ORGANIC - EGE 5E CH. 7 - NUCLEOPHILIC SUBSTITUTION AND ELIMINATION REACTIONS
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2 CONCEPT: INTRODUCTION TO SUBSTITUTION Previously, we discussed the various ways that acids could react with bases: Recall that in these mechanisms, electrons always travel from density to density Bronsted-Lowry Reactions: When a nucleophile and electrophile react to exchange a Lewis Acid/Base Reactions: When a nucleophile and electrophile with an empty orbital react to form a covalent bond Substitution Reactions take place when a nucleophile reacts with an electrophile that does not have an empty orbital. This generates the need to analyze a new type of conjugate base: the leaving group. EXAMPLE: Predict the product. Identify all of the chemical species in the following reaction. In typical acid and base reactions, we used stability of the conjugate base to determine chemical equilibrium In substitution, we use the stability of the leaving group to help determine reaction rate. Page 2
3 CONCEPT: LEAVING GROUPS Leaving groups break a bond with the electrophile to make it reactive. They are molecules that will remain stable after accepting an extra electron pair. We use factors affecting acidity to determine which atoms will be most stable after gaining extra electrons. Recall that the element effect consists of two trends: EXAMPLE: Predict which of the following pairs of electrophiles possesses the best leaving group a. b. c. d. Due to their high electronegativity, will be the primary leaving groups for this chapter. Page 3
4 CONCEPT: THE SN 2 MECHANISM A negatively charged nucleophile reacts with an accessible leaving group to produce substitution in one-step. SN 2 Properties (Circle One) Nucleophile = Strong / Weak Leaving Group = Unsubstituted / Highly Substituted Reaction coordinate = Transition State / Intermediate Reaction = Concerted / Two-Step Rate = Unimolecular / Bimolecular Rate = k[rx] / k[nu][rx] Stereochemistry = Inversion / Retention / Racemic Nickname = EXAMPLE: Rank the following alkyl halides in order of reactivity toward an SN 2 reaction. Page 4
5 PRACTICE: Provide the mechanism and final products for the following reactions. a. b. Page 5
6 CONCEPT: THE SN 1 MECHANISM A neutral nucleophile reacts with an inaccessible leaving group to produce substitution in two-steps. The more -R groups, the more substituted the carbocation, the more SN 1 Properties (Circle One) Nucleophile = Strong / Weak Leaving Group = Unsubstituted / Highly Substituted Reaction coordinate = Transition State / Intermediate Reaction = Concerted / Two-Step Rate = Unimolecular / Bimolecular Rate = k[rx] / k[nu][rx] Stereochemistry = Inversion / Retention / Racemic Nickname = Page 6
7 PRACTICE: Provide the mechanism and final products for the following reactions. NOTE: Substitution reactions with neutral nucleophiles require an additional deprotonation step. a. b. Page 7
8 CONCEPT: SUBSTITUTION COMPARISON When given a substitution reaction, use the following two factors to determine the mechanism: 1. Nucleophile Strength SN 1 = SN 2 = 2. Leaving Group Substitution SN 1 = SN 2 = EXAMPLE: Provide the mechanism and final products for the following reactions. a. b. Page 8
9 CONCEPT: THE E2 MECHANISM A strong nucleophile reacts with an inaccessible leaving group to produce beta-elimination in one-step. E2 Properties (Circle One) Nucleophile = Strong / Weak Leaving Group = Unsubstituted / Highly Substituted Reaction coordinate = Transition State / Intermediate Reaction = Concerted / Two-Step Rate = Unimolecular / Bimolecular Rate = k[rx] / k[nu][rx] Stereochemistry = EXAMPLE: Rank the following alkyl halides in order of reactivity toward an E2 reaction. Page 9
10 CONCEPT: RECOGNIZING DIFFERENT β-hydrogens Elimination reactions remove β-hydrogens to create double bonds. The number of non-equivalent β-carbons with at least one determines the number of possible products. EXAMPLE: Identify the number of unique products that could be obtained through elimination. a. b. c. d. Page 10
11 CONCEPT: THE ANTI-COPLANAR REQUIREMENT E2 reactions require an anti-coplanar arrangement in order for the orbitals to overlap and create a new pi bond. When this occurs on cyclohexane, the leaving group and beta-proton must be DIAXIAL to each other. EXAMPLE: Identify which of the following E2 mechanisms would react to completion. Do not draw final products. a. b. c. d. Page 11
12 CONCEPT: THE E1 MECHANISM A weak nucleophile reacts with an inaccessible leaving group to produce beta-elimination in two-steps. E1 Properties (Circle One) Nucleophile = Strong / Weak Leaving Group = Unsubstituted / Highly Substituted Reaction coordinate = Transition State / Intermediate Reaction = Concerted / Two-Step Rate = Unimolecular / Bimolecular Rate = k[rx] / k[nu][rx] Stereochemistry = Page 12
13 PRACTICE: Provide the full mechanism and draw the final product for the following reactions. a. b. Page 13
14 CONCEPT: SOLVENTS Solvents are mostly inert compounds that provide a medium for the reaction to take place in. Although extremely important in lab, they rarely affect the outcome of a written reaction in Orgo 1 Classification of Solvents Polar solvents are solvents which contain a Aprotic solvents are solvents that cannot display Protic solvents are solvents that display (stabilize carbocations, hinder nucleophiles) Therefore, we will prefer to run & in protic solvents, and & in aprotic solvents. EXAMPLE: Identify the following solvents as apolar, polar aprotic or polar protic Page 14
15 CONCEPT: LEAVING GROUPS AND NUCLEOPHILES Leaving Groups: Alkyl halides are the most common leaving groups of organic chemistry, but there are others. Sulfonate Esters are the name given to a group of leaving groups with the general formula -SO3R Water is also a common leaving group, usually formed after alcohol is protonated with a strong acid Page 15
16 Nucleophile: Molecule that can easily donate electrons Base: Molecule that can easily remove a proton Relative Strength Rules: 1. A negative charge will always be a stronger nucleophile than its neutral counterpart. 2. The bulkier the substrate, the more and less it is. 3. Page 16
17 CONCEPT: THE BIG DADDY MECHANISM FLOWCHART Mechanisms are rarely given. We use nucleophile and leaving group information to determine the favored mechanism. Page 17
18 PRACTICE: Predict the mechanism for the following reactions. Provide the full mechanism and draw the final product. a. b. c. Page 18
19 PRACTICE: Predict the mechanism for the following reactions. Provide the full mechanism and draw the final product. d. e. f. Page 19
20 CONCEPT: CARBOCATION INTERMEDIATES STABILITY Carbocations are stabilized by a phenomenon called Hyperconjugation is the delocalization of charge by the interaction of an empty p-orbital with an adjacent, eclipsed σ-bond Since this is only possible with -R groups, the more substituted the carbocation, the more EXAMPLE: Which of the following alkyl halides would generate the most stable carbocation? Page 20
21 CONCEPT: ACID-CATALYZED DEHYDRATION Alcohols are terrible leaving groups, but in the presence of acid, they can be converted into an awesome leaving group The more R groups on the alcohol, the easier to dehydrate: The specific elimination mechanism depends on how easily the molecule will form a. E2 Dehydration: 1 o Alcohol Mechanism: Protonation: E2 β-hydrogen Elimination: Page 21
22 CONCEPT: ACID-CATALYZED DEHYDRATION Alcohols are terrible leaving groups, but in the presence of acid, they can be converted into an awesome leaving group E1 Dehydration: 2 o and 3 o Alcohol Mechanism: Protonation: Carbocation Formation: E1 β-hydrogen Elimination: Page 22
23 PRACTICE: Provide the mechanism and major product for the following dehydration reactions: a. b. Page 23
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