6.4 Electrophilic Addition of Hydrogen Halides to Alkenes 213 This methyl group block the double bond to th IGURE 6.3 The catalyst surface sa methyl group that lies over he double bond of a-pinene e surface of the catalyst. Hydrogenation of a-pinene he bottom face of the dou Hydrogen is transferred from yst surface to the bottom he double bond--this is hindered side hydrogen can in principle lead to either cis-pinane or trans-pinane, depending on which face of the double bond accepts the hydrogen atoms(shown in red in the equation) CH3 CH cis-Pinane ch are common na tionship be tween the pair of methyl groups on the bridge and the third methyl group a-Pinene cis-Pinane rans-Pinane ( not formed) product obtained is cis-pinane. None of the stereoisomeric trans-pinane is forme only In practice, hydrogenation of a-pinene is observed to be 100% stereoselective. The The stereoselectivity of this reaction depends on how the alkene approaches the catalyst surface. As the molecular model in Figure 6.3 shows, one of the methyl group on the bridge carbon lies directly over the double bond and blocks that face from easy access to the catalyst. The bottom face of the double bond is more exposed, and both hydrogens are transferred from the catalyst surface to that face. Reactions such as catalytic hydrogenation that take place at the"less hindered side of a reactant are common in organic chemistry and are examples of steric effects on reactivity. We have previously seen steric effects on structure and stability in the case of cis and trans stereoisomers and in the preference for equatorial substituents on cyclo- hexane rings 6.4 ELECTROPHILIC ADDITION OF HYDROGEN HALIDES TO ALKENES In many addition reactions the attacking reagent, unlike H2, is a polar molecule. Hydro- gen halides are among the simplest examples of polar substances that add to alkenes Alkene Alkyl halid Back Forward Main MenuToc Study Guide ToC Student o MHHE Websitehydrogen can in principle lead to either cis-pinane or trans-pinane, depending on which face of the double bond accepts the hydrogen atoms (shown in red in the equation). In practice, hydrogenation of -pinene is observed to be 100% stereoselective. The only product obtained is cis-pinane. None of the stereoisomeric trans-pinane is formed. The stereoselectivity of this reaction depends on how the alkene approaches the catalyst surface. As the molecular model in Figure 6.3 shows, one of the methyl groups on the bridge carbon lies directly over the double bond and blocks that face from easy access to the catalyst. The bottom face of the double bond is more exposed, and both hydrogens are transferred from the catalyst surface to that face. Reactions such as catalytic hydrogenation that take place at the “less hindered” side of a reactant are common in organic chemistry and are examples of steric effects on reactivity. We have previously seen steric effects on structure and stability in the case of cis and trans stereoisomers and in the preference for equatorial substituents on cyclo￾hexane rings. 6.4 ELECTROPHILIC ADDITION OF HYDROGEN HALIDES TO ALKENES In many addition reactions the attacking reagent, unlike H2, is a polar molecule. Hydro￾gen halides are among the simplest examples of polar substances that add to alkenes. H C C X Alkyl halide H X Hydrogen halide C C Alkene  H2 Ni H H H CH3 CH3 CH3 trans-Pinane (not formed) CH3 H H H H CH3 CH3 cis-Pinane (only product) CH3 H CH3 CH3 -Pinene 6.4 Electrophilic Addition of Hydrogen Halides to Alkenes 213 This methyl group blocks approach of top face of the double bond to the catalyst surface Hydrogen is transferred from the catalyst surface to the bottom face of the double bond—this is the “less hindered side” FIGURE 6.3 The methyl group that lies over the double bond of -pinene shields one face of it, pre￾venting a close approach to the surface of the catalyst. Hydrogenation of -pinene occurs preferentially from the bottom face of the dou￾ble bond. cis-Pinane and trans-pinane are common names that de￾note the relationship be￾tween the pair of methyl groups on the bridge and the third methyl group. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website