Could somebody please explain the concept of “cross-conjugation” (Organic Chemistry) to me? In what kind of conjugated systems is it observed, what are its effects on resonance of the system as a whole and so on? Also, I read this line in my book, possibly with respect to the same topic-
If any group has more than one π bond in conjugation, then only one π bond will take part in delocalisation.
Example:- In But-1-en-3-yne, only one π bond will take part in resonance.
Does this mean, that in every compound that shows cross-conjugation, only one, not all π bonds are actually involved in delocalisation? Or it it just that cross-conjugation nullifies the much-expected stabilising effect of resonance on the concerned organic compound? I am quite confused and my textbook is not really helping.