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ALKYL HALIDES

ALKYL HALIDES. by Parinya Theramongkol Department of Chemistry Khon Kaen University. Structure : The Functional Group. R -X. Alkyl group. Halogen atom. เป็นตัวกำหนดลักษณะเชิงโครงสร้างและควบคุมคุณสมบัติของสาร. The Functional Group. Classification & nomenclature.

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ALKYL HALIDES

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  1. ALKYL HALIDES by Parinya Theramongkol Department of Chemistry Khon Kaen University

  2. Structure: The Functional Group R-X Alkyl group Halogen atom เป็นตัวกำหนดลักษณะเชิงโครงสร้างและควบคุมคุณสมบัติของสาร The Functional Group

  3. Classification & nomenclature แบ่งตามชนิดของอะตอมคาร์บอนที่แฮโลเจนเกาะอยู่

  4. Common vs IUPACnames common IUPAC n-Butyl bromide 1-Bromobutane (1o) Isopropyl chloride 2-Chloropropane (2o) Isobutyl chloride 1-Chloro-2-methylpropane (1o)

  5. (2o) 3-Chloro-2-methylpentane (3o) 4-Bromo-2,4-dimethylhexane ?

  6. PREPARATION 1. From alcohols

  7. 2. Halogenation of certain hydrocarbons

  8. 3. Addition of hydrogen halide to alkenes 4. Addition of halogens to alkenes and alkynes

  9. 5. Halide exchange Notes on preparation : • The most general and practical way to make RX is to prepare from alcohols. • RXs are almost neverprepared by direct halogenation of alkanes. • RI is often prepared from the corresponding bromide or chloride by treatment with a solution of NaI in acetone.

  10. REACTIONS 1.Nucleophilic aliphatic substitution d- d+ Electrophilic site Nucleophilic site

  11. Nucleophilic substitution See more examples on text p.174

  12. 2.Dehydrohalogenation : elimination 3. Preparation of Grignard reagent

  13. 4.Reduction M= Li , Na, K

  14. substitution nucleophilic bimolecular The SN2 Reaction: Kinetics : the concentrations of both reactants the reaction ratea Second - order kinetics

  15. Nucleophile attacks on the back-side of the C-X bond Bond-making and bond-breaking occur simultaneously Product has a complete inversion of configuration Mechanism & stereochemistry of SN2 reaction

  16. SN2 Reactivity. Steric hindrance Reactivity : CH3W > 1o > 2o > 3o

  17. substitution nucleophilic unimolecular The SN1 Reaction: Kinetics : the concentration of alkyl halide the reaction ratea First - order kinetics

  18. Mechanism & stereochemistry of SN1 reaction Step 1 : ionization = rate determining step Step 2 : combination

  19. CARBOCATION Structure of carbocation A reactive intermediate which is a group of atoms that contains a carbon atom bearing only 6 electrons. sp2 empty p-orbital

  20. Ionization of the C-X bond to generate acarbocation is the rate determining step. Reaction proceeds with racemization. Optically active Opposite configuration ; Lower optical purity Mechanism & stereochemistry of SN1 reaction SN1 : racemization plus inversion

  21. (a) Inversion (predominates) H2O (b) retension

  22. 3o > 2o > 1o > CH3W Reactivity in SN1 : 3o > 2o > 1o > CH3+ Rate of formation of C+ : SN1 Reactivity. Ease of formation of carbocation Relative Rate (SN1)

  23. Stabilization of C+ : Polar effects Polar effects : ผลที่เกิดขึ้นตรงจุดเกิดปฏิกริยา อันเนื่องมาจาก การให้หรือรับอิเล็คตรอนของกลุ่มข้างเคียง Charge dispersion stability G = e- donating group G = e- withdrawing group Disperses charge intensifies charge Stabilizes cation Destabilizes cation

  24. Rearrangement of carbocations A less stable C+ can rearrange itself in order to become a more stable C+. rearrangement No rearrangement rearrangement

  25. Migratory mode : 1,2-shifts Two common migrations are a hydride shiftand an alkyl shift A hydride shift An alkyl shift

  26. SN2vs. SN1 SN2 • second-order kinetics • complete stereochemical inversion • absence of rearrangement • the reactivity sequence CH3W > 1o > 2o > 3o SN1 • first-order kinetics • racemization • rearrangement • the reactivity sequence 3o > 2o > 1o > CH3W

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