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 porbital • 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