Cross-conjugation

Question

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.

Answer 1

Cross conjugation is a phenomena due to which (in simple words) resonance from two groups in opposite direction cancel out. It is a destabilising effect.

It is something like this:

These are some examples which will help you understand why this is a destabilising effect:

In the above two examples the stability of carbanion decreases drastically as the two "resonances" cannot happen simultaneously (if it did, we would have 5 bonds with carbon and we don't want that).

Therefore, only one of the conjugated system out of the two present resonate. That implies the effective resonance of one conjugate system is decreased to half (where the molecule is symmetrical).

I hope this helps.