Substitution and Elimination-Week 8
Substitution and Elimination-Week 8 CHEM 2030 - 01
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This 5 page Class Notes was uploaded by Shannon Z. on Sunday October 18, 2015. The Class Notes belongs to CHEM 2030 - 01 at University of Missouri - Columbia taught by Rainer Glaser in Fall 2015. Since its upload, it has received 13 views. For similar materials see Survey of Organic Chemistry in Chemistry at University of Missouri - Columbia.
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Date Created: 10/18/15
1 Classification of Nucleophiles a b Can be classified by the atom in the nucleophile that will be forming a bond with the substrate i OH 1 The O in this nucleophile will bond to the substrate It is called a hydroxide ion For a complete list of the names used to classify nucleophiles see table 61 from the text 2 Nucleophilic substitution a b c d e A reaction that alkyl halides undergo The nucleophile will remove and replace the halide leaving group Process i The nucleophile reacts with the substrate removing a halogen atom from it 1 The halogen that leaves the substrate is called a leaving group ii The leaving group will take both electrons from the bond that previously attached it to the substrate a Leaving groups are also nucleophiles They have a lone pair iii The nucleophile will provide two electrons with which the substrate and nucleophile will bind This reaction is reversible i To force the reaction to occur in which the original leaving group is removed and a new nucleophile is added make sure the nucleophile being added is a stronger nucleophile than the leaving group 1 A stronger nucleophile may have a H charge Charges i If the nucleophile and substrate are neutral the product will be positively charged ii If the nucleophile is a negatively charged and the substrate is neutral the product will be neutral 3 Types of nucleophilic reactions a b The type seen depends on the structure of the nucleophile and alkyl halide solvet used reaction temp ect 5N2 i A one step process 1 The nucleophile attacks the substrate and a leaving group exits ii There is a transition state in which the nucleophile and leaving group are attached by partial bonds shown by a dashed line iii Process 1 The nucleophile attacks the rear side of the CL bond 2 There is a transition state in which the nucleophile and leaving group are attached by partial bonds shown by a dashed line 3 The leaving group leaves taking two electrons from the bond that attached it to the substrate 4 The nucleophile donates electrons to complete the bond between it and the substrate 5 One sigma bond is broken and one sigma bond is formed iv This is a bimolecular reaction v The reaction energy diagram for SNZ is exothermic and occurs in one step vi Reaction speed 1 Depends on the concentration of the nucleophile and substrate a The higher the concentration of either the faster the reaction 2 Primary vs Tertiary a Fastest when the substrate s alkyl group is primary or a methyl group b Slowest when the substrate s alkyl group is tertiary vii Walden Reaction 1 The reaction will flip the arrangement of the substituents like the wind flips an umbrella HIE Er H 51 rljl I tfi itiifjlb inlg r11 I jg Ig39tli ill39 tj a Same reaction just without the umbrella drawn on it CI i 32H It a 7 HIE t 1539 h Hit lm i Hr ElI if it 1 1 filial i if l iglf H H 112 1 I I tfj mijjlbum r11 I jl lg iatijj l l ijl b c SNl i Atwostep reaction ii Process 1 Step one a Slow b The bond between the carbon and the leaving group and the substrate ionizes c The electrons from the bond that was between the leaving group and carbon will both go with the leaving group d A carbocation is formed 2 Step Two a Fast b The carbocation combines with the nucleophile go crate the final product 3 When the nucleophile is neutral there will be a third step a A proton will be lost from the nucleophile to give the final product iii Step one is unimolecular iv The energy reaction diagram shows that step one requires a lot of energy to initiate d Nucleophiles that take place in SNl reactions are e Reaction speed i The reaction speed does not depend on the concentration of the nucleophile or substrate unlike SNZ reactions ii What does determine the speed of this reaction is the rate of formation of the carbocation The rate at which the nucleophile is added is nearly instantaneous f If the carbon that was attached to the leaving group sis stereogenic the reaction may result in a loss of optical activity 4 Determining if a reaction is SNl or SNZ a Primary halides almost always react by SNZ b Tertiary halides react by SNl c Polar protic solvents speed up SNl reactions but slow down SNZ reactions i Aprotic polar solvents speed up SNZ reactions d Strong nucleophiles tend to lead to SNZ reactions 5 How to tell if a nucleophile is strong a Negative ions create a stronger nucleophile b Elements located geographically lower on the periodic table tend to be stronger nucleophiles c More electropositive atoms are stronger nucleophiles 6 Elimination Reactions a When an alkyl halide that has a H attached to the carbon adjacent the halogen bearing carbon reacts with a nucleophile there are two reaction paths possible substitution or elimination Milli quotlUEIE phJiIE39 ll I1 tlufx Nu 39 atquot IIE a 7 if llTHl lIi um Ir V39VCC El NMEH 1 K I halogen H sulltastl39tutlon Iiiii l TT 393 I alu W l if i Substitution 1 The nucleophile replaces the halogen ii Elimination 1 The nucleophile eliminates the halogen and the hydrogen from the carbon adjacent to the halogen b One of the two substitution or elimination may predominate based the structure of the nucleophile the structure of the substrate and other reaction conditions c There are two types of elimination i E1 1 Two step process a Step one i Slow ii The bond between the carbon and the leaving group and the substrate ionizes iii The electrons from the bond that was between the leaving group and carbon will both go with the leaving group iv A carbocation is formed b Step 2 i There are two possible reactions at this point ii The carbocation may combine with the nucleophile in a SNl reaction OR iii The carbocation may lose the hydrogen atom from the carbon atom adjacent to the that held the leaving group A double bond will from between the two carbon atoms making an alkene H H NH 2 L L F r 5quot 2 l l l H H quota f L L H 1221 1 cai39lmcn tinn if at 2 Step one of this process is rate determining the amount of time it takes for this step to occur determines how long the reaction will take to complete ii E2 1 One step process a The nucleophile acts as a base and removes the hydrogen atom on the carbon adjacent to the carbon bonded to the leaving group halogen b At the same time the leaving group leave the compound and a double bond is formed between the two carbons who had substituents eliminated 7 Substitution and Elimination in Competition a b Tertiary Halides i Substitution can only occur via SNl ii Elimination can occur by E1 or E2 iii When weak nucleophiles and polar solvents are used E1 and SNl compete iv Strong nucleophiles and less polar solvents favor E2 Primary Halides i Substitution can only occur via SN2 ii Elimination can only occur via E2 iii Mostly substitution occurs iv E2 only occurs with bulky strongly basic nucleophiles Secondary Halides i Substitution occurs via SN2 and SNl ii Elimination occurs via E2 and E1 iii The product is sensitive to the strength of the nucleophile how much of a base the nucleophile is the solvent and the temperature iv Substitution is generally favored with 1 Strong nucleophiles that are not strong bases a 5N2 2 Weak nucleophiles in polar solvents a 5N1 v Elimination is favored by strong bases 8 Misc a Polar protic solvents are solvents like water or alcohols that donate protons