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
MO Rules: Constructive overlap bonding Destructive overlap=antibonding Number of n MO's=Number of p orbitals The MO energies are symmetrically distributed above and below the initial energies of the p orbitals Half the MO's are bonding,the other half antibonding.) So,now for 1.3-butadiene: There are 4 p orbitals to consider. The lowest energy MO will have the greatest number of favorable interactions (i.e.with all the p lobes interacting constructively). This is calledπ. &1&8 (Although 1,3-butadiene is not linear,it is easier and convenient to represent it as such). This MO places electron density on all four carbon atoms. There are 3 bonding interactions and this MO delocalizes the electrons over four nuclei. This explains why there is partial double bond character between C2 and C3. Ch15 Conjugated Systems (landscape) Page 6
Ch15 Conjugated Systems (landscape) Page 6 MO Rules: Constructive overlap = bonding Destructive overlap = antibonding Number of MO’s = Number of p orbitals The MO energies are symmetrically distributed above and below the initial energies of the p orbitals. Half the MO’s are bonding, the other half antibonding.) So, now for 1,3-butadiene: There are 4 p orbitals to consider. The lowest energy MO will have the greatest number of favorable interactions (i.e. with all the p lobes interacting constructively). This is called 1. (Although 1,3-butadiene is not linear, it is easier and convenient to represent it as such). This MO places electron density on all four carbon atoms. There are 3 bonding interactions and this MO delocalizes the electrons over four nuclei. This explains why there is partial double bond character between C2 and C3. =
The next MO has a node in the center of the molecule. &&?8 This is a classic diene MO,with bonding between C1-C2,and C3-C4. There is an antibonding interaction between C2-C3. This MO has two bonding and one antibonding interactions-so we expect it to be overall bonding,but higher in energy than 1. The third MO has two nodes. &?98 With two antibonding and one bonding interaction,overall this is an antibonding MO,and is denoted by an asterix (*) The fourth MO has 3 nodes,and is completely antibonding &9t9 元4 This is the highest energy MO. Ch15 Conjugated Systems (landscape) Page 7
Ch15 Conjugated Systems (landscape) Page 7 The next MO has a node in the center of the molecule. This is a classic diene MO, with bonding between C1-C2, and C3-C4. There is an antibonding interaction between C2-C3. This MO has two bonding and one antibonding interactions – so we expect it to be overall bonding, but higher in energy than 1. The third MO has two nodes. With two antibonding and one bonding interaction, overall this is an antibonding MO, and is denoted by an asterix (*). The fourth MO has 3 nodes, and is completely antibonding. This is the highest energy MO. 2 3 * 4 *
1,3-Butadiene has 4 n electrons to accommodate,and each MO can hold two electrons,with the lower energy MO's being filled first. &?↓9 antibonding &??8 isolated具. p orbital O bonding &-98 十&1-88m Both bonding MO's are filled,and the antibonding MO's are vacant Generally (and unsurprisingly!)stable molecules tend to have filled bonding MO's and vacant antibonding MO's. The planar conformation of 1,3-butadiene is the most stable conformation since it allows the overlap of the 4 p orbitals. There are two planar conformations that 1,3-butadiene can adopt.(s-trans and s-cis conformations). H (H H s-trans S-Cis They are single bond analogues of cis/trans isomers. Ch15 Conjugated Systems (landscape) Page 8
Ch15 Conjugated Systems (landscape) Page 8 1,3-Butadiene has 4 electrons to accommodate, and each MO can hold two electrons, with the lower energy MO’s being filled first. Both bonding MO’s are filled, and the antibonding MO’s are vacant. Generally (and unsurprisingly!) stable molecules tend to have filled bonding MO’s and vacant antibonding MO’s. The planar conformation of 1,3-butadiene is the most stable conformation since it allows the overlap of the 4 p orbitals. There are two planar conformations that 1,3-butadiene can adopt. (s-trans and s-cis conformations). They are single bond analogues of cis/trans isomers. 4 * 3 * 2 1 isolated p orbital antibonding bonding H H H H H H H H H H H H s-trans s-cis
The s-trans(single-trans)conformation is about 2.3kcal lower in energy than the s-cis conformation,which arises from the steric repulsions of the hydrogens. The barrier to rotation for these conformers is 4.9kcal This low energy difference means at room temperature these conformations are easily and rapidly interconverting. Allylic Cations Allylic cations are stabilized by resonance with the adjacent double bond,which delocalizes the positive charge over two carbon atoms. 入Br The delocalized cation can be represented by the two resonance structures or the combined structure. 1.2-and 1.4-Additions Allylic cations are often intermediates when there is electrophilic addition to conjugated dienes. Consider the case of electrophilic H-Br addition to 1.3-butadiene: Br Br H-Br 1,2-addition 1,4-addition 1,2-Addition and 1,4-addition refer to the relationship of the carbon atoms to which the H and Br are added. Ch15 Conjugated Systems (landscape) Page 9
Ch15 Conjugated Systems (landscape) Page 9 The s-trans (single-trans) conformation is about 2.3kcal lower in energy than the s-cis conformation, which arises from the steric repulsions of the hydrogens. The barrier to rotation for these conformers is 4.9kcal. This low energy difference means at room temperature these conformations are easily and rapidly interconverting. Allylic Cations Allylic cations are stabilized by resonance with the adjacent double bond, which delocalizes the positive charge over two carbon atoms. The delocalized cation can be represented by the two resonance structures or the combined structure. 1,2- and 1,4-Additions Allylic cations are often intermediates when there is electrophilic addition to conjugated dienes. Consider the case of electrophilic H-Br addition to 1,3-butadiene: 1,2-Addition and 1,4-addition refer to the relationship of the carbon atoms to which the H and Br are added. Br + -Br - + = 1 /2+ 1 /2+ H-Br Br Br 1,2-addition 1,4-addition
Mechanism for 1.2-and 1.4-Addition of HBr The mechanism is the same as for any electrophilic addition(protonation gives a cation,nucleophile attacks the carbocation)except now the cation is allylic. 1,2-addition 1,4-addition The allylic cation is resonance stabilized and the positive charge is spread over two carbon atoms. The nucleophile(bromide ion)can now attack either of the positively charged carbons,generating the mixture of products. Ch15 Conjugated Systems (landscape) Page 10
Ch15 Conjugated Systems (landscape) Page 10 Mechanism for 1,2- and 1,4-Addition of HBr The mechanism is the same as for any electrophilic addition (protonation gives a cation, nucleophile attacks the carbocation) except now the cation is allylic. The allylic cation is resonance stabilized and the positive charge is spread over two carbon atoms. The nucleophile (bromide ion) can now attack either of the positively charged carbons, generating the mixture of products