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Reactions of Alkenes and Alkynes. Chapter 5

Reactions of Alkenes and Alkynes. Chapter 5. Contents of Chapter 4. Electrophilic Addition Reactions Carbocations Various Addition Reactions. Mechanism for Electrophilic Addition to Alkenes. Reaction of 2-butene with hydrogen bromide is typical of electrophilic addition to alkenes

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Reactions of Alkenes and Alkynes. Chapter 5

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  1. Reactions of Alkenes and Alkynes. Chapter 5 Chapter 5

  2. Contents of Chapter 4 • Electrophilic Addition Reactions • Carbocations • Various Addition Reactions Chapter 5

  3. Mechanism for Electrophilic Addition to Alkenes • Reaction of 2-butene with hydrogen bromide is typical of electrophilic addition to alkenes • The reaction starts with the slow addition of an electrophile to an sp2 carbon, resulting in formation of a carbocation • The next step is the rapid addition of a nucleophile to the other sp2 carbon Chapter 5

  4. Addition of Hydrogen Halides to Alkenes The more substituted carbocation is preferred Chapter 5

  5. Stability of Carbocations Alkyl groups (“R”s) tend to stabilize the positive charge on the sp2carbon of a carbocation Chapter 5

  6. Transition State Stability Formation of a tertiary carbocation should be faster than formation of a primary carbocation Chapter 5

  7. Markovnikov’s Rule • Modern equivalent statement: • The electrophile adds to the sp2carbon that forms the least stable carbocation • The other carbon forms the carbocation intermediate. • Predicting product: put + and – signs under carbons and electrophile and nucleophile and swap partners. Chapter 5

  8. Addition of Water Chapter 5

  9. Addition of Alcohol Initiating the reaction: generating a carbocation H + e l e c t r o p h i l i c a t t a c k o f H H C + H C H C H 3 3 2 + C H C H O H C C H 3 2 2 H C s l o w C H 3 3 2-methylpropene carbocation Chapter 5

  10. Addition of Alcohol The resulting carbocation is attacked by the nucleophilic alcohol H n u c l e o p h i l i c a t t a c k o f e t h a n o l C H H C 2 H 3 C H H C 3 C H 3 2 H O + C H C H O H 3 2 f a s t H C C H C H ethanol 2 3 3 Chapter 5

  11. Product Analysis Chapter 5

  12. Alkynes Common names of alkynes are based on substitution of the simplest alkyne, acetylene Chapter 5

  13. IUPAC Nomenclature of Alkynes • Find the longest chain containing the triple bond and change the corresponding “-ane” ending to “-yne” • The chain is numbered in direction that gives the triple bond, the lower number • If the same number for the triple bond is obtained in numbering from both directions, the number for the substituent nearest the chain end is minimized Chapter 5

  14. IUPAC Multifunctional Compound Nomenclature Chapter 5

  15. IUPAC Multifunctional Compound Nomenclature • The longest chain has to go past the highest-priority functional group • High prio group has lowest possible number • If not highest priority NH2 is amino and OH is hydroxy substituent • General form is n-substit-n-alken-n-yn-n-groupsuffix Chapter 5

  16. Reactivity Considerations • The hydrohalogenation product is an alkene which can undergo a second electrophilic addition reaction • First halogen follows alkene hydrohalogenation regioselectivity rules • Second halogen goes on same carbon as first halogen Chapter 5

  17. Relative Stabilities of Carbocations Vinyl cations are one level less stable than alkyl cations Chapter 5

  18. Halogen Addition to Alkynes • Halogens add to alkynes as well as to alkenes • Excess halogen leads to the addition of a second equivalent Chapter 5

  19. Addition of Water to Alkynes Water adds to alkynes in the presence of acid to yield an enol Chapter 5

  20. Addition of Water to Alkynes However the initially formed enol reacts further to produce a ketone Such isomers, differing only in the placement of a hydrogen atom, are called tautomers Chapter 5

  21. Addition of Hydrogen to an Alkyne Chapter 5

  22. Acidity of a Hydrogen Bonded to an sp Carbon The conjugate bases have the following relative base strength because the stronger the acid, the weaker the conjugate base Chapter 5

  23. Syntheses Using Acetylide Ions Alkylation reactions work best with primary alkyl halides and methyl halides Chapter 5

  24. Introduction to Multistep Synthesis The thought process is known as retrosynthetic analysis and is indicated by using open arrows Chapter 5

  25. Introduction to Multistep Synthesis - Considerations • Alkene products made from alkynes • Carbonyl products made from alkynes • Alkane products made from alkenes • Understand issues involved in proper choice of reagents thoroughly Chapter 5

  26. Introduction to Multistep Synthesis – Reagent Issues • Regioselectivity of HX + alkene/alkyne rxn • Regioselectivity of 2 HX + alkyne rxn • Stereospecificity of H2/Lindlar reduction • H2 with Pd/C, Pt/C, or Ni does complete reduction • RX + NaNH2 + H-C=C-R rxn works best with primary halide Chapter 5

  27. Introduction to Multistep Synthesis – Practice Use retrosynthetic analysis to make these: Chapter 5

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