A Question about the Nonrestrictive Clause

[Orris]

Hey, professors,

I met some difficulties in nonrestrictive clause. When we use "which" to modify a noun, must the noun mentioned be the one closest to "which"?

Like the sentence: Tom just argued with his best friend, who was just promoted to general several days ago.
In this sentence, who was promoted, Tom or his friend?

And another sentence: The fact that the zinc in this reaction is oxidized by giving up electrons makes it possible for electrons to be gained by the copper, which is acting as a reducing agent.
In this sentence, which is acting as the reducing agent, the zinc or the copper?

I sense that the "which" in the first refers to Tom's friend while in the second refers to the zinc, but I'm not sure and cannot figure out why. Doesn't "which" always modify the noun close to it?

[JohnParis]

Hello Orris - welcome to Using English.

Hey, professors,

I met some difficulties in nonrestrictive clause. When we use "which" to modify a noun, must the noun mentioned be the one closest to "which"?

Like the sentence: Tom just argued with his best friend, who was just promoted to general several days ago.
In this sentence, who was promoted, Tom or his friend? His friend.

And another sentence: The fact that the zinc in this reaction is oxidized by giving up electrons makes it possible for electrons to be gained by the copper, which is acting as a reducing agent.
In this sentence, which is acting as the reducing agent, the zinc or the copper? Copper

I sense that the "which" in the first refers to Tom's friend while in the second refers to the zinc, but I'm not sure and cannot figure out why. Doesn't "which" always modify the noun close to it? Generally, yes.
If zinc is the reducing agent, then you might consider the following sentence:
Zinc, which acts as a reducing agent, is oxidized and gives up its electrons to be gained by the copper. I'm not quite sure what "to be gained by the copper" means, but this is how the sentence might be constructed.

John

[Bennevis]

Tom just argued with his best friend (,) who was just promoted to general several days ago.
In this sentence, who was promoted, Tom or his friend?

Compare ->

Tom, who was just promoted to general several days ago, just argued with his best friend.

Do the same thing to the sentence below and you'll see how easy it is to figure out what is what once you make it easier to read.

The fact that the zinc in this reaction is oxidized by giving up electrons makes it possible for electrons to be gained by the copper (,) which is acting as a reducing agent.
In this sentence, which is acting as the reducing agent, the zinc or the copper?

[Orris]

I guess you are both right. Thanks for your help.