![]() ![]() Which is more electron withdrawing CN or F? ![]() ![]() If it decreases the rate relative to H it is called deactivating. If a substituent increases the rate of reaction relative to H it is called activating. How do you tell if a group is activating or deactivating? The EWG removes electron density from a π system, making it less reactive in this type of reaction, and therefore called deactivating groups. Are electron withdrawing groups more reactive?Īn electron withdrawing group (EWG) will have the opposite effect on the nucleophilicity of the ring. NADH is the electron donor and O2 is the electron acceptor. For example, the mitochondrial electron transport chain can be described as the sum of the NAD+/NADH redox pair and the O2/H2O redox pair. Since electron transport chains are redox processes, they can be described as the sum of two redox pairs. How do you determine electron acceptor and donor? The strongest EWGs are groups with pi bonds to electronegative atoms: Nitro groups (-NO2) How can you tell LUMO? Which is the strongest electron withdrawing group? If it ends up with the electrons going into the benzene ring, then it is an electron donating group. If the resonance structure ends up taking the electrons out of the benzene ring (for instance, a ketone bonded to the benzene ring), then the group is an electron withdrawing group. How do you tell if a group is electron withdrawing or donating? Thus, it has a stronger effect on the HOMO than on the LUMO (at least in organic molecules). An electron-donating group usually acts through an occupied nonbonding orbital. Why do electron withdrawing groups lower the LUMO?Īn electron-withdrawing group removes electrons and, therefore decreases the HOMO and LUMO energies. Protonation generates a positive charge (on the Oxygen atom), which lowers the electron energy for all MOs in the vicinity, including that of the LUMO. Note that for a correct LUMO energy you should have a larger than minimal basisset. This is called Negative Electron Affinity. Moreover, if your LUMO energy is positive, it means that it costs energy to bind an extra electron to your molecule. Among these anti-bonding orbitals, the lowest energy orbital is LUMO. On the other hand, the orbital without any electrons is anti-bonding orbitals. This is the meaning of the Highest Occupied Molecular Orbital (HOMO). The orbit with the highest energy among these bonding orbitals is the HOMO. Yes the molecule with lower HOMO energy is more stable than molecule with higher HOMO energy. This is because the energy of these orbitals is very high and electrons tend to occupy in low energy levels first. As its name implies, these orbitals are unoccupied thus, contains no electrons. These molecular orbitals can receive electrons from HOMO. LUMO stands for lowest unoccupied molecular orbital. This creates a bond between the acid and base. What does a low LUMO mean?Ī low LUMO usually means an empty valence orbital, like on B or on a cation.) We can make a bonding and anti-bonding combination of the base HOMO and acid LUMO, and that will stabilize the electrons from the base’s HOMO, lowering the total energy. When you have a large aromatic system in particular, small HOMO-LUMO gaps lead to mobile π electrons since it is easy for the electron to jump to a higher energy level that is close in energy. ![]() 24 Which is more activating OH or OCH3? Why is LUMO energy high? ![]()
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