Indene as a Carbocyclic Paradigm

In the above we considered indane derivatives that lacked endocyclic double bonds, i.e., indene and its derivatives. However, we recall the invocation of partial double bond character between the X, Y, and/or Z groups in many of the above species. Why don t we now consider indene and its derivatives More precisely, let us consider indene (XXVII) itself, with [-X-Y-] = -CH=CH- and Z = CH2 and the related reaction  

This reaction is exothermic by 8.6 kJ mob1. We are not surprised that this reaction is less exothermic than the one for indane itself because we have lost the conjugation energy associated with conjugation of the endocyclic double bond attached to a benzene ring. 

Consider now indole (XXVIII), benzol b furan (XXIX) and benzo[fc]thiophene (XXX) with -X-Y- = -CH = CH- and Z=-NH- -0-, and -S- and the reactions  

The enthalpy of formation of none of these 1-propenyl derivatives are known. However, they all may be reliably estimated from combining the rearrangement enthalpies  

There is no enthalpy of formation data for methyl //-propylamine  

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