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

3. Structure : The Functional Group R-X Alkyl groupHalogen atom เป็นตัวกำหนดลักษณะ เชิงโครงสร้างและ ควบคุมคุณสมบัติของ สาร The Functional Group

4. Classification & nomenclature แบ่งตามชนิดของอะตอมคาร์บอนที่ แฮโลเจนเกาะอยู่

5. Common vs IUPAC names commonIUPAC n-Butyl bromide 1-Bromobutane Isopropyl chloride2-Chloropropane Isobutyl chloride 1-Chloro-2-methylpropane (1o)(1o) (1o)(1o) (2o)(2o)

6. 3-Chloro-2-methylpentane 4-Bromo-2,4-dimethylhexane (2 o ) (3 o ) ?

7. PREPARATION 1. From alcohols

8. 2. Halogenation of certain hydrocarbons

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

10. 5. Halide exchange Notes on preparation : •The most general and practical way to make RX is to prepare from alcohols. •RXs are almost never prepared by direct halogenation of alkanes. •RI is often prepared from the corresponding bromide or chloride by treatment with a solution of NaI in acetone.

11. REACTIONS 1.Nucleophilic aliphatic substitution   Nucleophilic site Electrophilic site

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

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

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

15. The S N 2 Reaction: Kinetics : the concentrations of both reactants the reaction rate  Second - order kinetics substitution nucleophilic bimolecular

16. Mechanism & stereochemistry of S N 2 reaction 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

18. S N 2 Reactivity.Steric hindrance Reactivity : CH 3 W > 1 o > 2 o > 3 o

19. The S N 1 Reaction: substitution nucleophilic unimolecular Kinetics : the concentration of alkyl halide the reaction rate  First - order kinetics

20. Mechanism & stereochemistry of S N 1 reaction Step 1 : ionization = rate determining step Step 2 : combination

21. A reactive intermediate which is a group of atoms that contains a carbon atom bearing only 6 electrons. CARBOCATION sp 2 empty p-orbital Structure of carbocation

22. Mechanism & stereochemistry of S N 1 reaction Ionization of the C-X bond to generate a carbocation is the rate determining step. Reaction proceeds with racemization. Optically active Opposite configuration ; Lower optical purity S N 1 : racemization plus inversion

23. H2OH2O (b) retension (a) Inversion (predominates)

24. S N 1 Reactivity.Ease of formation of carbocation Relative Rate (S N 1) Reactivity in S N 1 : 3 o > 2 o > 1 o > CH 3 W Rate of formation of C+ : 3 o > 2 o > 1 o > CH 3 +

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

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

27. 1,2-shifts Two common migrations are a hydride shift and an alkyl shift Migratory mode : A hydride shift An alkyl shift

28. SN1SN1S N 2 vs. (a) second-order kinetics (b) complete stereochemical inversion (c) absence of rearrangement (d) the reactivity sequence CH 3 W > 1 o > 2 o > 3 o (a) first-order kinetics (b) racemization (c) rearrangement (d) the reactivity sequence 3 o > 2 o > 1 o > CH 3 W SN2SN2 SN1SN1