Chem 261 1st Edition Lecture 17Outline of Last Lecture:I. Alkene Naming ReviewII. Cis-Trans Isomers of AlkenesIII. E, Z Designation in AlkenesIV. Alkene Stability and SubstitutionOutline of Current Lecture:I. Revisiting Alkene StabilityII. Substitution of CycloalkenesIII. Electrophile AdditionRevisiting Alkene Stability:1. In linear alkenes, trans configurations are more stable than cis configurations2. With constitutional isomers, more substitutions (ex. A tetra-substituted configuration) is favorable/more stable than less substitutions (ex. Mono-substitution)3. Cycloalkene stability: HyperconjugationDef: The atomic overlap of C-R bonds, where R is not H, stabilizes the pi bonds in the system (the more substituted an alkene, the more stable. When the R group isn’t a hydrogen, there are more orbitals for overlap)C3-C4 bond: lots of angle strainC5-C6 bond: a bit of angle strainC7 and up: very little angle strain. Capable of trans C=C **C10 or bigger = most stable form is trans)Substitution of Cycloalkenes: -For C5 and larger, the more substituted C=C is best**most stable is when the double bond is on the OUTSIDE of the ring!!**for C4 and under, strange stabilities, many factors contribute to overall stabilityElectrophile Addition:-The pi bond attracts an electrophile produces carbocation intermediate reacts with the nucleophile (alkene = nuc:-)-With a reaction intermediate, there will be two transition states. The first transition state is the rate determining step of the reaction-The reaction produces a carbocation intermediate (2 different options in the below example!)“A” is bound to the most substituted end of the alkene/ most stable C+**hyperconjugation: C-C bonds have more overlap, which stabilizes the carbocation!Most stable = tertiary carbon (bound to 3 R groups)(Secondary carbon= bound to 2 R groups, 1 H…Primary carbon= bound to 1 R group, 2 hydrogens)Markonikov’s Rule: For HX addition, H+ adds to the lest substituted end of C=C (least stable carbocation)Hammond Postulate: 2 starting materials, 2 intermediates, 2 products. The more stable products form
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