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cyclohexane

cyclohexane. cis -. 1,2-. dimethyl. cyclo. hexane. cis -1,2-dimethylcyclohexane. axial - equatorial. trans -. 1,2-dimethylcyclohexane. trans -1,2-dimethylcyclohexane. axial - axial. equatorial - equatorial. Optical isomerism. Stereocenter 4 different substituents. C * =. *. Br.

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cyclohexane

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  1. cyclohexane cis- 1,2- dimethyl cyclo hexane cis-1,2-dimethylcyclohexane axial-equatorial trans- 1,2-dimethylcyclohexane trans-1,2-dimethylcyclohexane axial-axial equatorial-equatorial

  2. Optical isomerism Stereocenter 4 different substituents C*= *

  3. Br Br Fischer projections * C C * I Cl Cl I H H (+) (-) Br Br Enantiomers * * I Cl Cl I H H non-superimposable mirror images

  4. Enantiomers identical in most properties differ in: 1.interaction with polarized light 2. interaction with chiral environments Light interacts with molecules when it passes through them [interaction of electrical fields] When light encounters mirror image of molecule, interaction is reversed

  5. Polarized light (-) Oneenantiomer - rotate light to the left (+) Otherenantiomer - rotate light to the right in 50/50 mix - no net rotation racemic mixture one enantiomer - polarized light will be rotated optical activity

  6. Optical isomerism Stereocenter 4 different substituents C*= * * 3-methyl hexane bromo chloro iodo methane * * bromo cyclo pentane trans- 1,3-dibromo cyclopentane no C*

  7. Alkane Summary 1. Alkanes -sp3 hybridized 2. Relatively unreactive Substitution with halogens Combustion 3. Non-polar IMF = London Dispersion Forces size structure

  8. 4. Free rotation around C-C bonds conformations 5. Non-cyclic alkanes - structural isomers 6. Cyclic alkanes - geometric isomers cis-, trans- 7. Alkanes -optical isomers stereocenters C*

  9. alkyl halides 1o uv + Cl2 1o 3o 1o 2o .. .. :Cl:Cl: .. .. .. 2 :Cl. half-arrow = 1e- step 1 .. form mostly stability of free radicals 3o > 2o > 1o

  10. 3-methylhexane no reaction + OH- * stereocenter * + Br- + OH- C+ electrophile e- deficient + .. - : OH- nucleophile e- rich ..

  11. Nucleophilic Substitution SN1 * * + Br- + OH- reaction is 1st order in C7H15Br zero order in OH- reactants are optically active rate = k [R – X] products are optically inactive

  12. CH3 CH3 . . . . : : C2H5 – C – Br : C2H5 – C+ Br- . . . . C3H7 C3H7 Nucleophilic Substitution SN1 rate determining step unimolecular step 1 slow C+ carbocation (4 – ½ (6) – 0) = +1 reactants are optically active rate = k [R – X] products are optically inactive

  13. CH3 CH3 . . . . : : C2H5 – C – Br : C2H5 – C+ Br- . . . . C3H7 C3H7 . . CH3 . . : - O – H . . : H – O - . . C+ CH3 CH3 C3H7 HO C3H7 H7C3 OH C2H5 C2H5 C2H5 Nucleophilic Substitution SN1 step 1 slow step 2 * * reactants are optically active rate = k [R – X] products are optically inactive

  14. E C+ Nucleophilic Substitution SN1 Ea carbocation intermediate 1st order in R – X 2 products Nu – R + X- Nu- + R – X

  15. Nucleophilic Substitution SN2 * * + Br- + OH- reaction is 1st order in C4H9Br 1st order in OH- rate = k [R – X] [OH-] reactants are optically active products are optically active

  16. H H H . . . . : C – Br HO – C – CH3 H-O . . : ---C---Br . . CH3 C2H5 C2H5 CH3 C2H5 . . : H – O - . . C2H5 C2H5 . . :Br:- . . H Br HO H CH3 CH3 Nucleophilic Substitution SN2 rate determining step bimolecular step 1 * slow transition state stereocenter inverted reactants are optically active rate = k [R – X][OH-] products are optically active

  17.  -  - Nu---C---X E Nucleophilic Substitution SN2 Ea 1st order in R – X 1st order in Nu- 1 product Nu – R + X- Nu- + R – X

  18. SN1 or SN2 H R R R H R 3o R –X SN1 carbocation 3o > 2o > 1o yes 2o R –X ? 1o R –X no

  19. SN1 or SN2 1o R – X yes SN2 transition state 2o R –X ? 3o R – X no

  20. SN1 or SN2 3o R –X SN2 1o R – X yes yes SN1 2o R –X 2o R –X ? ? 1o R –X no 3o R – X no Other factors influencing rates: Nucleophile : SH- I- CN- charged OH- neutral NH3 H2O Solvent Temperature

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