ציטירנדיקער פּירוש אויף דברי הימים ב 2:4

The reason the Torah describes the river Euphrates as “the great river,” although it was mentioned only as the fourth and last of the rivers emanating from Gan Eden (2,14) is because it borders on the land of Israel. We have a popular parable which illustrates the point. “A commoner is liable to describe the valet of a king as ‘king,’ because such a person is continuously in the king’s presence” (compare Rashi on our verse). Seeing that the land of Israel is holy, a choice piece of real estate, the site of the Holy Temple to which is applied the term הבית הגדול, “the great house” (Chronicles II 2,4), the description “great” as applied to the river Euphrates which borders this land is not a misnomer.
Although G-d spoke here about the land being given to “your descendants” at a time when neither Ishmael nor Yitzchak had as yet been born, Ishmael, the son of a slave woman, is not included as a recipient of that promise. Ishmael did not constitute Avram’s principal “descendant” anymore than did the sons he begat after Sarah had died. The principal descendant is the son of the major wife of a husband, not that of concubines or minor wives. G-d had spelled this out after Ishmael had already been born when He said to Avraham (21,12) “for your descendants will be known through Yitzchak.”