Equilibrium, cycloalkyne/cycloallene

Table 4. The equilibrium composition of cycloalkyne-cycloallene mixtures...

The alternative /3,y-elim [nation-addition route, forming the cyclo-allenic intermediate (161, n = 2, 3) could not be dismissed, since the equilibrium between a cycloallene and the corresponding cycloalkyne... 

With decreasing ring size the isomeric cycloallenes become more stable than the corresponding cycloalkynes, because an allene requires only three colinear carbon atoms (see p. 191-192). This is shown by comparing the equilibrium constants for the cycloalkyne/1,2-cycloalkadiene equilibria at 100 °C in tert.butanol/potassium-tert.-butoxide (Table 7)4). 

Isomerization of stable cycloalkynes to the isomeric cyclic allenes under basic conditions indicates that the equilibrium shifts towards allene with decreasing ring sizes . The observed trend suggests that the short-life cycloallenes should be more stable than the corresponding cycloalkynes. The formation of short-life cycloallenes has been proved by the structures of the dimers and the trapping products - - - . ... 

---