = 6 trans Note that bothcis and trans exist for C8.

  39. Be Careful !!! The main chain determines cis / trans in the IUPAC name cis-3-methyl-2-pentene trans-3-methyl-2-pentene This compound is cis but the two methyl groups are ….trans to each other. This compound is trans but the two methyl groups are ….cis to each other. but the terms cis and trans are also used to designate the relative position of two groups: a new system is needed!

  40. Sect 4.12: E/Z nomenclature

  41. E/Z system of nomenclature To avoid the confusion between what the main chain is doing and the relationship of two similar groups ….. the IUPAC invented the E/Z system. cis ? Cl F trans ? I H This system also allows alkenes like the one above to be classified ….. an impossibility with cis / trans.

  42. E / Z Nomenclature In this system the two groups attached to each carbon are assigned a priority ( 1 or 2 ). If priority 1 groups are both on same side of double bond: Z isomer = zusammen = together (in German) same side opposite sides Z E If priority 1 groups on opposite sides of double bond: E isomer = entgegen = opposite (in German)

  43. Assigning priorities 1. Look at the atoms attached to each carbon of the double bond. 2. The atom of higher atomic number has higher (1) priority. 1 1 example I > Br F > H 2 2 Since the 1’s are on the same side, this compound is Z (Z)-1-bromo-2-fluoro-1-iodoethene notice use of parentheses

  44. Priorities in the E-Z Nomenclature system

  45. 3. If you can’t decide using the first atoms attached, go out to the next atoms attached. If there are non-equivalent paths, always follow the path with atoms of higher atomic number. Once you find a difference, you can stop. path goes to F not to H 1 1 comparison stops here 2 2 path goes to C not to H This molecule has Z configuration.

  46. Let’s give this compound a cis/trans name and an E/Z name C H C H F 3 2 H C H C H 2 3 trans-3-fluoromethyl-2-pentene (longest chain) (Z)-3-fluoromethyl-2-pentene (priorities)

  47. C C H C H C H C H 2 2 O C C O C O H H 4. C=C double bond: equivalent to having two carbons. C=O double bond: equivalent to having two oxygens. 2 C 1

  48. 1 2 1 2 (E)

  49. 2 2 1 1 (Z)

  50. More than one double bond: dienes

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