上海交通大学：《有机化学》教程教学资源（英文讲稿）04 Alkenes and Alkynes

4. Alkenes and alkynes 试剂 温度△ 压力 有机反应表示催化剂 溶剂 p22 产率 分步 A-B C=C C-C 底物(原料) 主产物

4. Alkenes and Alkynes 有机反应的表示: p.22 C C A B C C A B y. 温度 压力 催化剂 溶剂 产率 分步 试剂 底物(原料) 主产物

4. 1 Addition of hX to Alkene o 100 Hydrohalogenation General reaction mechanism: electrophilic addition Attack of electrophile(such as HBr)on T bond of alkene Produces carbocation and bromide ion Carbocation is an electrophile, reacting with nucleophilic bromide ion

4.1 Addition of HX to Alkenes: Hydrohalogenation • General reaction mechanism: electrophilic addition • Attack of electrophile (such as HBr) on p bond of alkene • Produces carbocation and bromide ion • Carbocation is an electrophile, reacting with nucleophilic bromide ion p.100

Electrophilic addition energy path Two step process First transition state is high energy point Carbocation intermediate First transition state Second transition state CH △G1 CH3 CCH B CH CH3C= CH2+ HBr CH CH3C-Br Reaction progress

• Two step process • First transition state is high energy point Electrophilic Addition Energy Path

Example of Electrophilic Addition Addition of hydrogen bromide to 2-Methyl- H3C C propene H3C H H-Br transfers proton to C=C H3C Forms carbocation H intermediate More stable cation forms · Bromide adds to H3CPC-C.H carbocation

Example of Electrophilic Addition • Addition of hydrogen bromide to 2-Methyl￾propene • H-Br transfers proton to C=C • Forms carbocation intermediate – More stable cation forms • Bromide adds to carbocation

4.2 Orientation of Electrophilic.101 Addition: markovnikoy's rule In an unsymmetrical alkene, HX reagents add in two different ways, but one way may be preferred over the other If one orientation predominates the reaction is regiospecific(位置选择性) Markovnikov observed in the 1gth century that in the addition of hx to alkene the h attaches to the carbon with the most hs and x attaches to the other end(to the one with the most alkyl substituents) This is markovnikoy's ruler(马氏规则)

4.2 Orientation of Electrophilic Addition: Markovnikov’s Rule • In an unsymmetrical alkene, HX reagents can add in two different ways, but one way may be preferred over the other • If one orientation predominates, the reaction is regiospecific(位置选择性) • Markovnikov observed in the 19th century that in the addition of HX to alkene, the H attaches to the carbon with the most H’s and X attaches to the other end (to the one with the most alkyl substituents) – This is Markovnikov’s rule(马氏规则) p.101

Example of markovnikov's rule Addition of Hcl to 2-methylpropene R egiospecific-one product forms where two are possible H3C H H一C-C0 H3C H3C C 1-Chloro-2-methyI-propane H3C H H3C H H-C C|—C-C 100% H 2-Chloro-2-methyl-propane

• Addition of HCl to 2-methylpropene • Regiospecific – one product forms where two are possible Example of Markovnikov’s Rule

Energy of Carbocations and Markovnikov's rule More stable carbocation forms faster Tertiary cations and associated transition states are more stable than primary cations Primary transition state △ Prim ((H3)2CHCH2 Tertiary transition state co △cert (CH3)3C+ Cr H2C=C(CH3)2 1 HCI (CH3)2CHCH2 (CH3)3CCI Reaction progress

Energy of Carbocations and Markovnikov’s Rule • More stable carbocation forms faster • Tertiary cations and associated transition states are more stable than primary cations

Mechanistic Source of Regiospecificityin Addition reactions If addition involves a carbocation intermediate and there are two H3C C-H possible ways to add the route producing the Much more stable more alkyl substituted cationic center is lower CH H in energy C、仓,H Alkyl groups stabilize H3C carbocation H

Mechanistic Source of Regiospecificityin Addition Reactions If addition involves a carbocation intermediate – and there are two possible ways to add – the route producing the more alkyl substituted cationic center is lower in energy – alkyl groups stabilize carbocation

4.3 Carbocation Structure p102 and stability Carbocations are planar and the tricoordinate carbon is surrounded by only 6 electrons in sp2 orbitals The fourth orbital on carbon is a vacant p- orbital The stability of the carbocation is increased by the presence of alkyl substituents Therefore stability of carbocations 3°>20>1°>+CH

4.3 Carbocation Structure and Stability • Carbocations are planar and the tricoordinate carbon is surrounded by only 6 electrons in sp2 orbitals • The fourth orbital on carbon is a vacant p￾orbital • The stability of the carbocation is increased by the presence of alkyl substituents • Therefore stability of carbocations: 3º > 2º > 1º > +CH3 p.102

Vacant p orbital R R′ R′ 120° 92004 Thomson-Brooks/Cole