1559T_ch12_220-24611/02/0521:55Pa9e220 ⊕ EQA 12 Reactions of Alkenes Alkenes s in the t are electrons in bonds.As a result,they are"available"in a"Lewis base" sense lone pai 黑义of teve adi中 known,and they allov es,inciu ing ones we've seen b an updated Functional Group Interconversions chart is presented in this chapter of the study guide. Outline of the Chapter 12-1 Thermodynamic Feasibility of Addition Reactions 12-2 Hydrogenation of Alkenes 12-3 through 12-7 12-8 ReacandSereosdle s by Hydroboration oup inter ving a 12.9 Diazomethane,Carbenes,and Cyclopropane Synthesis ng u 12-10,12-11,12-12 Oxidation of Alkenes of oxygen-containing electrophiles 12-13 Radical Additions to Alkenes d of mechanism for alkene reactions:radical additions 12-14,12-15 Dimerization,Oligomerization,and Polymerization of Alkenes 12-16 Ethene in Industry 12-17 Alkenes in Nature Keys to the Chapter 12-1.Thermodynamic Feasibility of Addition Reactions bond).but on reactions to are generally exothermi 22 use one
12 Reactions of Alkenes Alkenes are reactive and synthetically useful molecules. Their reactivity originates from the electrons in the bond. They are on the average further away from (and therefore electrostatically less tightly held to) the carbon nuclei than are electrons in bonds. As a result, they are “available” in a “Lewis base” sense, somewhat like the lone pair electrons on the oxygen of water or the nitrogen of ammonia. As you will see, attachment of an electrophile to these basic electrons has the effect of breaking the bond and is the first step in many reactions of alkenes, additions. Many of these addition reactions are known, and they allow conversion of alkenes to other organic molecules, including ones we’ve seen before (haloalkanes and alcohols) as well as new ones. These additions expand the scope of our synthetic possibilities significantly. Therefore, an updated Functional Group Interconversions chart is presented in this chapter of the study guide. Outline of the Chapter 12-1 Thermodynamic Feasibility of Addition Reactions 12-2 Hydrogenation of Alkenes 12-3 through 12-7 Electrophilic Additions to Alkenes The largest group of reactions characteristic of alkenes. 12-8 Regio- and Stereoselective Hydration of Alkenes by Hydroboration Another functional group interconversion involving addition. 12-9 Diazomethane, Carbenes, and Cyclopropane Synthesis Preparing three-membered rings. 12-10, 12-11, 12-12 Oxidation of Alkenes Additions of oxygen-containing electrophiles. 12-13 Radical Additions to Alkenes A new kind of mechanism for alkene reactions: radical additions. 12-14, 12-15 Dimerization, Oligomerization, and Polymerization of Alkenes Reactions of, mainly, industrial use. Or, “where everything in your kitchen that you don’t eat comes from.” 12-16 Ethene in Industry 12-17 Alkenes in Nature Keys to the Chapter 12-1. Thermodynamic Feasibility of Addition Reactions Carbon–carbon bonds are weaker than bonds. Addition reactions to alkenes are generally exothermic because one of the bonds broken is weak (the bond), but both new bonds formed are strong. 220 1559T_ch12_220-246 11/02/05 21:55 Page 220
15597ch12220-24611/02/0521:55Pag0221 EQA Keys to the Chapler·221 C=C+A-B一 △Hr°usualIy negative Bonds broken Bonds formed 12-2.Hydrogenation of Alkenes The simplest reaction of alkenes is the addition of hydrogento form alkanes.Hydro isa non process,unlik (Adams's catalyst),palladium on carbon (Pd-C).or Raney nickel (RaNi). 12-3 through 12-7.Electrophilic Additions to Alkenes othe larg hmtPpncalcdasofaditionreactions.ofalk bonded cabons.reacion forming a cationic intermediate.Combination of this cation with any avaib nucleophile gives the final addition product: From a synthetic point of view,addition reactions allow conversion of alkenes to molecules containing new functional groups at one or both of the original double-bonded carbons.These sections present many such Most of the"A-B"-typ e molecules that particinate in electrophilic addition to alkenes are str such as H Cl.Bra)fall into this category.A hough nonpolar mo if it contained an electr tion mechanism v dd double bonds.The cation then can do any of its usual reactions (e.gattach to a nucleophile or rearrange) Larger electrophiles.especially ones with lone pairs (e.g..":Br:"from Br).add to give positively charged three-mem red nngs (bro monium ior chloronium ion, 12-8.Regio-and Stereoselective Functionalization of Alkenes by Hydroboration
12-2. Hydrogenation of Alkenes The simplest reaction of alkenes is the addition of hydrogen to form alkanes. Hydrogenation is a nonpolar process, unlike many of the addition reactions to be described later, which involve electrophiles attaching to the nucleophilic electrons of the alkene. Hydrogenation reactions require catalysts, such as platinum oxide (Adams’s catalyst), palladium on carbon (Pd-C), or Raney nickel (Ra-Ni). 12-3 through 12-7. Electrophilic Additions to Alkenes In these sections we turn to the largest and most typical class of addition reactions of alkenes. These all proceed in two steps. The first step is attachment of an electrophilic atom to one of the alkene’s doublebonded carbons, a reaction forming a cationic intermediate. Combination of this cation with any available nucleophile gives the final addition product: (1) (2) From a synthetic point of view, addition reactions allow conversion of alkenes to molecules containing new functional groups at one or both of the original double-bonded carbons. These sections present many such examples. Most of the “A–B”-type molecules that participate in electrophilic addition to alkenes are strongly polarized, such as HOCl. Others are not but can still serve as sources of electrophilic atoms. Halogens (Cl2, Br2) fall into this category. Although nonpolar molecules like Br2 do not have permanent dipoles, recall that electron movement can give rise to “fleeting dipoles,” thus allowing the halogen molecule to behave as if it contained an electrophilic atom, namely, “Br”. The details of the addition mechanism vary somewhat with the nature of the electrophile. Thus, protons add regiospecifically to give the more stable of the two possible carbocations in the case of unsymmetrical double bonds. The cation then can do any of its usual reactions (e.g., attach to a nucleophile or rearrange). Larger electrophiles, especially ones with lone pairs (e.g., “)Br >)” from Br2), add to give positively charged three-membered rings (bromonium ion, chloronium ion, etc.). Nucleophiles react with these ions in much the same way that they open the rings of the cyclic alkyloxonium ions of Chapter 9. Addition occurs at the most highly substituted carbon (Markovnikov orientation) in an anti manner. 12-8. Regio- and Stereoselective Functionalization of Alkenes by Hydroboration This section describes the special characteristics and utility of the reaction between an alkene and a borane, a molecule containing BOH bonds. Boranes are electrophilic, and their reactions with alkenes are therefore very reasonable (note carefully, however, that borohydrides like BH4 are very different—they are anions, not electrophiles, and don’t react with alkenes). C A C C C A B B CC C A C A B or something ( similar ( Either as A Bor A B B C C bond (weak) C A B C H usually negative A B Bonds broken Bonds formed bond (strong) 2 bond (strong) Keys to the Chapter • 221 1559T_ch12_220-246 11/02/05 21:55 Page 221
1559T_ch12_220-24611/02/0521:55Pa9e222 EQA 222.Chapter 12 REACTIONS OF ALKENES useful because the new carbon-boron bond can be oxidized by basic HO to give an alcohol. --一-C-0m The chart that follows summarizes the new reactions of Sections 12-3 through 12-8,using 1-methylcyclo- 1.Hydroboration-oxidation(anti-Markovnikov).e.g.. -aH=at0a--a-at CH3 OH CH-C-CH-CH2-OH H02 2.Oxymercuration-demercuration (Markovnikov).e.g. CH3 CH3OH CH-C-CH-CH.CH-c-CH-CH CH3 HgOAc CHsOH :-CH-CHs 3.Acid-catalyzed hydration (Markovnikov,with carbocations that may rearrange).e.g.. CHs CHs CH;- -C-CH-CH2 H, H.O CH,- -CH-CH H CH CH;-C-CH-CHs CH;- CH-CH CH
Borane BOH bonds add across alkenes. The reaction (1) is regioselectively anti-Markovnikov, (2) is stereospecifically a syn addition, (3) goes by a concerted, one-step mechanism, and (4) is synthetically very useful because the new carbon–boron bond can be oxidized by basic H2O2 to give an alcohol. The chart that follows summarizes the new reactions of Sections 12-3 through 12-8, using 1-methylcyclohexene as a substrate. Note the three methods for adding H2O to a double bond, each with its own specific characteristics. 1. Hydroboration–oxidation (anti-Markovnikov), e.g., 2. Oxymercuration–demercuration (Markovnikov), e.g., 3. Acid-catalyzed hydration (Markovnikov, with carbocations that may rearrange), e.g., C CH3 CH3 CH2 CH3 CH H2SO2 H2O C CH CH3 CH3 CH3 CH3 C CH CH3 CH3 CH3 CH3 C CH CH3 CH3 OH CH3 CH3 C CH3 CH3 CH2 CH3 CH Hg(OAc)2 H2O THF NaBH4 NaOH H2O C CH CH3 CH3 CH3 OH HgOAc CH2 OH C CH CH3 CH3 CH3 CH3 C CH3 CH3 CH2 CH3 CH 3 ( ( BH3 THF C CH2 CH3 CH3 CH3 CH2 B NaOH H2O2 C CH2 OH CH3 CH3 CH3 CH2 C B C OH 222 • Chapter 12 REACTIONS OF ALKENES 1559T_ch12_220-246 11/02/05 21:55 Page 222
1559r.ch12.220-24611/02/0521:55Page223 EQA 88音导8 66格说销 ⊕ 223
223 CHART OF EXAMPLES Electrophilic Additions to 1-Methylcyclohexene Typical Regiochemistry Major Example reagent Electrophile Nucleophile Intermediate Stereochemistry Product Hydroboration BH3 B H None anti-Markovnikov syn addition HX addition HCl H Cl Markovnikov Hydration H2SO4, H2O H H2O Markovnikov Halogenation ICl I Cl Haloalcohol Cl2, H2O Cl H2O formation Chlorosulfenylation CH3SCl CH3S Cl Oxymercuration Hg(OCCH3)2, H2O CH3COHg H2O OH CH3 H O HgOCCH3 CH3 O HgOCCH3 Cl CH3 SCH3 H CH3 SCH3 OH CH3 Cl H CH3 Cl Cl CH3 I H CH3 I CH3 OH CH3 CH3 Cl CH3 CH3 H H B(H, R)2 O B O B Markovnikov anti addition Markovnikov anti addition Markovnikov anti addition Markovnikov anti addition 1559T_ch12_220-246 11/02/05 21:55 Page 223
1559T_ch12_220-24611/02/0521:55Pa9e22 EQA 224.Chapter 12 REACTIONS OF ALKENES 12-9. Diazomethane ,Carbe sand Cyclopropane Synthesis which have the h-energy spec cannot be synt In thand e a co ang an octet,ca stored but have【 section,look at the a vs in the mecha nism for this process.The electron pair in the bond of the alkene these two bond-forming processes happen together,making this an example of a concerted reaction.one in which multiple happen sim from alkenes these are called ca benoids they act like carbens without carbenes Like carbenes they contan a spcia supplying ar 12-10,12-11,and 12-12.Oxidation of Alkenes These sections present several reactions that attach oxygen to both the double-bonded carbons of alkenes Petch rea bonding changes occur in one step. R808 that adds to give oxacyclopropanes. abo The ing compound 安 cleavage of carbon-carbon b ompoun R 0 R4 12-13.Radical Additions to Alkenes the the radicalatom Unlike additions of positively chard ctophiles how hcaocation but ather radical.Like the procesese studied in Chapter 3.these l chain mechanisms
224 • Chapter 12 REACTIONS OF ALKENES 12-9. Diazomethane, Carbenes, and Cyclopropane Synthesis This section introduces an unusual electrophile, the carbene, which has a specific use in synthesis. Carbenes, which have the general structure R2C:, have a neutral but electron-deficient carbon capable of electrophilic additions. Lacking an octet, carbenes are high-energy species that cannot be synthesized and stored but have to be prepared in the presence of whatever substance they are intended to react with. In the presence of alkenes, carbenes react rapidly to form bonds with both alkene carbons, giving a cyclopropane as a result. In the text section, look at the arrows in the mechanism for this process. The electron pair in the bond of the alkene moves to make one bond between one of the alkene carbons and the electrophilic carbene carbon, and the electron pair on the carbene carbon itself moves to make the second bond with the other alkene carbon. Typically these two bond-forming processes happen together, making this an example of a concerted reaction, one in which multiple bonding changes happen simultaneously. Some alternative species are encountered that also make cyclopropanes from alkenes; these are called carbenoids because they act like carbenes without actually being carbenes. Like carbenes they contain a special carbon atom capable of reacting as an electrophile with the bond of the alkene to form one carbon–carbon bond and also supplying an electron pair to form a second carbon–carbon bond. 12-10, 12-11, and 12-12. Oxidation of Alkenes These sections present several reactions that attach oxygen to both the double-bonded carbons of alkenes. Each reaction is an example of a concerted process, in which several bonding changes occur in one step. Peroxycarboxylic acids such as MCPBA contain an electrophilic oxygen O B ROCOOOO OH that adds to give oxacyclopropanes. Osmium tetroxide and ozone both participate in a process where three pairs of electrons move in a circle to simultaneously form new COO bonds at both the double-bonded carbons. The products of these concerted cycloadditions are ring compounds: Notice, for synthetic purposes, that ozonolysis is the first process you’ve seen that can lead to complete cleavage of carbon–carbon bonds. Ozonolysis followed by reduction breaks double bonds, giving two carbonyl compounds: 12-13. Radical Additions to Alkenes Although typical radicals are neutral, they are electron deficient in the sense that they are one electron short of having a full octet. Reaction between the electrons of an alkene double bond can take place, completing the octet around the radical atom. Unlike additions of positively charged electrophiles, however, the result of this addition is not a carbocation but another radical. Like the processes we studied in Chapter 3, these reactions follow radical chain mechanisms. C C R4 R1 R2 R3 1. O3, CH2Cl2 2. Zn, CH3COH O C O O R1 R2 C R4 R3 O O Os O O O C C O O C C 1559T_ch12_220-246 11/02/05 21:55 Page 224