Reactions of Alkenes: Electrophilic Additions and Regioselectivity, Study Guides, Projects, Research of Chemistry

Download Reactions of Alkenes: Electrophilic Additions and Regioselectivity and more Study Guides, Projects, Research Chemistry in PDF only on Docsity! Chapter 8 Lecture Organic Chemistry, 9th Edition L. G. Wade, Jr. Reactions of Alkenes © 2017 Pearson Education, Inc. Reactivity of the Carbon– Carbon Double Bond • The most common reactions of double bonds transform the pi bond into a sigma bond. Electrophilic Addition • Step 1: Pi electrons attack the electrophile. • Step 2: Nucleophile attacks the carbocation. Types of Additions TABLE 8-1 Types of Additions to Alkenes \ f Type of Addition P “ \ [Elements Added]? roduct H OH x X hydration a halogenation _ | _ | _ [H,O] | [X,], an oxidation | | H 4H X OH hydrogenation _ be de halohydrin formation be de [H,], a reduction | I [HOX], an oxidation | | OH OH H X dihydroxylation | | HX addition | | aT coon Comes One ae eg (a [HOOH}], an oxidation | | (HxX] | | (hydrohalogenation) oxidative cleavage ar C=O O=C. [O,], an oxidation NA a cyclopropanation _ br» epoxidation [O], an oxidation [CH] “These are not the reagents used, but simply the groups that appear in the product. Addition of HX to Alkenes • Step 1 is the protonation of the double bond. • The protonation step forms the most stable carbocation possible. • In step 2, the nucleophile attacks the carbocation, forming an alkyl halide. • HBr, HCl, and HI can be added through this reaction. Markovnikov’s Rule The acid proton will bond to carbon 3 in order to produce the most stable carbocation possible. Markovnikov’s Rule (Continued) • The bromide anion then adds to the carbocation. The Reaction-Energy Diagram The first step is rate-determining in the electrophilic addition to an alkene. More Examples CH, | CH,—C=CH—CH,CH, + HCI + HCl CH, 1" — CH, —C—CH—CH,CH, cl H Free-Radical Addition of HBr • In the presence of peroxides, HBr adds to an alkene to form the “anti-Markovnikov” product. • Peroxides produce free radicals. • Only HBr has just the right reactivity for each step of the free-radical chain reaction to take place. • The peroxide effect is not seen with HCl or HI because the reaction of an alkyl radical with HCl or HI is strongly endothermic. Free-Radical Initiation • The peroxide bond breaks homolytically to form the first radical: (Not Responsible for this mechanism on my exam) • Hydrogen is abstracted from HBr. This synthesis requires the conversion of an alcohol to an alkyl bromide with the bromine atom at the neighboring carbon atom. This is the anti-Markovnikov product, which could be formed by the radical-catalyzed addition of HBr to 1-methylcyclohexene. 1-Methylcyclohexene is easily synthesized by the dehydration of 1-methylcyclohexanol. The most substituted alkene is the desired product. Solved Problem 2 Solution Convert 1-methylcyclohexanol to 1-bromo-2-methylcyclohexane. The two-step synthesis is summarized as follows: Solved Problem 2 (Continued) Solution (Continued) Hydration of Alkenes 3 ways to add water to alkene (1st method here, Markovnikov with possibility of rearrangement) • Addition of water to the double bond forms an alcohol. • The addition follows Markovnikov’s rule. • This is the reverse of the dehydration of alcohol. • It uses dilute solutions of H2SO4 or H3PO4 to drive equilibrium toward hydration. Rearrangements Are Possible • A methyl shift after protonation will produce the more stable tertiary carbocation. Methyl Shift: Oxymercuration– Demercuration Reaction 3 ways to add water to alkene (2nd method here, Markovnikov without possibility of rearrangement) • Markovnikov addition of water to the double bond • Milder conditions than direct hydration • No rearrangements or polymerization Oxymercuration Reaction • The reagent is mercury(II) acetate, which dissociates slightly to form +Hg(OAc). • +Hg(OAc) is the electrophile that adds to the pi bond. • The intermediate is a three-membered ring called the mercurinium ion. • Overall, the addition of water follows Markovnikov’s rule. Hydroboration of Alkenes 3 ways to add water to alkene (3rd method here, anti-Markovnikov ) • H. C. Brown of Purdue University discovered that diborane (B2H6) adds to alkenes with anti-Markovnikov orientation to form alkylboranes, which, after oxidation, give anti-Markovnikov alcohols. • Brown received the Nobel Prize in Chemistry in 1979 for his work in the field of borane chemistry. Mechanism of Hydroboration • Borane adds to the double bond in a single step, with boron adding to the less substituted carbon and hydrogen adding to the more highly substituted carbon. • This orientation places the partial positive charge in the transition state on the more highly substituted carbon atom. Do NOT need to know this slide Oxidation to Alcohol • Oxidation of the alkyl borane with basic hydrogen peroxide produces the alcohol. • Orientation is anti-Markovnikov. Do NOT need to know this slide The Role of the Hydroperoxide Ion Do NOT need to know this slide Oxidation of a Trialkylborane Do NOT need to know this slide Show how you would convert 1-methylcyclopentanol to 2-methylcyclopentanol. Working backward, use hydroboration–oxidation to form 2-methylcyclopentanol from 1-methylcyclopentene. The use of (1) and (2) above and below the reaction arrow indicates individual steps in a two-step sequence. The 2-methylcyclopentanol that results from this synthesis is the pure trans isomer. This stereochemical result is discussed in Section 8-7C. 1-Methylcyclopentene is the most substituted alkene that results from dehydration of 1-methylcyclopentanol. Dehydration of the alcohol would give the correct alkene. Solved Problem 3 Solution