CHEM 2211 1st Edition Lecture 29 Outline of Last Lecture I II III IV V VI VII VIII Introduction to Sn2 reactions Mechanism for Sn2 reactions What affects Sn2 reactions Solvent effects on Nucleophilicity Aprotic Solvents Mechanism for Sn1 Steric effects Factors that effect Sn1 reactions Outline of Current Lecture I II III IV V Benzylic Halides Allylic Halide Vinylic halides and Aryl Halides Competition between Sn2 and Sn1 Reactions Role of solvent in Sn1 and Sn2 reactions Intermolcular forces vs Intramolecular forces Methylating agents used by chemist and by nature Lecture Notes I II Benzylic Halides Allylic Halide Vinylic halides and Aryl Halides A Primary and Secondary benzylic and allylic halides undergo Sn2 reactions B Non mirror image resonance contributors produce different products C Aryl compound has the halogen attached to the benzene ring i Aryl and vinylic halides do undergo neither Sn1 nor Sn2 reactions 1 Forms a carbocation that is too unstable for Sn1 2 Nucleophile is repelled by double bonds and electron density for Sn2 Competition between Sn2 and Sn1 Reactions A Only benzylic nd allylic halides can undergo both Sn1 and Sn2 reactions B Chemists prefer Sn2 reactions because they produce one specific product i Sn1 produces enantiomers and constitutional isomers because they create a carbocation C Factors that affect the likelihood of an Sn1 or Sn2 reaction occurring i Concentration of nucleophile 1 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 III IV V a Sn2 prefers a GOOD nucleophile in high concentrations b Sn1 prefers a POOR nucleophile in high concentrations ii Reactivity of nucleophile iii Solvent Role of solvent in Sn1 and Sn2 reactions A The dielectric constant is a measure of how well solvent molecules insulate other molecules form opposite charges i Polar solvents have high dielectric molecules ii Sn1 reactions MUST occur in POLAR solvents B For Sn1 reactions i Neutral compounds have a HIGH rate of reaction in polar solvent ii Charged compounds have LOW rate of reaction in polar solvent C For Sn2 reactions i Good nucleophiles are hindered by polar solvents ii Neutral nucleophiles are accelerated by polar solvents Intermolcular forces vs Intramolecular forces A A bifunctional molecule has two functional groups B INTRAmolecular reactions occur within one molecule i Favored with low cnocentrations ii Favored when a 5 6 membered ring will form C INTERmolecular reactions occur between two molecules i Favored by high concentrations ii When angle strain exists in ring structures 3 4 membered rings iii Favored when ring extends beyond six members Methylating agents used by chemist and by nature A Scientists use alkyl iodides to methylate most compounds because alkyl iodides i Undergo Sn2 reactions ii Are liquid at room temperature iii Are the most easily displaced leaving group B Alkyl iodides are not soluble in water so cells don t use it i They use S adenosylmethionine SAM instead
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