Chapter 9 Substitution Reactions of Alkyl Halides.pdf

Chapter 9 Substitution Reactions of Alkyl Halides.pdf - •...

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4 groups, focus on Group II Group II: electronegative atom (or e withdrawing group) attached to sp^3 carbon that creates a polar bond (+1/2)RCH_2 - X(-1/2) Substitution reaction: atom/group replaced by another atom/group o RCH_2CH_2X + Y > RCH_2CH2Y + X- Elimination reaction: atom/group removed alone with H on an adjacent carbon o RCH_2CH_2X + Y > RCH=CH_2 + HY + X- Leaving group: atom/group that is substituted/eliminated o in this chapter, group is halide (F-, Cl-, Br-, I-) o R - F (alkyl fluoride) R - Cl (alkyl chloride) R - Br (alkyl bromide) R - I (alkyl iodide) Alkyl halides have good leaving groups (easily displaced) Substitution allows us to convert alkyl halides into many other compounds and are important in cells Alkyl halides are insoluble in water (biological systems use other compounds) 9.1 The Mechanism for an Sn2 Reaction Reaction of nucleophile with an electrophile ... nucleophilic substitution reaction Rate depends on concentration o 2x concentration of alkyl halide > 2x ROR o 2x both reactants > 4x ROR rate= k [alkyl halide][nucleophile] o both alkyl halide and nucleophile involved in RDS Larger k = lower energy barrier Sn2: substitution nucleophilic bimolecular o Bimolecular: 2 molecules involved in TS for RDS Larger alkyl group = slower substitution reaction Relative rates for Sn2 o CH_3 - Br > CH_3CH_2 - Br > CH_3CH_2CH_2 - Br > CH_3C(CH_3)H - Br > CH_3C(CH_3)_2 - Br o methyl halide > 1* alkyl halide > 2* alkyl halide > 3* alkyl halide (too unreactive) Substitution reaction where halogen is bonded to chiral center > only one stereoisomer where configuration is inverted relative to reactant (see drawing in text) Sn2 is concerted (takes place in a single step), so no intermediates Mechanism in book o HO- + CH_3 - Br > CH_3 - OH + Br- o Nucleophile attacks back side ( back-side attack ) of carbon that has LG and displaces it o LG blocks approach from front side o Front-side attack > bonding + antibonding cancel each other o MO Theory: HOMO(non-bonding) must interact with LUMO(antibonding sigma*) See image in book TS contains both alkyl halide and nucleophile (see image in book)
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  • Fall '09
  • AUE
  • Nucleophilic substitution, LG

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