nucSub - TOPIC NUCLEOPHILIC SUBSTITUTIONS TOPIC 6....

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TOPIC 6. NUCLEOPHILIC SUBSTITUTIONS (chapter 6 and parts of chapter 11) S N 1S N 2 Mechanisms E1 E2 H H Nuc Topic 7 LG H Substitution Topic 6 slide 1 LG Elimination OBJECTIVES 1. Describe two pathways (mechanisms) to account for substitution at sp 3 carbons bearing an electronegative atom (leaving group) 2. Discuss the effect of starting material (substrate), leaving group, reagent (a nucleophile) and reaction conditions on the course of a reaction 3. Recognize functional group transformations and synthesis of new molecules in one step by substitution of an appropriate material 4. Explore the substitution chemistry of alcohols and ethers Topic 6 slide 2 CHEM 2311 B
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OVERVIEW: NUCLEOPHILIC SUBSTITUTIONS ( S N ) S:6.1-6.5 Topic 6 slide 3 CHEM 2311 B CL Nu CN u L We are only considering sp 3 (alkyl) substrates in this section. Molecules with leaving groups on sp 2 (e.g., vinyl, aryl, acyl (C=O)) or sp (ethynyl) carbons do not react in fashion described in this section Topic 6 slide 4 CHEM 2311 B
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Nucleophiles Donate a pair of electrons: to an electrophile (lone pair or pi bond) Neutral H 2 O Anionic (R)HO Hal NC C HC Electrophiles Receive a pair of electrons: from a nucleophile Cationic C Polar, electrophilic CX δ δ Topic 6 slide 5 Effect of Leaving Groups on Electrophilicity C L Nu C Nu L C-L bond must be relatively weak C-L bond must be polarizable (the ease with which the electron distribution in the bond is distorted in an electric field in the bond is distorted in an electric field) L needs to be able to accommodate a pair of electrons - Good leaving groups are weak bases Topic 6 slide 6
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Topic 6 slide 7 CHEM 2311 B TWO CLASSES OF REACTION Substitution reactions can be performed under different conditions which give rise to dramatically different outcomes. Nucleophilic substitution reactions can rise to dramatically different outcomes. Nucleophilic substitution reactions can be classified as one of two types, based on these experimental observations. Unimolecular and Bimolecular In order to develop predictive tools, we need to understand reasons why these Nucleophilic substitution observations are important. That is, we need to develop proposals for two different mechanisms which are consistent with the two sets of data and which we can use to predict the outcome of other reactions. 2 mechanisms: S 1andS 2 Topic 6 slide 8 S N 1 and S N 2 CHEM 2311 B
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SUBSTITUTION AT 1° SUBSTRATES: BI MOLECULAR NUCLEOPHILIC Prob: 6.13-15, S:6.6-6.8 SUBSTITUTION ( S N 2 ) Examples 17aj,25,27, 31,33,36 3 2 3 3 2 3 an ether I 3 3 a thiol Topic 6 slide 9 CHEM 2311 B Experimental Observations Kinetics (observations Kinetics (observations) CH 3 CH 2 Br + NaOMe CH 3 CH 2 OMe + NaBr [CH 3 CH 2 Br] [NaOMe] relative rate 1.) 0.01 M 0.01 M 1 2.) 0.02 M 0.01 M 2 3.) 0.01 M 0.02 M 2 4.) 0.02 M 0.02 M 4 Conclusion: Rate = k [R-L] 1 [Nu] 1 Overall Reaction Rate is 2nd order Interpretation: 1.) Substrate and Nucleophile are both involved in the Rate Determining Step 2.) Bimolecular collision involved in mechanism Topic 6 slide 10 CHEM 2311 B
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Mechanism of Nucleophilic Substitution of 1° Alkyl Halides: The S 2 Reaction Halides: The S N
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nucSub - TOPIC NUCLEOPHILIC SUBSTITUTIONS TOPIC 6....

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