216 CHAPTER SIX Reactions of alkenes Addition reactions H3C HaC 2-Methyl-2-butene Chlorine becomes attached Hydrogen becomes attached CH CH3—C—CH2CH3 2-Chloro-2-methylbutane (major product from Markovnikov addition of hydrogen chloride to 2-methyl-2-butene) when we examine the mechanism of electrophilic addition in more detal wort? Markovnikov's rule, like Zaitsev's, organizes experimental observations in a form 6.6 MECHANISTIC BASIS FOR MARKOVNIKOV'S RULE Let's compare the carbocation intermediates for addition of a hydrogen halide(hx)to an unsymmetrical alkene of the type RCH=CH2(a) according to Markovnikov's rule and(b)opposite to Markovnikov's rule (a) Addition according to Markovnikov's rule: RCH=CHa RCH—CH, → RCHCH Secondary Halide Observed product (b) Addition opposite to Markovnikov 's rule: RCH=CH2→>RCH—CH2+:X RCH,CH X-H Halide The transition state for protonation of the double bond has much of the character of a carbocation, and the activation energy for formation of the more stable carbocation (secondary)is less than that for formation of the less stable(primary)one. Figure 6.5 uses a potential energy diagram to illustrate these two competing modes of addition. Both car- bocations are rapidly captured by X to give an alkyl halide, with the major product derived from the carbocation that is formed faster. The energy difference between a pri mary carbocation and a secondary carbocation is so great and their rates of formation are so different that essentially all the product is derived from the secondary carbocation Back Forward Main MenuToc Study Guide ToC Student o MHHE WebsiteMarkovnikov’s rule, like Zaitsev’s, organizes experimental observations in a form suitable for predicting the major product of a reaction. The reasons why it works appear when we examine the mechanism of electrophilic addition in more detail. 6.6 MECHANISTIC BASIS FOR MARKOVNIKOV’S RULE Let’s compare the carbocation intermediates for addition of a hydrogen halide (HX) to an unsymmetrical alkene of the type RCHœCH2 (a) according to Markovnikov’s rule and (b) opposite to Markovnikov’s rule. (a) Addition according to Markovnikov’s rule: (b) Addition opposite to Markovnikov’s rule: The transition state for protonation of the double bond has much of the character of a carbocation, and the activation energy for formation of the more stable carbocation (secondary) is less than that for formation of the less stable (primary) one. Figure 6.5 uses a potential energy diagram to illustrate these two competing modes of addition. Both car￾bocations are rapidly captured by X to give an alkyl halide, with the major product derived from the carbocation that is formed faster. The energy difference between a pri￾mary carbocation and a secondary carbocation is so great and their rates of formation are so different that essentially all the product is derived from the secondary carbocation. X  Halide ion RCH   CH2 H Primary carbocation RCH2CH2 X Not formed X H RCH CH2 X  Halide ion RCH   CH2 H Secondary carbocation H X RCH CH2 RCHCH3 X Observed product 2-Methyl-2-butene C H3C H3C CH3 H C (major product from Markovnikov addition of hydrogen chloride to 2-methyl-2-butene) 2-Chloro-2-methylbutane CH3 CH3 Cl C CH2CH3 Hydrogen becomes attached to this carbon Chlorine becomes attached to this carbon 216 CHAPTER SIX Reactions of Alkenes: Addition Reactions Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website