10.7 Bonding in Conjugated Dienes CH,=C=CH,+ 2H CH3CH2CH3△H°=-295kJ(-70.5kca) Hydrog P CH3CH-CH2+ H2 CH3CH2CH3△H°=-125kJ(-299kcal) Hydrogen Propane PROBLEM 10.7 Another way in which energies of isomers may be compared is by their heats of combustion. Match the heat of combustion with the appropri ate diene 1, 2-Pentadiene,(E)-1, 3-pentadiene, 1, 4-pentadiene Heats of combustion: 3186 kJ/mol, 3217 kJ/mol 3251 kl/mol 761.6 kcal/mol. 768.9 kcal/mol, 777.1 kcal/mol Thus, the order of alkadiene stability decreases in the order: conjugated diene(most stable)- isolated diene -cumulated diene (least stable). To understand this ranking. we need to look at structure and bonding in alkadienes in more detail 10.7 BONDING IN CONJUGATED DIENES At 146 pm the C-2--C-3 distance in 1, 3-butadiene is relatively short for a carbon-car- bon single bond. This is most reasonably seen as a hybridization effect. In ethane both carbons are sp-hybridized and are separated by a distance of 153 pm. The carbon-car bon single bond in propene unites sp'-and sp-hybridized carbons and is shorter than that of ethane. Both C-2 and C-3 are sp-hybridized in 1, 3-butadiene, and a decrease in bond distance between them reflects the tendency of carbon to attract electrons more strongly as its s character increases. CH3-CH3 CH3-CH=CH, CH2=CH-CH=CH 153pm 151pm 146 The factor most responsible for the increased stability of conjugated double bonds is the greater delocalization of their T electrons compared with the T electrons of iso- lated double bonds. As shown in Figure 10.4a, the T electrons of an isolated diene sys- em occupy, in pairs, two noninteracting T orbitals. Each of these T orbitals encompasses two carbon atoms. An sp-hybridized carbon isolates the two orbitals from each other, preventing the exchange of electrons between them. In a conjugated diene, however, mutual overlap of the two T orbitals, represented in Figure 10. 4b, gives an orbital sys- tem in which each tr electron is delocalized over four carbon atoms Delocalization of electrons lowers their energy and gives a more stable molecule FIGURE 10.4(a) Isolated from each other by one or more sp-hybridized carbons and cannot overlap to give an extended tr orbital. (b)In of two Tr orbitals (a) Isolated double bonds (b)Conjugated double bonds ing four carbon Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
PROBLEM 10.7 Another way in which energies of isomers may be compared is by their heats of combustion. Match the heat of combustion with the appropriate diene. Dienes: 1,2-Pentadiene, (E)-1,3-pentadiene, 1,4-pentadiene Heats of combustion: 3186 kJ/mol, 3217 kJ/mol, 3251 kJ/mol 761.6 kcal/mol, 768.9 kcal/mol, 777.1 kcal/mol Thus, the order of alkadiene stability decreases in the order: conjugated diene (most stable) → isolated diene → cumulated diene (least stable). To understand this ranking, we need to look at structure and bonding in alkadienes in more detail. 10.7 BONDING IN CONJUGATED DIENES At 146 pm the C-2±C-3 distance in 1,3-butadiene is relatively short for a carbon–carbon single bond. This is most reasonably seen as a hybridization effect. In ethane both carbons are sp3 -hybridized and are separated by a distance of 153 pm. The carbon–carbon single bond in propene unites sp3 - and sp2 -hybridized carbons and is shorter than that of ethane. Both C-2 and C-3 are sp2 -hybridized in 1,3-butadiene, and a decrease in bond distance between them reflects the tendency of carbon to attract electrons more strongly as its s character increases. The factor most responsible for the increased stability of conjugated double bonds is the greater delocalization of their electrons compared with the electrons of isolated double bonds. As shown in Figure 10.4a, the electrons of an isolated diene system occupy, in pairs, two noninteracting orbitals. Each of these orbitals encompasses two carbon atoms. An sp3 -hybridized carbon isolates the two orbitals from each other, preventing the exchange of electrons between them. In a conjugated diene, however, mutual overlap of the two orbitals, represented in Figure 10.4b, gives an orbital system in which each electron is delocalized over four carbon atoms. Delocalization of electrons lowers their energy and gives a more stable molecule. CH3 sp3 sp3 CH3 153 pm CH3 CH sp3 sp2 CH2 151 pm CH2 CH sp2 sp2 CH CH2 146 pm CH2 C CH2 Allene CH3CH2CH3 Propane 2H H° 295 kJ (70.5 kcal) 2 Hydrogen CH3CH CH2 Propene CH3CH2CH3 Propane H H° 125 kJ (29.9 kcal) 2 Hydrogen 10.7 Bonding in Conjugated Dienes 375 (a) Isolated double bonds (b) Conjugated double bonds FIGURE 10.4 (a) Isolated double bonds are separated from each other by one or more sp3 -hybridized carbons and cannot overlap to give an extended orbital. (b) In a conjugated diene, overlap of two orbitals gives an extended system encompassing four carbon atoms. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
CHAPTER TEN Conjugation in Alkadienes and Allylic Systems Additional evidence for electron delocalization in 13-butadiene can be obtained by considering its conformations. Overlap of the two T electron systems is optimal when the four carbon atoms are coplanar. Two conformations allow this coplanarity: they are alled the s-cis and s-trans conformations ∠H s-Cis conformation of 1.3-butadiene s-Trans conformation of 1.3-butadiene The letter s in s-cis and s-trans refers to conformations around the c-c single bond in the diene. The s-trans conformation of 1, 3-butadiene is 12 kJ/mol(2.8 kcal/mol)more (2.8 kcal/mol) s-Cis FIGURE 10.5 Conformations and electron aligned parallel to. The s-cis an 1, 3-but one another for maxi mum Tr electron delocalization the s-trans s-cis Stabilize- ion resulting from tm electron delocalization le perpendicular conformation, which is a transition state for rotation about the c-2--C-3 single bond Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
Additional evidence for electron delocalization in 1,3-butadiene can be obtained by considering its conformations. Overlap of the two electron systems is optimal when the four carbon atoms are coplanar. Two conformations allow this coplanarity: they are called the s-cis and s-trans conformations. The letter s in s-cis and s-trans refers to conformations around the C±C single bond in the diene. The s-trans conformation of 1,3-butadiene is 12 kJ/mol (2.8 kcal/mol) more H H H H H H H s-Cis conformation of 1,3-butadiene H H H H H s-Trans conformation of 1,3-butadiene 376 CHAPTER TEN Conjugation in Alkadienes and Allylic Systems Energy Perpendicular 16 kJ/mol (3.9 kcal/mol) 12kJ/mol (2.8 kcal/mol) s-Cis s-Trans FIGURE 10.5 Conformations and electron delocalization in 1,3-butadiene. The s-cis and the s-trans conformations permit the 2p orbitals to be aligned parallel to one another for maximum electron delocalization. The s-trans conformation is more stable than the s-cis. Stabilization resulting from electron delocalization is least in the perpendicular conformation, which is a transition state for rotation about the C-2±C-3 single bond. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website