CHAPTER SIX Reactions of alkenes Addition reactions Ethylene, which has no substituents to stabilize its double bond, has the highest heat of hydrogenation hes that are similar in structure to one another have similar heats of hydrogenation. For example, the heats of hydrogenation of the monosubstituted (terminal) alkenes propene, 1-butene, and 1-hexene are almost identical. Cis-disubsti tuted alkenes have lower heats of hydrogenation than monosubstituted alkenes but higher heats of hydrogenation than their more stable trans stereoisomers. Alkenes with trisub- stituted double bonds have lower heats of hydrogenation than disubstituted alkenes, and tetrasubstituted alkenes have the lowest heats of hydrogenation. PROBLEM 6.2 Match each alkene of problem 6. 1 with its correct heat of hydro- genation. Heats of hydrogenation in kJlmol(kcal/mol): 112(26.7):118(28.2): 126(30.2) 6.3 STEREOCHEMISTRY OF ALKENE HYDROGENATION In the mechanism for alkene hydrogenation shown in Figure 6.1, hydrogen atoms are transferred from the catalyst's surface to the alkene. Although the two hydrogens are not transferred simultaneously, it happens that both add to the same face of the double bond, as the following example illustrates H +H2 H CO, CH3 I cyclohexene-1, 2-dicarboxylate The term syn addition describes the stereochemistry of reactions such as catalytic hydro- genation in which two atoms or groups add to the same face of a double bond. When atoms or groups add to opposite faces of the double bond, the process is called anti addition A second stereochemical aspect of alkene hydrogenation concerns its stereoselec- tivity. A reaction in which a single starting material can give two or more stereoisomeric products but yields one of them in greater amounts than the other(or even to the exclu- elimination reactions(Sec sion of the other) is said to be stereoselective. The catalytic hydrogenation of a-pinene (a constituent of turpentine) is an example of a stereoselective reaction. Syn addition of Back Forward Main MenuToc Study Guide ToC Student o MHHE WebsiteEthylene, which has no alkyl substituents to stabilize its double bond, has the highest heat of hydrogenation. Alkenes that are similar in structure to one another have similar heats of hydrogenation. For example, the heats of hydrogenation of the monosubstituted (terminal) alkenes propene, 1-butene, and 1-hexene are almost identical. Cis- disubsti￾tuted alkenes have lower heats of hydrogenation than monosubstituted alkenes but higher heats of hydrogenation than their more stable trans stereoisomers. Alkenes with trisub￾stituted double bonds have lower heats of hydrogenation than disubstituted alkenes, and tetrasubstituted alkenes have the lowest heats of hydrogenation. PROBLEM 6.2 Match each alkene of Problem 6.1 with its correct heat of hydro￾genation. Heats of hydrogenation in kJ/mol (kcal/mol): 112 (26.7); 118 (28.2); 126 (30.2) 6.3 STEREOCHEMISTRY OF ALKENE HYDROGENATION In the mechanism for alkene hydrogenation shown in Figure 6.1, hydrogen atoms are transferred from the catalyst’s surface to the alkene. Although the two hydrogens are not transferred simultaneously, it happens that both add to the same face of the double bond, as the following example illustrates. The term syn addition describes the stereochemistry of reactions such as catalytic hydro￾genation in which two atoms or groups add to the same face of a double bond. When atoms or groups add to opposite faces of the double bond, the process is called anti addition. A second stereochemical aspect of alkene hydrogenation concerns its stereoselec￾tivity. A reaction in which a single starting material can give two or more stereoisomeric products but yields one of them in greater amounts than the other (or even to the exclu￾sion of the other) is said to be stereoselective. The catalytic hydrogenation of -pinene (a constituent of turpentine) is an example of a stereoselective reaction. Syn addition of syn addition anti addition Pt CO2CH3 CO2CH3 Dimethyl cyclohexene-1,2-dicarboxylate CO2CH3 CO2CH3 H H Dimethyl cyclohexane-cis-1,2-dicarboxylate (100%)  H2 212 CHAPTER SIX Reactions of Alkenes: Addition Reactions Stereoselectivity was defined and introduced in connec￾tion with the formation of stereoisomeric alkenes in elimination reactions (Sec￾tion 5.11). Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website