Deuteronomy 21:16

it shall be in the day that he deviseth to his sons the inheritance of the wealth that may be his, he shall not be allowed to give the birthright portion to the son of the beloved, over the head of the son of the hated wife, to whom the birthright belongs;

he bequeathes his estate to his children whether while still of sound body, whether on the day of his death as he lies on his deathbed, or whether posthumously, by the court.

make the firstborn by declaration, or by elevating him in deed.

The word he cannot is here to be interpreted as it is in similar instances [see comment on 17: 15].

LO YUCHAL’ [literally: “he could not” but rendered: HE MAY NOT] DECLARE THE SON OF THE BELOVED FIRSTBORN. This is an admonition against doing so. Likewise are all such verses as: ‘Lo thuchal’ (thou mayest not) eat within thy gates etc.; ‘lo thuchal’ (thou mayest not) put a foreigner over thee., 17:15. And in all these cases, Onkelos translates “you have no right” [which proves that the verse before us, too, constitutes an admonition], and the purport of these expressions is, “you could not allow yourself the possibility of doing so,” emphasizing the importance of the admonitions. Similarly, But he shall acknowledge the firstborn, the son of the hated, by giving him a double portion constitutes a positive commandment that he bequeath a double portion to the firstborn. Thus he who equalizes the firstborn with his brothers [i.e., he assigns him a share equal to theirs] violates both a negative commandment and a positive commandment, even though his words are not legally valid. He surely violates both [commandments] if, knowing he [the son of the hated one] is the firstborn, and did not wish to make it known that he is the firstborn and claimed that he is an ordinary son and caused him to inherit like any one of his sons. Now, these are newly-declared commandments. And from that which Scripture states [he may not declare the son of the beloved firstborn] in the face of the firstborn, the son of the hated it would appear to me that this commandment and this law are binding only during the lifetime of the firstborn. But, if the firstborn died during the life of his father, even though he [the deceased son] inherits his share as the firstborn in the grave and bequeathes it by law to his children, yet if the father wished [to deviate] and said, “My sons shall inherit such-and-such of my belongings, and the children of my [late] son, the firstborn, shall take such-and-such of my belongings,” his instructions are valid just as they would be valid in a case where there is no firstborn son. Similarly, the father would not be violating this negative commandment if he did not acknowledge the firstborn son only after his death, for I have never found the expression ‘al pnei’ (in the face of) except with reference to the living, such as: ‘al pnei’ (in the presence of) Aaron their father; ‘al pnei’ (in the presence of) his father Terah, and similarly all such expressions. [Therefore, in this case, since it states that the father may not do it ‘al pnei’ the firstborn of the hated, it follows that the prohibition is binding only during the lifetime of the firstborn and that the father had acknowledged him as such.].

לא יוכל לבכר את בן האהובה על פני בן השנואה הבכור. The firstborn son’s entitlement to an extra share in his father’s inheritance must not be transferred on account of his father loving his mother more or “hating” her, i.e. loving her less than his second wife. If the reason the father wants to transfer the extra portion of the inheritance away from the chronologically entitled son due to that son’s misconduct¸ this is in order, as we know from Baba Batra 133 אם לא היה נוהג כשורה זכור לטוב, íf the chronologically oldest did not conduct himself properly his inheritance may be transferred to another better one. [opinion of Rabbi Shimon ben Gamliel only, not accepted by codifiers. Ed.] It appears that this is what Yaakov did when he took birthright privileges away from Reuven (Chronicles I 5,1) transferring it to Joseph.

לא יוכל לבכר את בן האהובה, “he must not favour the son of his beloved wife, (over the son of the wife he has grown to hate,” (by treating him as his first born) Yaakov having treated Joseph as his firstborn is somewhat different as he had never meant to marry Leah, and when he did so in order not to shame her, she subsequently felt hated (having been guilty of deceiving him.). Yaakov “hated her” as she had deprived him of Joseph becoming the true first born from the wife he had worked for, for seven years. Yaakov should have brought Reuven to trial, if he had been guilty of a sin against him instead of depriving him of his rights as a firstborn.