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The DA isa __: Two molecules combine in a one-step, __ to form a new ring. __ such as the DA are one class of __, which involve the concerted forming and breaing of bonds within a closed ring of interacting orbitals.
- cycloaddition
- concerted reaction
- Cycloadditions
- pericyclic reactions
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A __ has a __, whose Ea may be supplied by heat or UV light. Some __ proceed only under thermal induction, and others proceed only under photochemical induction. Some __ take place under both thermal and photochemical conditions, but the two sets of conditions give different products.
- concerted pericyclic reaction
- single TS
- pericyclic reactions x2
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What does the conservation of orbital symmetry say?
In that case, there will be __ to help stabilizethe TS. Without these bonding interactions int eh TS, the __ is much higher, and the concerted cyclic reaction cannot occur. Conservation of symmetryhas been used to develop "rules" to predict which __ are feasible and what products will result. These rules are often called the __.
the molecular orbitals of the reactants must flow smoothly into the MOs of the products without any drastic changes in symmetry
- bonding interactions
- Ea
- pericyclic reactions
- Woodward-Hoffman rules
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In the DA reaction, the diene acts as the __ and the dienophile acts as the __.
The electrons in the highest-energy occupied orbital, called the __ are the most important because they are __. The orbital that receives these electrons is the lowest-energy orbital available, called __. If the electrons can flow smoothly from __ to __, a __ may take place.
- electron-rich nucleophile
- electron-poor electrophile
- Highest Occupied MO (HOMO)
- Lowest Unoccupied MO (LUMO)
- HOMO
- LUMO
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Having the correct symmetry means the orbital sthat form the new bonds can __: plus with plus and minus with minus. These bonding interactions stabilize the TS and promote the concerted reaction. This favorable result predicts that the reaction is __. The __ is common, and this theory correctly predicts a favorable TS.
- overlap constructively
- symmetry-allowed
- DA reaction
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If a cycloaddition produces an overlap of positive phase orbitals iwth negative phase orbitals, __ are generated, which raise the Ea so the reaction is classified as __.
- antibonding interactions
- symmetry-forbidden
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When __ rather than heat is used to induce __ our predictions must be reversed.
When a photon with the correct energy strikes ethylene, one of the pi electrons is excited to the next higher molecular orbital. This higher orbital, formally hte __, is now occupied: It is the new __, the __ of teh excited molecule.
- UV light
- pericyclic reactions
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The __ ahs teh same symmetry as the LUMO of a ground-state ethylene. An excited molecule can react with a ground state molecule to give __. The __ is therefore __ bit __.
- HOMO*
- cyclobutane
- [2+2] cycloaddition
- photochemically allowed but thermally forbidden
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In most cases, photochemically allowed reactions are __, and thermally allowed reaections are __
- thermalyl forbidden
- photochemically forbidden
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