CHEM 2211 1st Edition Lecture 29Outline of Last LectureI. Introduction to Sn2 reactionsII. Mechanism for Sn2 reactionsIII. What affects Sn2 reactionsIV. Solvent effects on NucleophilicityV. Aprotic SolventsVI. Mechanism for Sn1VII. Steric effectsVIII. Factors that effect Sn1 reactionsOutline of Current LectureI. Benzylic Halides, Allylic Halide, Vinylic halides, and Aryl HalidesII. Competition between Sn2 and Sn1 ReactionsIII. Role of solvent in Sn1 and Sn2 reactionsIV. Intermolcular forces vs. Intramolecular forcesV. Methylating agents used by chemist and by natureLecture NotesI. Benzylic Halides, Allylic Halide, Vinylic halides, and Aryl HalidesA. Primary and Secondary benzylic and allylic halides undergo Sn2 reactionsB. Non-mirror image resonance contributors produce different productsC. Aryl compound has the halogen attached to the benzene ringi. Aryl and vinylic halides do undergo neither Sn1 nor Sn2 reactions1. Forms a carbocation that is too unstable for Sn12. Nucleophile is repelled by double bonds and electron density for Sn2 II. Competition between Sn2 and Sn1 ReactionsA. Only benzylic nd allylic halides can undergo both Sn1 and Sn2 reactionsB. Chemists prefer Sn2 reactions because they produce one specific producti. Sn1 produces enantiomers and constitutional isomers because they create a carbocationC. Factors that affect the likelihood of an Sn1 or Sn2 reaction occurringi. Concentration of nucleophile1. Increasing the concentration of the nucleophile increases the likelihood that it will run into the back of the electrophile (Sn2) before the leaving group leaves (Sn1)a. Sn2 prefers a GOOD nucleophile in high concentrationsb. Sn1 prefers a POOR nucleophile in high concentrationsii. Reactivity of nucleophileiii. SolventIII. Role of solvent in Sn1 and Sn2 reactions A. The dielectric constant is a measure of how well solvent molecules insulate othermolecules form opposite chargesi. Polar solvents have high dielectric moleculesii. Sn1 reactions MUST occur in POLAR solventsB. For Sn1 reactionsi. Neutral compounds have a HIGH rate of reaction in polar solventii. Charged compounds have LOW rate of reaction in polar solventC. For Sn2 reactionsi. Good nucleophiles are hindered by polar solventsii. Neutral nucleophiles are accelerated by polar solventsIV. Intermolcular forces vs. Intramolecular forcesA. A bifunctional molecule has two functional groupsB. INTRAmolecular reactions occur within one moleculei. Favored with low cnocentrationsii. Favored when a 5-6 membered ring will form C. INTERmolecular reactions occur between two moleculesi. Favored by high concentrationsii. When angle strain exists in ring structures (3-4 membered rings)iii. Favored when ring extends beyond six membersV. Methylating agents used by chemist and by natureA. Scientists use alkyl iodides to methylate most compounds because alkyl iodides i. Undergo Sn2 reactionsii. Are liquid at room temperatureiii. Are the most easily displaced leaving groupB. Alkyl iodides are not soluble in water so cells don’t use iti. They use S-adenosylmethionine (SAM)
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