Thought for the Day: When the Obligating Event Occurs Before One is Obligated

The obligation to say birkas hamazon ("bentch") is based on the verse, "[when] you have eaten and been sated, you shall bless the Lord your G-d for the land and the food." (D'varim 8:10)  Chazal tell us that means that to be obligated on a d'oraisa level, one must eat enough bread to be sated.  In halacha, that means a k'beitza; approximately two middle slices from a one lb rye according to R' Fuerst.  It also must be eaten in a short enough period of time, k'dei achilas pras (around 8 - 11 minutes) to qualify as an "eating" event.  After eating, one must bentch before the food has been digested; approximately 72 minutes, give or take.

Now for the fun.  What if something happens between the eating and the bentching that changes the level of obligation?  Here are three examples where there is no d'oraisa obligation to bentch at the time of eating, but before the digestion has finished one becomes qualified to be able to bentch.

1. A 12 year old boy eats a good meal, but will become 13 before the allowed time to bentch expires.  The question here is really should he wait to become a gadol and therefore be able to fulfill a Torah obligation, or bentch right away to fulfill "z'rizim makdimim l'mitzvos"?
2. An onain eats a good meal before the burial of his close relative, but the burial will be finished before the time has expired to bentch.  Since he was not allowed to say brachos when he ate, is he now allowed to bentch for that meal?
3. A goy has a nice meal, then goes to bais din and converts before the digestion has completed.  Should he now (as a Jew) bentch on the meal he ate as a goy?

In reverse order: The Chazon Ish paskens that the Jew (ex-goy) does not bentch.  The Mishna Brura in the Biyur Halacha on siman 71 paskens that the mourner would, in fact, bentch.  One could learn this as a machlokes Chazon Ish and Mishna Brura, saying the Chazon Ish holds it is the feeling of being sated that enjoins one to bentch, while the Mishna Brura holds it is the eating  (since at the time of eating the onain was patur, there is not obligation to bentch later).  In fact, the Dirshu Mishna Brura does learn this as a machlokes.

I would like to propose, however, that there may not be a machlokes.  It could be that the cases are based on another factor.  Namely, the goy was not even a member of the club that is obligated to bentch when he ate; that's why he doesn't bentch when he converts and the Mishna Brura would agree.  The onein, on the other hand, was really obligated to bentch, but was prevented from bentching because he was not allowed to make brachos when he ate.  Now that he is allowed to make brachos, automatically he is obligated now to bentch; and the Chazon Ish could agree.

What abotu the bar mitzvah bachur?  Good question.

Perhaps you are wondering, "doesn't he have better things to do with his time than think up bizarre situations like this?"  Nope.