Is "not" a conjunction in "I want Joe, not him"?

Question

I have been studying the word not lately and noticed it is not a conjunction just by itself and it is actually an adverb. Now, when we use following types of sentences, what is not's grammatical function:

1. If I was the boss, I would buy good quality stuff, not the shit one.
2. I wanna live like a G, not like a coward.
3. I want real money mate, not fake.

Oxford dictionary says, not is used with auxiliary verb. In the above mentioned examples, I don't think not modifies AV, it more likely looks like a comparison operator, like rather. It appears like not is a conjunction like rather. Please explain where am I getting it wrong.

Answer 1

I want [Joe], [not him].

This is essentially a "but not" coordination, where it is possible to have 'X not Y' without a "but"

The meaning is not quite the same as that of 'X but not Y. In "I want Joe, but not him" we understand that they might in principal both be wanted, but in fact only Joe is wanted.

In the above example, however, the issue is which of Joe or him it is that is wanted.

A similar coordination applies to your other examples.

Note: "but not" is not a constituent. The "not" belongs in the second coordinate forming the noun phrase "not him".

Answer 2

Wiktionary's entry for "not" is excellent and possibly fully answers your question:

Conjunction

not

1. And not.

   I wanted a plate of shrimp, not a bucket of chicken.
   He painted the car blue and black, not solid purple.

Usage notes

• The construction “A, not B” is synonymous with the constructions “A, and not B”; “not B, but A”; and “not B, but rather A”.

Note that there is also an adverbial definition for "not":

Adverb

not (not comparable)

1. Negates the meaning of the modified verb.

   Did you take out the trash? No, I did not.
   Not knowing any better, I went ahead.

2. To no degree.

   That is not red; it's green.
   It's not you, it's me.

3. Used to modify superlatives to indicate the opposite or near opposite, often in a form of understatement.

   That day was not the best day of my life.
   It was not my favorite movie of all time.

However, some linguists have noted that "adverbs" is not really a cohesive group because not all adverbs are grammatically interchangeable, and that "adverb" has been used as a catchall group for words that don't fit anywhere else. Some linguists also propose that "not" is grammatically unique and deserves a group of its own since there are no adverbs that can go everywhere that "not" can go.