Commentaire sur La Genèse 42:38

לא ירד בני עמכם MY SON SHALL NOT GO DOWN WITH YOU — He did not accept Reuben’s offer. He said, “What a fool is this oldest son of mine! He suggests that I should kill his sons. Are they his only and not mine also?” (Genesis Rabbah 91:9)

My son. Yaakov refers to Binyamin in this way because he was the only son left from his primary wife [Rochel].

כי אחיו מת והוא לבדו נשאר, "for his brother is dead, and he alone remains, etc. Jacob felt the need to explain how he evaluated the certain loss of Shimon who was presently a hostage in an Egyptian jail- if Benjamin were not to go to Egypt, compared to the possible loss of Benjamin if the latter were to go to Egypt. Jacob explained that if he were to lose Benjamin he would not have a son of Rachel left at all. If he were to lose Shimon, however, he would at least have other sons of Leah left to him. Besides, Jacob described an accident which would happen to Benjamin as practically certain; this is why he spoke about וקרהו אסון in the past tense instead of יקראהו אסון, something in the future. He added the words בדרך אשר תלכו בו, "on the journey you will undertake." He hinted that what happened to Joseph happened to him although he travelled on a route which he was familiar with and had travelled many times without encountering anything dangerous. Jacob was afraid that something similar could happen to the brothers even on a route they considered safe.

כי אחיו מת, not a statement of fact but an assumption. Yaakov reasoned that if Joseph were alive he would have heard from him at some time during the over twenty years that he had been missing.

לא ירד בני עמכם, “my son will not go down (there) with you.” Yaakov did not say a word in response to Reuven’s offer of his sons as a guarantee. He had no confidence in Reuven seeing that he had previously sinned against his father in the matter with Bilhah’s couch. He did, however, have confidence in Yehudah, seeing Yehudah was the strongest and had displayed mastery over the other brothers (Chronicles I 5,2). Yehudah’s argument presented Yaakov with two choices. According to Bereshit Rabbah 91,10 Yehudah told his brothers not to argue with their aged father but to wait until their shortage of food would make a trip to Egypt an absolute necessity. When that point was reached he told his father that whereas it was not certain Binyamin would not be arrested in Egypt, it was certain that if they did not travel to Egypt they would all perish from the famine. It was better therefore to have at least a chance of all of them surviving by fulfilling Joseph’s conditions.

He did not accept Reuvein’s offer... [How does Rashi know this? The answer is:] It was not because misfortune might befall Binyamin that Yaakov prevented him from going with them, although the verse states as much. Because if so, why did Yaakov not say he feared misfortune might occur on the way, when Yehudah offered (43:9), “I will be security for him”? Perforce, the argument of misfortune was merely an excuse, as Yaakov did not want to shame Reuvein by rejecting his offer. The reason he rejected Reuvein’s offer was he feared the master of the land would imprison Binyamin, as he did to Shimon, and no one could stop it. But when Yehudah assumed responsibility when he offered to take Binyamin, Yaakov allowed it because he knew that if Yehudah was involved, he would utterly dedicate himself to rescue Binyamin.2

וקרהו אסון, “and some mishap would befall him;” the letter ו in the word קרהו has the vowel shuruk, whereas the letter ק has the semivowel sh’va.