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cyclohexane cis-1,2-dimethylcyclohexane cis-1,2-dimethylcyclohexane axial-equatorial trans-1,2-dimethylcyclohexane trans-1,2-dimethylcyclohexane equatorial-equatorial axial-axial Optical isomerism * C* = Stereocenter 4 different substituents Br Fischer projections C* I C* Cl Cl I H H (+) (-) Br Br I Br * H Cl Enantiomers Cl * H non-superimposable mirror images I 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 Polarized light One enantiomer - rotate light to the left (-) Other enantiomer - rotate light to the right (+) in 50/50 mix - no net rotation racemic mixture one enantiomer - polarized light will be rotated optical activity Optical isomerism * C* = Stereocenter 4 different substituents bromochloro iodomethane * 3-methyl hexane * * bromocyclopentane no C* trans-1,3-dibromocyclopentane Alkane Summary 1. Alkanes - sp3 hybridized 2. Relatively unreactive Substitution with halogens Combustion 3. Non-polar IMF = London Dispersion Forces size structure 4. Free rotation around C-C bonds conformations 5. Non-cyclic alkanes - structural isomers 6. Cyclic alkanes - geometric isomers cis-, trans7. Alkanes - optical isomers stereocenters C* alkyl halides 1o 1o 3o 1o + Cl2 uv 2o .. .. step 1 :Cl:Cl: .. .. .. . 2 :Cl .. half-arrow = 1e- form mostly stability of free radicals 3o > 2o > 1o 3-methylhexane + OH- * no reaction stereocenter * + - + OH- C+ electrophile .. :OH nucleophile .. + Br- e- deficient e- rich Nucleophilic Substitution SN1 * + OH- * + Br- reaction is 1st order in C7H15Br zero order in OH- reactants are optically active products are optically inactive rate = k [R – X] Nucleophilic Substitution SN1 rate determining step unimolecular step 1 CH3 .. C2H5 – C – Br . .: C3H7 CH3 C2H5 – C+ .. -: : Br .. slow C3H7 C+ carbocation (4 – ½ (6) – 0) = +1 reactants are optically active products are optically inactive rate = k [R – X] Nucleophilic Substitution SN1 step 1 CH3 .. C2H5 – C – Br . .: C3H7 step 2 CH3 HO * .. : H–O .. C3H7 CH3 .. -: : Br .. C2H5 – C+ slow C3H7 CH3 C+ .. . .– H -:O C2H5C3H7 C2H5 reactants are optically active products are optically inactive CH3 H7C3 * OH C2H5 rate = k [R – X] Nucleophilic Substitution SN1 Ea C+ carbocation intermediate 1st order in R – X E 2 products Nu- + R – X Nu – R + X- Nucleophilic Substitution SN2 * + * OH- + Br- reaction is 1st order in C4H9Br 1st order in OH- rate = k [R – X] [OH-] reactants are optically active products are optically active Nucleophilic Substitution SN2 rate determining step bimolecular H step 1 H .. .. H–O . .: C – Br . .: C2H5 CH3 C2H5 H C2H5 Br HO .. H-O---C---Br . .: C2H5 CH3 transition state H HO – C *– CH3 C2H5 .. :Br: .. slow H stereocenter inverted CH3 CH3 reactants are optically active products are optically active rate = k [R – X][OH-] - Nucleophilic Substitution SN2 Ea Nu---C---X 1st order in R – X 1st order in Nu- E 1 product Nu- + R – X Nu – R + X- SN1 or SN2 H R H R R SN1 carbocation 3o > 2o > 1o 3o R –X yes 2o R –X ? 1o R –X no SN1 or SN2 SN2 transition state 1o R – X yes 2o R –X ? 3o R – X no SN1 or SN2 SN1 3o R –X yes 2o R –X ? 1o R –X no SN2 1o R – X yes 2o R –X ? 3o R – X no Other factors influencing rates: Nucleophile : charged OH- SH- I- CNneutral H2O NH3 Solvent Temperature