Frontier Molecular Orbital Theory

This is about the applications of FMO.

1. FRONTIER MOLECULAR ORBITAL THEORY applications CHEM- 311 UNIVERSITY OF GUJRAT

2. APPLICATIONS Frontier molecular orbital theory has following applications:

3. CYCLO-ADDITION REACTIONS A cycloaddition is a reaction that simultaneously forms at least two new bonds, and in doing so, converts two or more open-chain molecules into rings . These reactions can be predicted by the Woodward–Hoffmann rules and thus are closely approximated by FMO Theory. Ex: The famous Diels-Alder reaction , where a diene of 4pi electrons reacts with a dienophile of 2 pi electrons is [4+2] cycloaddition. The dimerization of ethylene is a combination of two olefin units and have a [2+2] cycloaddition.

4. FMO Approach for cycloaddition reactions The orbitals of one molecule must overlap with the orbital of second molecule . Modes for orbital overlap Suparafacial: When both σ bonds form from same side of π system . 4,5,6 member ring involve this mode . Antrafacial: When two σ bonds form from opposite side of π system . reactions involving large rings have this mode.

5. [4,2] cycloaddition reactions Frontier orbital analysis of these reactions show that overlap of in-phase orbital to form the two new sigma bonds require suparafacial orbital overlap . There are bonding interaction at the termini. The [ Π 2 a + Π 2 S] addition is thermally or symmetry allowed. Ex:

6. [2,2] Cycloaddition Reactions These reactions does not occur under thermal conditions but take place under photochemical conditions. The reason is

7. B . SIGMA-TROPIC REACTIONS A sigma tropic rearrangement is a reaction in which a sigma bond is broken and new sigma bind is formed , pi electrons rearrange. Word tropic is from Greek word “tropos”, which means change. These results can be predicted with FMO theory by observing the interaction between the HOMO and LUMO of the species. Following points must be noted: No. of pi bond does not change , same should be for both products and reactants. The pi bond that breaks is bonded to an allylic carbon. The sigma bond that cleaves can be in the middle of pi system. A specific numbering system is used to describe a sigma-tropic rearrangement.

8. Types of rearrangements h

9. Modes for sigma-tropic rearrangements : If the migrating groups remain on same face of the pi-system rearrangement is suparafacial , if it move to opposite face it is antarafacial .

10. Woodward-Hoffmann rules for sigma-tropic reactions

11. C. ELECTRO-CYCLIC REACTIONS An electro cyclic reaction is a pericyclic reaction which is characterized by opening or closing a ring within a single olefinic molecule through conversion of sigma to pi bond or vice versa. EX; This reaction occur by heat or sunlight.

12. FMO approach for electro-cyclic reactions: For electro-cyclic ring closure the HOMO of the acyclic polyene while for electro-cyclic ring opening the LUMO of cyclo-alkene is considered. During cyclization, two terminal p-orbitals of the polyene HOMO overlap in phase in order to have bonding interactions in favour of C-C sigma bond. During ring opening, the HOMO of C-C sigma bond of cyclo-alkene interacts with the LUMO of alkene part of the compound so that bonding interaction is maintained in the newly formed pi bond. The direction of rotation of terminal lobes depends on symmetry property of HOMO of polyene. Similarly, ring opening depends on orbital structure of LUMO of alkene part.

13. LIMITATIONS It should be recognized that the frontier orbital theory has its limitations.

14. references Fukui, Kenichi; Yonezawa , Teijiro ; Shingu , Haruo (1952). "A Molecular Orbital Theory of Reactivity in Aromatic Hydrocarbons". The Journal of Chemical Physics.  20  (4): 722.  Bibcode : 1952JChPh..20..722F .  doi : 10.1063/1.1700523 . ^  Jump up to: a   b   c   d   Fleming, Ian  (1978). Frontier Orbitals and Organic Chemical Reactions. London: Wiley. pp. 24–109.  ISBN   0-471-01819-8 . ^   Miller, Bernard (2004). Advanced Organic Chemistry: Reactions and Mechanisms. Upper Saddle River, NJ: Pearsons . pp. 53–54.  ISBN   0-13-065588-0 .