Alkenes, Reactions

NR   some R-H R-X R-OH R-O-R Alkenes Acids Bases Metals Oxidation Reduction Halogens

Alkenes, reactions. Addition ionic free radical Reduction Oxidation Substitution

Reactions, alkenes: Addition of hydrogen (reduction). Addition of halogens. Addition of hydrogen halides. Addition of sulfuric acid. Addition of water (hydration). Addition of aqueous halogens (halohydrin formation). 7. Oxymercuration-demercuration.

8. Hydroboration-oxidation. 9. Addition of free radicals. 10. Addition of carbenes. 11. Epoxidation. 12. Hydroxylation. 13. Allylic halogenation 14. Ozonolysis. 15. Vigorous oxidation.

Addition of hydrogen (reduction). | | | | — C = C — + H2 + Ni, Pt, or Pd  — C — C — | | H H a) Requires catalyst. #1 synthesis of alkanes CH3CH=CHCH3 + H2, Ni  CH3CH2CH2CH3 2-butene n-butane

Alkanes Nomenclature Syntheses 1. addition of hydrogen to an alkene 2. reduction of an alkyl halide a) hydrolysis of a Grignard reagent b) with an active metal and acid 3. Corey-House Synthesis Reactions 1. halogenation 2. combustion (oxidation) 3. pyrolysis (cracking)

heat of hydrogenation: CH3CH=CH2 + H2, Pt  CH3CH2CH3 + ~ 30 Kcal/mole ethylene 32.8 propylene 30.1 cis-2-butene 28.6 trans-2-butene 27.6 isobutylene 28.4

fats & oils: triglycerides CH2—O—CCH2CH2CH2CH2CH2CH2CH2CH2CH3 | O CH—O—CCH2CH2CH2CH2CH2CH2CH2CH2CH3 CH2—O—CCH2CH2CH2CH2CH2CH2CH3 “saturated” fat

O CH2—O—CCH2CH2CH2CH2CH=CH2CH2CH2CH3 | O CH—O—CCH2CH2CH2CH2CH=CH2CH2CH2CH3 CH2—O—CCH2CH2CH=CHCH2CH2CH3 Ω - 3 “unsaturated” oil

Saturated triglycerides are solids at room temperature and are called “fats”. butter fat, lard, vegetable shortening, beef tallow, etc. Unsaturated triglycerides have lower mp’s than saturated triglycerides. Those that are liquids at room temperature are called “oils”. (All double bonds are cis-.) corn oil, peanut oil, Canola oil, cottonseed oil, etc.

polyunsaturated oils + H2, Ni  saturated fats liquid at RT solid at RT oleomargarine butter substitute (dyed yellow) Trans-fatty acids formed in the synthesis of margarine have been implicated in the formation of “bad” cholesterol, hardening of the arteries and heart disease. 

2. Addition of halogens. | | | | — C = C — + X2  — C — C — | | X X X2 = Br2 or Cl2 test for unsaturation with Br2 CH3CH2CH=CH2 + Br2/CCl4  CH3CH2CHCH2 Br Br 1-butene 1,2-dibromobutane

Addition of hydrogen halides. | | | | — C = C — + HX  — C — C — | | H X HX = HI, HBr, HCl Markovnikov orientation CH3CH=CH2 + HI  CH3CHCH3 I CH3 CH3 CH2C=CH2 + HBr  CH3CCH3 Br

Markovnikov’s Rule: In the addition of an acid to an alkene the hydrogen will go to the vinyl carbon that already has the greater number of hydrogens.

CH3CH2CH=CH2 + HCl  CH3CH2CHCH3 CH3CH=CCH3 + HBr  CH3CH2CCH3 Br CH3CH=CHCH3 + HI  CH3CH2CHCH3 I

An exception to Markovikov’s Rule: CH3CH=CH2 + HBr, peroxides  CH3CH2CH2Br CH3 CH3 CH3C=CH2 + HBr, peroxides  CH3CHCH2Br “anti-Markovnikov” orientation note: this is only for HBr.

Markovnikov doesn’t always correctly predict the product! CH3 CH3 CH2=CHCHCH3 + HI  CH3CH2CCH3  I Rearrangement!

why Markovinkov? CH3CH=CH2 + HBr  CH3CHCH2 1o carbocation |   H or? CH3CHCH2 2o carbocation  | more stable + Br-  CH3CHCH3 | Br

In ionic electrophilic addition to an alkene, the electrophile always adds to the carbon-carbon double bond so as to form the more stable carbocation.

Addition of sulfuric acid. | | | | — C = C — + H2SO4  — C — C — | | H OSO3H alkyl hydrogen sulfate Markovnikov orientation. CH3CH=CH2 + H2SO4  CH3CHCH3 O O-S-O OH

Addition of water. | | | | — C = C — + H2O, H+  — C — C — | | H OH a) requires acid Markovnikov orientation low yield  CH3CH2CH=CH2 + H2O, H+  CH3CH2CHCH3 OH

| | H+ | | — C = C — + H2O  — C — C — | | OH H Mechanism for addition of water to an alkene to form an alcohol is the exact reverse of the mechanism (E1) for the dehydration of an alcohol to form an alkene.

How do we know that the mechanism isn’t this way? One step, concerted, no carbocation

CH3CH=CH2 + Br2 + H2O + NaCl  CH3CHCH2 + CH3CHCH2 + CH3CHCH2 Br Br OH Br Cl Br

Some evidence suggests that the intermediate is not a normal carbocation but a “halonium” ion: | | — C — C — Br  The addition of X2 to an alkene is an anti-addition.

Addition of halogens + water (halohydrin formation): | | | | — C = C — + X2, H2O  — C — C — + HX | | OH X X2 = Br2, Cl2 Br2 = electrophile CH3CH=CH2 + Br2(aq.)  CH3CHCH2 + HBr OH Br

7. Oxymercuration-demercuration. | | | | — C = C — + H2O, Hg(OAc)2  — C — C — + acetic | | acid OH HgOAc | | | | — C — C — + NaBH4  — C — C — OH HgOAc OH H alcohol

oxymercuration-demercuration: #1 synthesis of alcohols. Markovnikov orientation. 100% yields.  no rearrangements  CH3CH2CH=CH2 + H2O, Hg(OAc)2; then NaBH4  CH3CH2CHCH3 OH

With alcohol instead of water: alkoxymercuration-demercuration: | | | | — C =C — + ROH, Hg(TFA)2  — C — C — | | OR HgTFA | | | | — C — C — + NaBH4  — C — C — OR HgTFA OR H ether

alkoxymercuration-demercuration: #2 synthesis of ethers. Markovnikov orientation. 100% yields.  no rearrangements  CH3CH=CH2 + CH3CHCH3, Hg(TFA)2; then NaBH4  OH CH3 CH3 CH3CH-O-CHCH3 diisopropyl ether Avoids the elimination with 2o/3o RX in Williamson Synthesis.

Ethers nomenclature syntheses 1. Williamson Synthesis 2. alkoxymercuration-demercuration reactions 1. acid cleavage

8. Hydroboration-oxidation. | | | | — C = C — + (BH3)2  — C — C — | | diborane H B — | | | | | — C — C — + H2O2, NaOH  — C — C — H B — H OH alcohol

hydroboration-oxidation: #2 synthesis of alcohols. Anti-Markovnikov orientation.  100% yields.  no rearrangements  CH3CH2CH=CH2 + (BH3)2; then H2O2, NaOH  CH3CH2CH2CH2-OH

CH3 CH3C=CH2 + H2O, Hg(OAc)2; then NaBH4  Markovnikov CH3CCH3 OH CH3C=CH2 + (BH3)2; then H2O2, NaOH  anti-Markovnikov CH3CHCH2

Alcohols: nomenclature syntheses 1. oxymercuration-demercuration 2. hydroboration-oxidation 3. 4. hydrolysis of a 1o / CH3 alcohol 5. 6. 8.

9. Addition of free radicals. | | | | — C = C — + HBr, peroxides  — C — C — | | H X anti-Markovnikov orientation. free radical addition HI, HCl + Peroxides CH3CH=CH2 + HBr, peroxides  CH3CH2CH2-Br

Mechanism for free radical addition of HBr: Initiating steps: 1) peroxide  2 radical• 2) radical• + HBr  radical:H + Br• (Br• electrophile) Propagating steps: 3) Br• + CH3CH=CH2  CH3CHCH2-Br (2o free radical) • 4) CH3CHCH2-Br + HBr  CH3CH2CH2-Br + Br• 3), 4), 3), 4)… Terminating steps: Br• + Br•  Br2 Etc.

In a free radical addition to an alkene, the electrophilic free radical adds to the vinyl carbon with the greater number of hydrogens to form the more stable free radical. In the case of HBr/peroxides, the electrophile is the bromine free radical (Br•). CH3CH=CH2 + HBr, peroxides  CH3CH2CH2-Br

10. Addition of carbenes. | | | | — C = C — + CH2CO or CH2N2 , hν  — C — C —  CH2 •CH2• “carbene” adds across the double bond | | — C = C —   •CH2•

| | | | — C = C — + CHCl3, t-BuOK  — C— C — CCl2  -HCl •CCl2• dichlorocarbene | | — C = C —   •CCl2•

11. Epoxidation. | | C6H5CO3H | | — C = C — + (peroxybenzoic acid)  — C— C — O epoxide Free radical addition of oxygen diradical. | | — C = C —   •O•

12. Hydroxylation. (mild oxidation) | | | | — C = C — + KMnO4  — C — C — syn | | OH OH OH — C = C — + HCO3H  — C — C — anti peroxyformic acid | | glycol

CH3CH=CHCH3 + KMnO4  CH3CH-CHCH3 OH OH 2,3-butanediol test for unsaturation purple KMnO4  brown MnO2 CH2=CH2 + KMnO4  CH2CH2 ethylene glycol “anti-freeze”

13. Allylic halogenation. | | | | | | — C = C — C — + X2, heat  — C = C — C — + HX | | H  allyl X CH2=CHCH3 + Br2, 350oC  CH2=CHCH2Br + HBr a) X2 = Cl2 or Br2 b) or N-bromosuccinimide (NBS)

CH2=CHCH3 + Br2  CH2CHCH3 Br Br addition CH2=CHCH3 + Br2, heat  CH2=CHCH2-Br + HBr allylic substitution

14. Ozonolysis. | | | | — C = C — + O3; then Zn, H2O  — C = O + O = C — used for identification of alkenes CH3 CH3CH2CH=CCH3 + O3; then Zn, H2O  CH3CH2CH=O + O=CCH3

15. Vigorous oxidation. =CH2 + KMnO4, heat  CO2 =CHR + KMnO4, heat  RCOOH carboxylic acid =CR2 + KMnO4, heat  O=CR2 ketone

CH3CH2CH2CH=CH2 + KMnO4, heat  CH3CH2CH2COOH + CO2 CH3 CH3 CH3C=CHCH3 + KMnO4, heat  CH3C=O + HOOCCH3

CH3CH=CHCH3 + KMnO4  CH3CHCHCH3 OHOH mild oxidation  glycol CH3CH=CHCH3 + hot KMnO4  2 CH3COOH vigorous oxidation

Reactions, alkenes: Addition of hydrogen Addition of halogens Addition of hydrogen halides Addition of sulfuric acid Addition of water/acid Addition of halogens & water (halohydrin formation) 7. Oxymercuration-demercuration

8. Hydroboration-oxidation 9. Addition of free radicals 10. Addition of carbenes 11. Epoxidation 12. Hydroxylation 13. Allylic halogenation 14. Ozonolysis 15. Vigorous oxidation

CH3 CH3 CH3C=CH2 + H2, Pt  CH3CHCH3 isobutylene CH3 “ + Br2/CCl4  CH3C-CH2 Br Br “ + HBr  CH3CCH3 Br “ + H2SO4  CH3CCH3 O SO3H

CH3 CH3 CH3C=CH2 + H2O, H+  CH3CCH3 isobutylene OH CH3 “ + Br2(aq.)  CH3C-CH2Br OH CH3 CH3 CH3C=CH2 + H2O,Hg(OAc)2; then NaBH4  CH3CCH3 “ + (BH3)2; then H2O2, OH-  CH3CHCH2

CH3 CH3 CH3C=CH2 + HBr, peroxides  CH3CHCH2 isobutylene Br CH3 “ + CH2CO, hv  CH3C–CH2 CH2 “ + PBA  CH3C–CH2 O

CH3 CH3 CH3C=CH2 + KMnO4  CH3C–CH2 isobutylene OH OH CH3 “ + Br2, heat  CH2C=CH2 + HBr Br “ + O3; then Zn/H2O  CH3C=O + O=CH2 “ + KMnO4, heat  CH3C=O + CO2