Summary of Reactions  301 

organoborane with alkaline H2O2. The two hydration methods are complementary: oxymercuration–demercuration gives the product of Markovnikov addition, whereas hydroboration–oxidation gives the product with non-Markovnikov syn stereochemistry.

Alkenes are reduced by addition of H2 in the presence of a catalyst such as platinum or palladium to yield alkanes, a process called catalytic hydrogenation. Alkenes are also oxidized by reaction with a peroxyacid to give epoxides, which can be converted into trans-1,2-diols by acid-catalyzed hydrolysis. The corre-sponding cis-1,2-diols can be made directly from alkenes by hydroxylation with OsO4. Alkenes can also be cleaved to produce carbonyl compounds by reaction with ozone, followed by reduction with zinc metal. In addition, alkenes react with divalent substances called carbenes, R2C:, to give cyclopropanes. Nonhalo-genated cyclopropanes are best prepared by treatment of the alkene with CH2I2 and zinc–copper, a process called the Simmons–Smith reaction.

Alkene polymers—large molecules resulting from repetitive bonding together of many hundreds or thousands of small monomer units—are formed by chain-reaction polymerization of simple alkenes. Polyethylene, polypropylene, and poly-styrene are examples. As a general rule, radical addition reactions are not common in the laboratory but occur much more frequently in biological pathways.

Many reactions give chiral products. If the reactants are optically inactive, the products are also optically inactive. If one or both of the reactants is opti-cally active, the products can also be optically active.

Summary of ReactionsNote: No stereochemistry is implied unless specifically indicated with wedged, solid, and dashed lines.

1. Addition reactions of alkenes(a) Addition of HCl, HBr, and HI (Sections 7.7 and 7.8)

Markovnikov regiochemistry occurs, with H adding to the less highly substituted alkene carbon and halogen adding to the more highly substituted carbon.

Ether

HXCC

H X

CC

(continued)

Key words—cont’dreduction, 277Simmons–Smith reaction, 

288stereospecific, 288syn stereochemistry, 273

Learning ReactionsWhat’sseventimesnine?Sixty-three,ofcourse.Youdidn’thavetostopandfigureitout;youknewtheanswerimmediatelybecauseyoulongagolearnedthemultiplicationtables.Learningthereactionsoforganicchemistryrequiresthesameapproach:reac-tionshavetobelearnedforimmediaterecalliftheyaretobeuseful.

Differentpeopletakedifferentapproachestolearningreactions.Somepeoplemakeflashcards;othersfindstudyingwith friends tobehelpful.Tohelpguideyourstudy,mostchaptersinthisbookendwithasummaryofthereactionsjustpresented.Inad-dition,theaccompanyingStudy Guide and Solutions Manualhasseveralappendixesthatorganizeorganicreactionsfromotherviewpoints.Fundamentally,though,therearenoshortcuts.Learningorganicchemistrydoestakeeffort.

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302  CHAPTER 8 | Alkenes: Reactions and Synthesis

(continued)

(b) Addition of halogens Cl2 and Br2 (Section 8.2)Anti addition is observed through a halonium ion intermediate.

CH2Cl2

X2C C

X

CC

X

(c) Halohydrin formation (Section 8.3)Markovnikov regiochemistry and anti stereochemistry occur.

+ HXH2O

X2C C

OH

CC

X

(d) Addition of water by oxymercuration–demercuration (Section 8.4)Markovnikov regiochemistry occurs.

1. Hg(OAc)2, H2O/THF

2. NaBH4CC CC

HO H

(e) Addition of water by hydroboration–oxidation (Section 8.5)Non-Markovnikov syn addition occurs.

1. BH3, THF

2. H2O2, OH–C C

H OH

CC

(f) Catalytic hydrogenation (Section 8.6)Syn addition occurs.

H2

Pd/C or PtO2C C

H H

CC

(g) Epoxidation with a peroxyacid (Section 8.7)Syn addition occurs.

O

RCOOHCC

O

C C

(h) Hydroxylation with OsO4 (Section 8.7)Syn addition occurs.

1. OsO4

2. NaHSO3, H2Oor OsO4, NMO

C C

HO OH

CC

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Summary of Reactions  303 

(i) Addition of carbenes to yield cyclopropanes (Section 8.9)

(1) Dichlorocarbene addition

KOH+ CHCl3CC

ClCl

CC C

(2) Simmons–Smith reaction

Zn(Cu)

Ether+ CH2I2

CC

H H

CC C

2. Hydroxylation by acid-catalyzed epoxide hydrolysis (Section 8.7)

Anti stereochemistry occurs.

OH

HO

H3O+CC

O

CC

3. Oxidative cleavage of alkenes (Section 8.8)(a) Reaction with ozone followed by zinc in acetic acid

R R

R R

CC

R

R

OC +R

R

CO1. O3

2. Zn/H3O+

(b) Reaction with KMnO4 in acidic solution

R R

R R

CC

R

R

OC +R

R

COKMnO4, H3O+

R H

H H

CC + CO2KMnO4, H3O+

OHR

O

C

4. Cleavage of 1,2-diols (Section 8.8)

OC + COHIO4H2O

HO OH

CC

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