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My textbook had this question:

Hydrogenation of napthalene with excess hydrogen gas stops cleanly at 1,2,3,4-tetrahydronapthalene. What conclusion can be drawn from this experiment?

I observed that only one of the two rings was reduced. I did expect both the rings to reduce though, because the hydrogen gas was in excess.

In the solution manual, it is given that this is the "annelation effect", which isn't given in my textbook. I checked the internet, and could only find two research papers, which only detailed some typical aspect of annelation, not annelation itself. Wikipedia instead has no article on the "annelation effect" (but annulation instead, they definitely don't seem to be related though)

So, what is the annelation effect? I am not asking for the mechanism (unless it is as simple as catalytic reduction of alkenes, which I guess it isn't) nor the internal details of its procedure, but a simple overview of what it is and where it is applicable, with an example or two so I can easily identify it. And, how does the "annelation effect" help the fact that the above hydrogenation "stops cleanly" at one ring, even with excess hydrogen?

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  • $\begingroup$ Annulation/annelation is just adding another ring and can stabilise or destabilise a ring in effect, that's it. Also no idea why are you holding to overly laconic wording from your (not particularly helpful) book. $\endgroup$ – Mithoron Mar 17 '18 at 19:14

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