REACTION AND SYNTHESIS REVIEW A STUDENT SHOULD BE ABLE TO PREDICT PRODUCTS, IDENTIFY REACTANTS, GIVE REACTION CONDITIONS, PROPOSE SYNTHESES, AND PROPOSE MECHANISMS (AS LISTED BELOW). REVIEW THE MECHANISM FLOWCHART FOR SUBSTITUTIONS AND ELIMINATIONS, ALL REAGENTS AND REACTIONS COVERED ON THE SUMMARY OF ORGANIC CHEM I REACTIONS, AND THE SYNTHESIS CHAPTER IN YOUR TEXTBOOK. The seven major mechanisms to review are: Acid-Base (proton transfer), S N 2, S N 1, E2, E1, Electrophilic Addition, and Free Radical reactions. Knowledge of the mechanism is important in order to determine not only the product, but the correct regiochemistry and stereochemistry as well. Oxidation and Reduction reactions are also discussed, but you are not required to know these mechanisms. Acid-Base reactions: Acid-Base reactions (H + transfer) take place to produce a weaker base using the ARIO rules. Review ARIO and its exceptions. Substitution reactions: S N 2 is a bimolecular reaction that proceeds with inversion. S N 2 is the dominant mechanism on methyl (CH 3 ) substrates. S N 2 is the dominant mechanism on 1 substrates if the reagent is a good nucleophile/strong base; the exception on 1 substrates occurs with a bulky, strong base (e.g., (CH 3 ) 3 COK), which gives E2 elimination (see below). S N 2 will take place on 2 substrates if the reagent is a good nucleophile, but not a strong base (see below, E2). S N 2 will not take place on 3 substrates. S N 1 is a unimolecular reaction with a carbocation intermediate that proceeds with racemization. S N 1 will not take place on CH 3 -X or 1 substrates. On 2 or 3 substrates, the nucleophile must be a weak base (typically a neutral oxygen, such as water or alcohols), the solvent must be polar and protic (such as water or alcohol), and the reaction must be performed without heat for this reaction to predominate. Rearrangement of the carbocation (1,2-hydride or 1,2-methyl shift) can occur to stabilize the intermediate. Review carbocation stability. Acid-catalyzed substitutions are used when protonation of a group generates a good leaving group when one is not present (e.g., RX synthesis from ROH and HX). The reactions of alcohols with HX are S N 2 for 1 and 2 substrates and S N 1 for 3 substrates. Elimination reactions: E2 is a bimolecular anti elimination reactions. The regiochemistry of E2 reactions results in the more stable alkene as the major product unless a strong, bulky base (e.g., (CH 3 ) 3 COK) is used which gives the less sterically hindered, less substituted alkene as a product. 176
E2 reactions take place on 1 substrates only with a strong, bulky base (e.g., (CH 3 ) 3 COK). E2 reactions take place on 2 substrates only if the reagent acts as a strong base ( - OH or - OR). E2 reactions will take place on 3 substrates if the reagent is a strong base. E1 is a unimolecular reaction with a carbocation intermediate that may rearrange. E1 will not take place on CH 3 -X or 1 substrates. On 2 or 3 substrates, the nucleophile must be a weak base (typically a neutral oxygen, such as in water or alcohols), the solvent must be polar and protic (such as water or alcohol), and the reaction must be performed with heat for this reaction to predominate. Acid-catalyzed eliminations may occur by first protonating the substrate to make a good leaving group when one is not present, typically on 2 and 3 ROH. This reaction of alcohols gives the more stable alkene as the major product; rearrangements occur when favorable. 1 ROH do not dehydrate efficiently. Addition reactions: Electrophilic Addition reactions occur first with the addition of an electrophile (Ê) to an alkene creating the most stable carbocation intermediate. The regiochemistry that usually results is the Ê (added first) on the less substituted C of the alkene (or alkyne) and the carbocation intermediate on the more substituted C. These are also referred to as Markovnikov additions. Markovnikov additions: Addition of HF, HCl, HBr (no peroxides), HI, H 3 O +, etc.) to alkenes and alkynes. Anti-Markovnikov additions refer to those reactions with opposite regiochemistry, meaning the Ê is added to the more substituted C. Anti-Markovnikov additions: Addition of HBr with peroxides (Free Radical). Addition of water across a pi bond can be accomplished with a variety of regio- and stereochemistries: acid-catalyzed hydration (Markovnikov addition, rearrangement can occur), oxymercuration-demercuration (Markovnikov addition, no rearrangement), and hydroboration-oxidation (anti-markovnikov, overall syn addition). Addition of halogens (Br 2 and Cl 2 ), anti addition. Make note of the solvent (CCl 4, H 2 O, or ROH) which may change the product. Glycols: Addition of KMnO 4 (cold) or OsO 4 to make glycols (syn addition); epoxidation followed by hydrolysis of the epoxide (syn addition in the first step followed anti attack). Hydrogenation of alkenes and alkynes with metal catalysts (Pt, Pd, Ni) to give alkanes, and the reduction of alkynes to give both cis (H 2 with Lindlar s catalyst) and trans (Na or Li metal with liq. NH 3 ) alkenes. (These are reduction reactions.) Typically, additions to alkynes take place exactly like those for alkenes, only twice; note that the mechanisms may be different. Two exceptions are oxymercuration and hydroboration-oxidation, which generate carbonyl products. This is due to tautomerization. 177
Free-Radical reactions: Free radical halogenation with Cl 2 and Br 2 in the presence of heat or light; F 2 is overactive, and I 2 is unreactive. Br 2 is more selective than Cl 2. Addition of HBr or HCl with peroxides. Polymerization of alkenes Reduction reactions: Hydrogenation of alkenes and alkynes with metal catalysts (Pt, Pd, Ni) to give alkanes, and the hydrogenation of alkynes to give both cis and trans alkenes, using specialized reagents Oxidation reactions: Degradation of alkenes (to generate aldehydes and ketones) and alkynes (to generate carboxylic acids) via ozonolysis (oxidative cleavage). 178
Name Reaction Review Quiz (Sample A) 1. Supply the missing reactant(s), conditions, or product(s). 2. Propose a curved-arrow mechanism for the following reaction. Add in lone pairs and charges where necessary. 179
Name Reaction Review Quiz (Sample B) 1. Give the structure of the major organic product(s) or supply the missing reactant/conditions or starting material. Show the stereochemistry of the product(s) or starting material, if applicable. 2. Propose a curved-arrow mechanism for the following reaction. Add in lone pairs and charges where necessary. 180
Name Synthesis Review (Sample A) Propose syntheses for these compounds, using the given starting material and any other necessary reagents. You do not have to show reaction mechanisms. There may be more than one correct answer for some questions. 181
Name Synthesis Review (Sample B) Propose syntheses for these compounds, using the given starting material and any other necessary reagents. You do not have to show reaction mechanisms. There may be more than one correct answer for some questions. 182