《有机化学》课程PPT教学课件（Organic Chemistry with Biological Applications, 3th Edition, John McMurry, 2016）Chapter 08 Reactions of Alkenes and Alkynes

CNGNGE JOHN MCMURRY CHAPTER 8 Reactions of Alkenes and Alkynes T H I R D E DI TION Organic Chemistry with Biological Applications

CHAPTER 8 Reactions of Alkenes and Alkynes

Alkene Reactions Reactions of Alkenes Addition of a halogen to give 1,2-dihalide Addition of a hypohalous acid to give OH halohydrin Addition of water to give alcohol Alcohol Alkane Addition of hydrogen to give alkane Halohydrin 1,2-Diol Addition of single oxygen to give three- membered cyclic ether:epoxide Addition of two hydroxyl groups to give Carbonyl 1,2-diol 1.2-Dihalide Alkene compound Addition of a hydrogen halide to give halide Oxidative cleavage to form a carbonyl Halide Cyclopropane compound Epoxide Addition of carbene to form cyclopropane

▪ Addition of a halogen to give 1,2-dihalide ▪ Addition of a hypohalous acid to give halohydrin ▪ Addition of water to give alcohol ▪ Addition of hydrogen to give alkane ▪ Addition of single oxygen to give three￾membered cyclic ether: epoxide ▪ Addition of two hydroxyl groups to give 1,2-diol ▪ Addition of a hydrogen halide to give halide ▪ Oxidative cleavage to form a carbonyl compound ▪ Addition of carbene to form cyclopropane Reactions of Alkenes Alkene Reactions

8-1 Preparing Alkenes:A Preview of Elimination Reactions Preparation of alkenes:elimination reactions Precursors to alkenes Biological systems-usually alcohols Laboratory-either alcohols or alkyl halides Alkenes and alcohols are chemically related through addition and elimination reactions Alkenes add H2O to form alcohols Alkenes add HX to form halides Alcohols eliminate water to form alkenes Alcohols eliminate HX to form alkenes Addition X-Y Elimination

Preparation of alkenes: elimination reactions Precursors to alkenes ▪ Biological systems – usually alcohols ▪ Laboratory – either alcohols or alkyl halides Alkenes and alcohols are chemically related through addition and elimination reactions ▪ Alkenes add H2O to form alcohols ▪ Alkenes add HX to form halides ▪ Alcohols eliminate water to form alkenes ▪ Alcohols eliminate HX to form alkenes 8-1 Preparing Alkenes: A Preview of Elimination Reactions

Preparing Alkenes:A Preview of Elimination Reactions Dehydrohalogenation Loss of HX from alkyl halide Usually occurs by reaction of an alkyl halide with a strong base H Br KOH KBr CH3CH2OH H20 H H Bromocyclohexane Cyclohexene (81%)

Dehydrohalogenation ▪ Loss of HX from alkyl halide ▪ Usually occurs by reaction of an alkyl halide with a strong base Preparing Alkenes: A Preview of Elimination Reactions

Preparing Alkenes:A Preview of Elimination Reactions Dehydration Loss of water from an alcohol a Usually occurs by treatment of an alcohol with a strong acid CH3 CH3 OH H2S04,H20 THF 50C H20 1-Methylcyclohexanol 1-Methylcyclohexene (91%) Tetrahydrofuran (THF)-a common solvent

Dehydration ▪ Loss of water from an alcohol ▪ Usually occurs by treatment of an alcohol with a strong acid Preparing Alkenes: A Preview of Elimination Reactions

Preparing Alkenes:A Preview of Elimination Reactions In biological pathways dehydrations normally take place on substrates in which-OH is positioned two carbons away from a carbonyl group H 0 H3C ACP H30 ACP H20 H B-Hydroxybutyryl ACP trans-Crotonyl ACP

In biological pathways dehydrations normally take place on substrates in which –OH is positioned two carbons away from a carbonyl group Preparing Alkenes: A Preview of Elimination Reactions

8-2 Halogenation of Alkenes Halogenation -Addition reaction of alkenes Addition of Br,and Cl,to alkenes to yield 1,2-dihalides CI CI + Cl2 H一C一C一H H H Ethylene 1,2-Dichloroethane (ethylene dichloride)

Halogenation ▪ Addition reaction of alkenes ▪ Addition of Br2 and Cl2 to alkenes to yield 1,2-dihalides 8-2 Halogenation of Alkenes

Halogenation of Alkenes Halogenation of cycloalkenes Only trans-stereoisomer of dihalide product is formed Reaction occurs with anti stereochemistry -the two halogen atoms come from opposite faces of double- bond,one from top face and one form bottom face Br-Br Br Br Br Cyclopentene trans-1,2-Dibromo- cis-1,2-Dibromo- cyclopentane cyclopentane (sole product) (Not formed)

Halogenation of cycloalkenes ▪ Only trans-stereoisomer of dihalide product is formed ▪ Reaction occurs with anti stereochemistry – the two halogen atoms come from opposite faces of double￾bond, one from top face and one form bottom face Halogenation of Alkenes

Halogenation of Alkenes Reaction occurs through an intermediate bromonium ion (R2Br+),formed by interaction of the alkene with Br2 and simultaneous loss of Br Br: :Br: CC ---C- :Br: An alkene A bromonium ion

Reaction occurs through an intermediate bromonium ion (R2Br+), formed by interaction of the alkene with Br2 and simultaneous loss of Br￾Halogenation of Alkenes

Halogenation of Alkenes Bromonium ion shields one side of molecule so that reaction with Br ion occurs only from opposite side Top side open to attack .e :Br :Br: B Bottom side shielded from attack Cyclopentene Bromonium ion trans-1,2-Dibromo- intermediate cyclopentane

Bromonium ion shields one side of molecule so that reaction with Br￾ion occurs only from opposite side Halogenation of Alkenes