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Chapter 10. Nucleophilic Substitution: The S N 1 and S N 2 Mechanisms. Assignment for Chapter 10. We will cover all the sections in this chapter, except Sections 10.12 and 10.13. Problem Assignment for Chapter 10. In-Text Problems 1 - 15 17, 18 19 (S N 2 react)
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Chapter 10 Nucleophilic Substitution: The SN1 and SN2 Mechanisms WWU -- Chemistry
Assignment for Chapter 10 • We will cover all the sections in this chapter, except Sections 10.12 and 10.13 WWU -- Chemistry
Problem Assignment for Chapter 10 In-Text Problems 1 - 15 17, 18 19 (SN2 react) 20 (SN1 reaction), 21, 22, 24, 25, 26, 27, 28 End-of-Chapter Problems 30 - 37 39 - 42 44 – 49 51 - 55 WWU -- Chemistry
Sect. 10.1: Nomenclature of alkyl halides -- common names methylene chloride CH2Cl2 chloroform CHCl3 carbon tetrachloride CCl4 WWU -- Chemistry
More common and IUPAC names isopropyl chloride (2-chloropropane) sec-butyl chloride (2-chlorobutane) isobutyl chloride (1-chloro-2-methylpropane) tert-butyl chloride (2-chloro-2-methylpropane) allyl chloride (3-chloro-1-propene) vinyl chloride (chloroethene) benzyl chloride (chloromethylbenzene) phenyl chloride (chlorobenzene) WWU -- Chemistry
Sect. 10.2: Overview of nucleophilic substitution • The substitution reaction: SN1 and SN2 • Primary halides = SN2 • Secondary halides = both mechanisms! • Tertiary halides = SN1 • Leaving groups: halogens most common • There are a number of different nucleophiles!! WWU -- Chemistry
Nucleophilic Substitution (SN2) WWU -- Chemistry
Nitrogen as a nucleophile (SN2) WWU -- Chemistry
Carbon as a nucleophile (SN2) WWU -- Chemistry
d- d- energy Reaction coordinate WWU -- Chemistry
The SN1 Mechanism carbocation WWU -- Chemistry
d+ d+ d- d- + energy intermediate Reaction coordinate WWU -- Chemistry
Sect. 10.3: SN2 Mechanism • reaction and mechanism • kinetics • stereochemistry • substrate structure • nucleophiles • leaving groups • solvents WWU -- Chemistry
The SN2 Reaction Sterically accessible compounds react by this mechanism!! Methyl group is small WWU -- Chemistry
SN2 Mechanism: kinetics • The reactions follows second order (bimolecular) kinetics • Rate = k [R-Br]1 [OH-]1 WWU -- Chemistry
d- d- energy Reaction coordinate WWU -- Chemistry
SN2 Reaction: stereochemistry Inversion of configuration WWU -- Chemistry
For an SN2 Reaction: EVERY REACTION EVENT ALWAYS LEADS TO INVERSION OF CONFIGURATION WWU -- Chemistry
SN2 Reaction: substrate structure (Table 10-5) KI in Acetone at 25° WWU -- Chemistry
Chloromethane + Iodide as the Nucleophile Fast I- WWU -- Chemistry
tert-Butyl Chloride + Iodide as the Nucleophile No reaction I- WWU -- Chemistry
SN2 Reaction: substrate structure Reactivity order---- fastest to slowest! WWU -- Chemistry
SN2 Reaction: nucleophilicity WWU -- Chemistry
is morenucleophilic! Predict which is more nucleophilic WWU -- Chemistry
Relative Nucleophilicity 1) In general, stronger bases are better nucleophiles 2) However, iodide doesn’t fit that pattern (weak base, but great nucleophile!) 3) Cyanide is an excellent nucleophile because of its linear structure 4) Sulfur is better than oxygen as a nucleophile WWU -- Chemistry
SN2 Reaction: Leaving Groups • Best leaving groups leave to form weakLewis bases. • Good leaving groups: • Br, I, Cl, OTs, OH2+ • “Lousy” leaving groups: • OH, OR, NH2,, F WWU -- Chemistry
Sulfonate Leaving Groups WWU -- Chemistry
Tosylate leaving group WWU -- Chemistry
Inversion of Configuration WWU -- Chemistry
SN2 Reaction: solvents SN2 reactions are accelerated in polar, aprotic solvents. Consider Na+-OEt as an example of a nucleophile. Why are reactions accelerated? The Na+ cation is complexed by the negative part of the aprotic solvent molecule pulling it away from –OEt. Now that the sodium ion is complexed, the oxygen in the nucleophile –OEt is more available for attack. WWU -- Chemistry
Aprotic solvents • These solvents do not have OH bonds in them. They complex the cation through the lone pairs on oxygen or nitrogen: WWU -- Chemistry
How cations are complexed with aprotic solvents WWU -- Chemistry
Now that the Na+ is complexed, the –OEt can react more easily WWU -- Chemistry
SN2 Reaction: solvents SN2 reactions areretarded (slowed) in polar, protic solvents. Protic solvents have O-H groups. Why are reactions retarded? Nucleophile is hydrogen bonded to solvent! WWU -- Chemistry
Protic solvents abbreviations Typical protic solvents: WWU -- Chemistry
Sect. 10.4: SN1 Mechanism • reaction and mechanism • kinetics • stereochemistry • substrate structure • nucleophiles • leaving groups • solvents WWU -- Chemistry
Solvolysis of tert-Butyl Bromide Acetone is used to dissolve everything! Water is the solvent and nucleophile (solvolysis). WWU -- Chemistry
The SN1 Mechanism carbocation 1935: Hughes & Ingold WWU -- Chemistry
d+ d- intermediate + energy intermediate Reaction coordinate WWU -- Chemistry
SN1 Reaction: kinetics • The reactions follows first order (unimolecular) kinetics • Rate = k [R-Br]1 WWU -- Chemistry
SN1 Reaction: stereochemistry With chiral R-X compounds, the product will be racemic (50% of each enantiomer). WWU -- Chemistry
Stereochemistry in SN1 reactions – racemic product WWU -- Chemistry
d+ d- intermediate + energy intermediate Reaction coordinate WWU -- Chemistry
SN1 Reaction: substrate structure Solvolysis in water at 50°C WWU -- Chemistry
SN1 Reaction: substrate structure tertiary>secondary>primary > methyl Primary and methyl halides are very unreactive! They don’t go by SN1 reactions. WWU -- Chemistry
Nucleophiles • Usually SN1 reactions are run in polar protic solvents; compounds with O-H groups. • The polar protic solvent acts as BOTH nucleophile as well as the solvent. • Common solvent/nucleophiles include: water, ethanol, methanol, acetic acid, and formic acid. WWU -- Chemistry
A protic solvent acts as both a solvent and nucleophile in SN1 reactions - solvolysis: abbreviations WWU -- Chemistry
Typical solvolysis reaction Polar solvent stabilizes the carbocation! Solvent is the nucleophile WWU -- Chemistry
Leaving groups • Leaving groups are the same as in SN2 reactions: • Cl, Br, I, OTs are the usual ones. WWU -- Chemistry