Synnecrosis is a Blood-type move.
It has a Destiny Bond-like effect, in that it ties the fates of two opponents in a pokémon battle together. Unlike Destiny Bond, Synnecrosis does not fail if it's used in a turn before fainting, and is thus a popular move to use on Blood-typed pokémon if they're forced into battle with Poison-typed pokémon. Unlike Destiny Bond, it can fail for other reasons, however - specifically, it has a finite, suboptimal accuracy, and it requires brief body contact.
An immobilised target is not more likely to be afflicted by Synnecrosis - other than by proxy of body contact being practically guaranteed, of course, but that doesn't alter the base accuracy.