Chapter 5. Reactions of Alkenes and Alkynes

Transcription

1 Chapter 5. Reactions of Alkenes and Alkynes Learning objectives: 1. Identify the followings from a reaction coordinate diagram when applicable: endothermic or exothermic reactions, activation energy, heat of reaction, locations of transition states, locations of intermediates, and rate-limiting step. 2. Identify electrophiles and nucleophiles 3. Differentiate primary, secondary, and tertiary carbocations, and recognize the order of stability for these carbocations. 4. Write the electron-pushing (arrow-pushing) mechanisms for hydrohalogenation, hydration and halogenation. 5. Draw the reaction coordinate diagram and provide all the applicable information in hydrohalogenation, hydration and halogenation. 6. Explain the regio- and stereoselectivity issues involving in hydrohalogenation, hydration and halogenation of alkenes. 7. Know the products from hydrogenation of alkenes and alkynes. Sections to be covered (in the order of delivery): 5.1 Electron-pushing Mechanism and Reaction Coordinate Diagram 5.2 Electrophilic Addition: ydrohalogenation 5.3 ydration 5.4 alogenation and ydrogenation 5.5 An Introduction to Alkynes* *: section to be skipped 5.1 Electron-pushing Mechanism and Reaction Coordinate Diagram Electron-pushing mechanism is the key to understand organic reactions! Understand the movement of electron may help you understand organic reaction better! A. Arrows in rganic Reactions Use to represent two electrons movement. Use to represent one electron movement. 1

2 B. Before Deriving an Electron-pushing Mechanism (i) Identify electrophiles and nucleophile. Use Lew structures when needed. (ii) Recognize electron-rich and electron-deficient atoms. The sites where electron movement will occur. ctet rule! Inductive effect! Electronegativity! (iii) Compare starting compounds and products and recognize the chemical that need to be broken or formed. bonds Identify what needs to be changed (broken or formed). C. Derive an Electron-pushing Mechanism Example: electrophilic addition of alkenes Cl Cl 2

3 D. Reaction Coordinate Diagram Reaction coordinate diagram describes the energy changes that takes place during a reaction Important Terminologies: starting materials, products, reaction mechanism, energy diagram, reaction coordinate, heat of reaction ( ), exothermic reaction, endothermic reaction, activation energy (Ea) or free energy of activation ( G ), reaction intermediate, rate-determining step (rate-limiting step) and transition state (TS ). Cl II Cl potential energy A III IV B I C V reaction coordinate 3

4 E. Self-assessment Question Can you identify what are electrophile and nucleophile? Can you use arrow to indicate the movement of electron-pair for an electrophilic addition? Can you identify the followings from a reaction coordinate diagram when applicable: endothermic or exothermic reactions, activation energy, heat of reaction, locations of transition states, locations of intermediates, and ratelimiting step. Can you derive the structures of transition states and intermediates from an electron-pushing mechanism? 4

5 5.2 Electrophilic Addition: ydrohalogenation A. General Reaction: Know what is electrophilic addition reaction. RC C 2 + X RC C 3 X X = F, Cl, Br, I R: alkyl group (example: C3-, C3C2C2-) B. Electron-pushing Mechanism: C. Two possible products can be formed (Regioselectivity of ydrohalogenation) 5

6 D. Stability of Carbocation Intermediate Carbocation stability depends on the number of alkyl substituents attached to the positively charged carbon. Relative stability of carbocations: 6

7 E. Regioselectivity of ydrohalogenation Know what is regioselectivity. (i). Markovnikov s rule: For the electrophilic addition, hydrogen (atom prefers to attach to the doublebonded carbon with more hydrogen atoms. 7

8 (ii) Regioselectivity of governed by the stability of carbocation Examples F. Self-assessment Question Can you differentiate primary, secondary, and tertiary carbocations, and recognize the order of stability for these carbocations? Can you write the electron-pushing (arrow-pushing) mechanisms for hydrohalogenation? Can you explain the regioselectivity of hydrohalogenation? Can you describe what is Markovnikov s rule? 8

9 5.3. ydration (The addition of water and alcohols to an alkene) A. The addition of water to an alkene is an acid-catalyzed hydration General Reaction: RC C RC C 3 Commonly used acid catalysis: + (X), 3 + (acid in water), 2S4, 3P4 + B. Electron-pushing Mechanism: C. Reaction Energy Diagram 9

10 D. Self-assessment Questions Can you write the electron-pushing (arrow-pushing) mechanisms for acidcatalyzed hydration? Can you explain the regioselectivity of acid-catalyzed hydration? 5.4 alogenation and ydrogenation A. Addition of Bromine and Chlorine (alogenation) (i) General Reaction: RC C 2 + X X RC C 2 X X X = Cl, Br, I (ii) Electron-pushing Mechanism of halogenation: 10

11 (iii) Anti-selectivity (Anti addition): Issue in the Stereoselectivity of alogenation Selection between stereoisomers. alide attacks from the opposite of the X + ion. B. ydrogenation Addition of molecular hydrogen to double-bond or triple-bond. ydrogenation is a type of reduction. 11

12 (i) Stereoselectivity of ydrogenation C. Self-assessment Questions Can you write the electron-pushing (arrow-pushing) mechanisms for halogenation? Can you explain the stereoselectivity (anti-addition) of halogenation? Can you explain the stereoselectivity (syn-addition) of hydrogenation? 12

13 5.5 An introduction to alkynes N 2 N Parsalmide (analgesic) I Calicheamicin γ1 (anticancer) S S N N S S N A. The nomenclature of alkynes Br 13

14 B. The structure of alkynes C C C C C C C C C C C. The physical properties of unsaturated hydrocarbons D. The addition of a hydrogen halide to an alkyne R + X R X R + X R CX 2 C 3 X X = F, Cl, Br, I 14

15 omework for Chapter 5 (optional) Name: A number: 15