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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 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 แบ่งตามชนิดของอะตอมคาร์บอนที่แฮโลเจนเกาะอยู่
Common vs IUPACnames common IUPAC n-Butyl bromide 1-Bromobutane (1o) Isopropyl chloride 2-Chloropropane (2o) Isobutyl chloride 1-Chloro-2-methylpropane (1o)
(2o) 3-Chloro-2-methylpentane (3o) 4-Bromo-2,4-dimethylhexane ?
PREPARATION 1. From alcohols
3. Addition of hydrogen halide to alkenes 4. Addition of halogens to alkenes and alkynes
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.
REACTIONS 1.Nucleophilic aliphatic substitution d- d+ Electrophilic site Nucleophilic site
Nucleophilic substitution See more examples on text p.174
2.Dehydrohalogenation : elimination 3. Preparation of Grignard reagent
4.Reduction M= Li , Na, K
substitution nucleophilic bimolecular The SN2 Reaction: Kinetics : the concentrations of both reactants the reaction ratea Second - order kinetics
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
SN2 Reactivity. Steric hindrance Reactivity : CH3W > 1o > 2o > 3o
substitution nucleophilic unimolecular The SN1 Reaction: Kinetics : the concentration of alkyl halide the reaction ratea First - order kinetics
Mechanism & stereochemistry of SN1 reaction Step 1 : ionization = rate determining step Step 2 : combination
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
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
(a) Inversion (predominates) H2O (b) retension
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)
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
Rearrangement of carbocations A less stable C+ can rearrange itself in order to become a more stable C+. rearrangement No rearrangement rearrangement
Migratory mode : 1,2-shifts Two common migrations are a hydride shiftand an alkyl shift A hydride shift An alkyl shift
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