Orbital Interaction in the Diels-Alder Reaction

The HOMO of one ethylene molecule and the LUMO of another do not have the proper symmetry to permit two a bonds to be formed in the transition state for concerted cycloaddition. 

Use frontier orbital analysis to decide whether the dimerization of 1,3-butadiene shown here is symmetry-allowed or -forbidden. 

This chapter focused on the effect of a carbon-carbon double bond as a stabilizing substituent on a positively charged carbon in an allylic carbocation, on a carbon bearing an odd electron in an allylic free radical, on a negatively charged carbon in an allylic anion, and on a second double bond in a conjugated diene. 

Section 10.1 Allyl is the common name of the parent group H2C=CHCH2— and is an acceptable name in lUPAC nomenclature. 

Sections The carbocations formed as intermediates when allylic halides undergo SnI 

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The front cover shows the key orbital interactions in the Diels-Alder reaction. The highest-occupied molecular orbital (HOMO) of 1,3-butadiene and the lowest-unoccupied molecular orbital (LUMO) of ethylene overlap in phase with one another allowing the reaction to occur in a single step. 

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