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1. Presentation Date: 15-04-2024 IJAZ ALI by PhD student (2024-2028) Advance Organic Chemistry (1650117) 1 st Semester C entro de ciências químicas, farmacêuticas e de alimentos (CCQFA ) UFPel University, Brazil

2. Advance Organic Chemistry In this section we consider how we can arrive at similar impressions about molecules by using the underlying principles of MO theory in a Qualitative way. In MO theory, reactivity is related to: R elative energies b) Shapes of the orbitals Reactions that can take place through relatively stable intermediates and transition structures are more favorable than reactions that involve less stable ones . To begin, it is important to remember some fundamental relationships of quantum mechanics that are incorporated into MO theory. Qualitative application of MO theory to reactivity: Perturbation Theory, symmetry, the interaction between molecules and frontier orbitals

3. Advance Organic Chemistry A B C D E F G H A < B < C < D E < F < G < H Aufbau principle: Electrons occupy the MOs of lowest energy. Pauli exclusion principle : MO can have only two electrons, one of each spin. Each MO must be either symmetric or antisymmetric with respect to each element of molecular symmetry.

4. Advance Organic Chemistry Ethene molecular orbital energy levels . Energies are in atomic units. s * s

5. The symmetry of the molecular orbitals : Advance Organic Chemistry It is particularly important feature of many analyses of reactivity based on MO theory . Shapes of Orbitals (Atomic coefficients): Thus in the qualitative application of MO theory, it is important to consider the shape of the orbitals (as indicated quantitatively by their atomic coefficients ). The shapes of orbitals also affect the energy of reaction processes. The strongest interactions (bonding when the interacting orbitals have the same phase) occur when the orbitals have high coefficients on those atoms that undergo bond formation. The qualitative description of reactivity in molecular orbital terms begins with a basic understanding of the MOs of the reacting systems.

6. Advance Organic Chemistry One approach is called perturbation molecular orbital theory or PMO: The basic postulate of PMO theory are: The analysis of interactions of the orbitals in reacting molecules. Interactions are strongest between orbitals that are close in energy . The resulting changes in the MO energies are relatively small and can be treated as adjustments (perturbations) on the original MO system. As molecules approach one another and reaction proceeds there is a mutual perturbation of the orbitals. This process continues until the reaction is complete and the product (or intermediate in a multistep reaction) is formed . In PMO theory, the MO characteristics of the new system are deduced by analyzing how the change in structure affects the MO pattern. The type of changes that can be handled in a qualitative way include substitution of atoms by other elements, with the resulting change in electronegativity, as well as changes in connectivity that alter the pattern of direct orbital overlap.

7. Advance Organic Chemistry The concept of frontier orbital : The most important interactions are between a particular pair of orbitals. These orbitals are the highest filled orbital of one reactant (the HOMO) and the lowest unfilled (LUMO) orbital of the other reactant . Only MOs of matching symmetry can interact and lead to bond formation. We concentrate attention on these two orbitals because they are the closest in energy. Frontier orbital theory proposes that these strong initial interactions guide the course of the reaction as it proceeds to completion. Thus , analysis of a prospective reaction path focuses attention on the relative energy and symmetry of the frontier orbitals. Proposes that:

8. Advance Organic Chemistry Illustration by some simple Cases: These ideas can be illustrated by looking at some simple cases. Let us consider the fact that the double bonds of: Ethene b) Formaldehyde The π bond in ethene is more reactive toward electrophiles than the formaldehyde π bond. In the first place, the higher atomic number of oxygen provides two additional electrons, so that in place of the CH 2 group of ethene, the oxygen of formaldehyde has two pairs of nonbonding electrons. This introduces an additional aspect to the reactivity of formaldehyde. The oxygen atom can form a bond with a proton or a Lewis acid, which increases the effective electronegativity of the oxygen . One significant difference between the two molecules is the lower energy of the π and π ∗ orbitals in formaldehyde.

9. Advance Organic Chemistry Another key change has to do with the frontier orbitals, the π (HOMO) and π ∗ (LUMO) orbitals . These are illustrated in Figure: These are lower in energy than the corresponding ethene orbitals because they are derived in part from the lower-lying (more electronegative) 2pz orbital of oxygen. Because of its lower energy, the π ∗ orbital is a better acceptor of electrons from the HOMO of any attacking nucleophile than is the LUMO of ethene . We also see why ethene is more reactive to electrophiles than formaldehyde. In electrophilic attack, the HOMO acts as an electron donor to the approaching electrophile. 0.2426

10. Advance Organic Chemistry In this case, because the HOMO of ethene lies higher in energy than the HOMO of formaldehyde, the electrons are more easily attracted by the approaching electrophile . The unequal electronegativities of the oxygen and carbon atoms also distort electron distribution in the molecular orbital. In contrast to the symmetrical distribution in ethene, the formaldehyde MO has a higher atomic coefficient at oxygen. This results in a net positive charge on the carbon atom, which is favorable for an approach by a nucleophile . E stimates that the orbital has about 1.2 electrons associated with oxygen and 0.8 electrons associated with carbon, placing a positive charge of +0.2e on carbon. This is balanced by a greater density of the LUMO on the carbon atom.

11. Advance Organic Chemistry Two Principles of PMO theory : One principle of PMO theory is that the degree of perturbation is a function of the degree of overlap of the orbitals. Secondly , the strength of an interaction depends on the relative energy of the orbitals. The closer in energy, the greater the mutual perturbation of the orbitals. Reactivity toward nucleophilic species and an electrophilic species: Let us illustrate these ideas by returning to the comparisons of the reactivity of ethene and formaldehyde toward a nucleophilic species and an electrophilic species. The perturbations that arise as a nucleophile and an electrophile approach are sketched in following Figure. The electrophilic species E + must have a low-lying empty orbital. The strongest interaction will be with the ethene π orbital and this leads to a stabilizing effect on the complex since the electrons are located in an orbital that is stabilized.

12. Advance Organic Chemistry The same electrophilic species would lie further from the π orbital of formaldehyde since the formaldehyde orbitals are shifted to lower energy. As a result the mutual interaction with the formaldehyde HOMO will be weaker than in the case of ethane . The conclusion is that an electrophile will undergo a greater stabilizing attraction on approaching ethene than it will on approaching formaldehyde .

13. Advance Organic Chemistry Nucleophilic Attack In the case of Nu − , a strong bonding interaction with π ∗ of formaldehyde is possible . In the case of ethene, the strongest interaction is with the HOMO of the nucleophile, but this is a destabilizing interaction since both orbitals are filled and the lowering of one orbital is canceled by the raising of the other. Thus we conclude that a nucleophile with a high-lying HOMO will interact more favorably with formaldehyde than with ethene . The representations of nucleophilic attack on formaldehyde as involving the carbonyl LUMO and electrophilic attack on ethene as involving the HOMO also make a prediction about the trajectory of the approach of the reagents . The highest LUMO density is on carbon and it is oriented somewhat away from the oxygen.

14. Advance Organic Chemistry On the other hand, the ethene HOMO is the π orbital, which has maximum density at the midpoint above and below the molecular plane. Calculations of the preferred direction of attack of electrophilic and nucleophilic reagents are in accord with this representation, as shown below

15. Advance Organic Chemistry Substitution Effect: The ideas of PMO theory can also be used to describe substituent effects. Let us consider, for example, the effect of a π-donor substituent and a π-acceptor substituent on the MO levels and reactivity of substituted ethenes. π-Donor Substituent We can take the amino group as an example of a π-donor substituent. The nitrogen atom adds one additional 2pz orbital and two electrons to the π system. The overall shape of the π orbitals for ethenamine is very similar to those of an allyl anion. The highest charge density is on the terminal atoms, i.e , the nitrogen atom and the ß-carbon, because the HOMO has a node at the center carbon . The HOMO in ethenamine resembles of the allyl anion and is higher in energy than the HOMO of ethene. It is not as high as the allyl because ethenamine is neutral rather than anionic and because of the greater electronegativity of the nitrogen atom .

16. Advance Organic Chemistry Thus we expect ethenamine, with its higher-energy HOMO, to be more reactive toward electrophiles than ethene. Furthermore , the HOMO has the highest coefficient on the terminal atoms so we expect an electrophile to become bonded to the ß-carbon or nitrogen, but not to the α-carbon. The LUMO corresponds to the higher-energy of the allyl anion, so we expect ethenamine to be even less reactive toward nucleophiles than is ethene.

17. Advance Organic Chemistry π-Acceptor group: An example of a π-acceptor group is the formyl group as in propenal (acrolein). CH 2 = CHCH = O In this case, the π MOs resemble those of butadiene. Relative to butadiene, however, the propenal orbitals lie somewhat lower in energy because of the more electronegative oxygen atom. This factor also increases the electron density at oxygen at the expense of carbon. p MO energy levels in au for ethylene with a p -acceptor substituent

18. Advance Organic Chemistry The LUMO, which is the frontier orbital in reactions with nucleophiles, has a larger coefficient on the ß-carbon atom. W hereas the two occupied orbitals are distorted in such a way as to have larger coefficients on the oxygen. The overall effect is that the LUMO is relatively low lying and has a high coefficient on the ß-carbon atom. Frontier orbital theory therefore predicts that nucleophiles will react preferentially at the ß-carbon atom . HF/6-31G ∗∗ Calculation: MO orbital calculations at the HF/6-31G ∗∗ level have been done on both propenal and ethenamine. The resulting MOs were used to calculate charge distributions. The given Figure gives the electron densities calculated for butadiene, propenal , and aminoethyene . We see that the C(3) in propenal has a less negative charge than the terminal carbons in butadiene. On the other hand, C(2), the β-carbon in ethenamine, is more negative.

19. Advance Organic Chemistry Fig. Charge distribution in butadiene, acrolein, and aminoethylene based on HF/6-31G ∗ calculations. For example, enamines are protonated on the ß-carbon. Propenal is an electrophilic alkene, as predicted, and the nucleophile attacks the ß-carbon.  

20. Advance Organic Chemistry Resonance Effect: The MO approach gives the same qualitative picture of the substituent effect as described by resonance structures. The amino group is pictured by resonance as an electron donor, indicating a buildup of electron density at the ß-carbon, whereas the formyl group is an electron acceptor and diminishes electron density at the ß-carbon. The chemical reactivity of these two substituted ethenes is in agreement with the MO and resonance descriptions. Amino-substituted alkenes, known as enamines, are very reactive toward electrophilic species and it is the ß-carbon that is the site of attack

21. Advance Organic Chemistry Hard-Hard and Soft-Soft reactions: The concepts of PMO and frontier orbital theory can be related to the characteristics of hard-hard and soft-soft reactions. Recall that hard-hard reactions are governed by electrostatic attractions, whereas soft-soft reactions are characterized by partial bond formation. The hard-hard case in general applies to situations in which there is a large separation between the HOMO and LUMO orbitals. Under these conditions the stabilization of the orbitals is small and the electrostatic terms are dominant. In soft-soft reactions, the HOMO and LUMO are close together, and the perturbational stabilization is large.

22. Advance Organic Chemistry Thank you f or Your Attention