Conjugated Systems,Orbital Symmetry and UV Spectroscopy Introduction There are several possible arrangements for a molecule which contains two double bonds(diene): Isolated:(two or more single bonds between them) 入入、入 Conjugated:(one single bond between them) Cumulated:(zero single bonds between them:allenes) C=C=C Conjugated double bonds are found to be the most stable. Ch15 Conjugated Systems(landscape) Page I
Ch15 Conjugated Systems (landscape) Page 1 Conjugated Systems, Orbital Symmetry and UV Spectroscopy Introduction There are several possible arrangements for a molecule which contains two double bonds (diene): Isolated: (two or more single bonds between them) Conjugated: (one single bond between them) Cumulated: (zero single bonds between them: allenes) Conjugated double bonds are found to be the most stable. C C C
Stabilities Recall that heat of hydrogenation data showed us that di-substituted double bonds are more stable than mono- substituted double bonds. H2,Pt 入入 △H°=-30.0kcal H2,Pt △H°=-27.4kcal When a molecule has two isolated double bonds,the heat of hydrogenation is essentially equal to the sum of the values for the individual double bonds H2,Pt 个◇ 入入 AH°=-60.2kcal For conjugated dienes,the heat of hydrogenation is less than the sum of the individual double bonds. H2,Pt ·入入 △H°=-53.7kcal The conjugated diene is more stable by about 3.7kcal/mol. (Predicted-30+(-27.4)=-57.4kcal,observed-53.7kcal) Ch15 Conjugated Systems (landscape) Page 2
Ch15 Conjugated Systems (landscape) Page 2 Stabilities Recall that heat of hydrogenation data showed us that di-substituted double bonds are more stable than monosubstituted double bonds. When a molecule has two isolated double bonds, the heat of hydrogenation is essentially equal to the sum of the values for the individual double bonds. For conjugated dienes, the heat of hydrogenation is less than the sum of the individual double bonds. The conjugated diene is more stable by about 3.7kcal/mol. (Predicted -30 + (-27.4) = –57.4kcal , observed –53.7kcal) H2 , Pt H o = -30.0kcal H2 , Pt H o = -27.4kcal H2 , Pt H o = -60.2kcal H2 , Pt H o = -53.7kcal
Allenes,which have cumulated double bonds are less stable than isolated double bonds H H H2,Pt C=C=C △H°=-69.8kcal H CH2CH3 Increasing Stability Order(least to most stable) Cumulated diene -69.8kcal Terminal alkyne -69.5kcal Internal alkyne -65.8kcal Isolated diene -57.4kcal Conjugated Diene -53.7kcal cumulated terminal alkyne penta-1,2-diene pent-1-yne pent-2-yne isolated diene (69.5 kcal) 275kJ penta-1.4-diene solated iene (65,8kcal) frans-hexa-14-diene conjugated 2521 diene (60.2 kcal) 2421 (57.7kcl trans-penta-1,3-diene 25 (53.7keal alkane (pentane or hexane) Ch15 Conjugated Systems (landscape) Page 3
Ch15 Conjugated Systems (landscape) Page 3 Allenes, which have cumulated double bonds are less stable than isolated double bonds. Increasing Stability Order (least to most stable) Cumulated diene -69.8kcal Terminal alkyne -69.5kcal Internal alkyne -65.8kcal Isolated diene -57.4kcal Conjugated Diene -53.7kcal C C C H CH2CH3 H H H2 , Pt H o = -69.8kcal
Molecular Orbital (M.O)Picture The extra stability of conjugated double bonds versus the analogous isolated double bond compound is termed the resonance energy Consider 1,3-butadiene: H2,Pt △H°=-30.1kcal H2,Pt △H°=-56.6kcal (2 x-30.1=-60.2kcal).Resonance energy of 1,3-butadiene is 3.6kcal. small amount of overlap partial double bond H 1.34A H 1.48A 1.34A H The C2-C3 bond is much shorter than a normal alkane single bond(1.48A vs.1.54A).This is mainly due to the n bonding overlap(resulting in some double bond character). The planar arrangement,and the alignment of the p orbitals allows overlap between the two double bonds.The electrons are delocalized over the full length of the molecule. This delocalization of electrons creates partial double bond character between C2 and C3. Lewis structures cannot accurately depict delocalized structures,and we turn to molecular orbital theory Chl5 Conjugated Systems (landscape) Page 4
Ch15 Conjugated Systems (landscape) Page 4 Molecular Orbital (M.O) Picture The extra stability of conjugated double bonds versus the analogous isolated double bond compound is termed the resonance energy. Consider 1,3-butadiene: (2 x –30.1 = -60.2kcal). Resonance energy of 1,3-butadiene is 3.6kcal. The C2-C3 bond is much shorter than a normal alkane single bond (1.48Å vs. 1.54Å). This is mainly due to the bonding overlap (resulting in some double bond character). The planar arrangement, and the alignment of the p orbitals allows overlap between the two double bonds. The electrons are delocalized over the full length of the molecule. This delocalization of electrons creates partial double bond character between C2 and C3. Lewis structures cannot accurately depict delocalized structures, and we turn to molecular orbital theory. H2 , Pt H o = -30.1kcal H2 , Pt H o = -56.6kcal
M.O.'s of 1,3-butadiene All four carbons are sphybridized,and in the planar conformation,all the p orbitals overlap. But first,let us recap simple MO theory using ethene: Each p orbital has two lobes,with differing wavefunction sign (+/-black/white,shaded/unshaded-not electrical charges). A nt bonding orbital is formed by overlap of p lobes with the same wavefunction sign.(Constructive overlap). A nt anti-bonding orbital is formed by overlap of p lobes with opposite wavefunction sign.(Destructive overlap). antibonding destructive overlap energy of the isolated p orbitals on CI and C2 energy (bonding)= constructive overlap Electrons have a lower potential energy in the bonding MO than in the original p orbitals,and a higher potential energy in the anti-bonding orbitals. In the ground state of ethene,two electrons fill the bonding MO,and the antibonding MO is empty Ch15 Conjugated Systems (landscape) Page 5
Ch15 Conjugated Systems (landscape) Page 5 M.O.’s of 1,3-butadiene All four carbons are sp 2 hybridized, and in the planar conformation, all the p orbitals overlap. But first, let us recap simple MO theory using ethene: Each p orbital has two lobes, with differing wavefunction sign (+/-, black/white, shaded/unshaded – not electrical charges). A bonding orbital is formed by overlap of p lobes with the same wavefunction sign. (Constructive overlap). A anti-bonding orbital is formed by overlap of p lobes with opposite wavefunction sign. (Destructive overlap). Electrons have a lower potential energy in the bonding MO than in the original p orbitals, and a higher potential energy in the anti-bonding orbitals. In the ground state of ethene, two electrons fill the bonding MO, and the antibonding MO is empty