'To' vs 'in order to' in negative clauses

Question

The answers to this related question suggest that to and in order to are pretty much interchangeable, the former being preferred in informal contexts. My question is about negative clauses.

According to the answers to the linked questions, the following two sentences are fine and mean the same thing:

We were speaking loudly to wake up Mr.Smith.

We were speaking loudly in order to wake up Mr.Smith.

Can the same be said about the following sentences?

We were speaking quietly not to wake up Mr. Smith.

We were speaking quietly in order not to wake up Mr. Smith.

I am asking because when I used the first variant a friend of mine corrected me and said that I must use either in order or so as to make the sentence grammatical.

Answer 1

It's true that so as not to and in order not to are much preferred among the majority of English speakers. I couldn't (without some deliberation) go so far as to say that not to is ungrammatical, but it sounds wrong to me, i.e. not fine.

As to why it's not consistent from a positive context to a negative: that's a good question, and even a broad site like Wikipedia doesn't address it:

Full [Infinitive]

It can be used like an adjective or adverb, expressing purpose or intent. So, "The letter says I'm to wait outside", or "He is the man to talk to", or "[In order] to meditate, one must free one's mind."

I googled infinitive purpose negative and came up with pages like this and this, which say essentially that we use so as not to or in order not to, rather than just not to, without explaining why:

Answer 2

I think this is a case of taking analysis of minutiae a little too far. In the words of the Bard...

• I come to bury Caesar, not to praise him (Julius Caesar Act 3, Scene 2)

Presumably no-one thinks Shakespeare should have written "...not in order to praise him".

It's simply that we usually do things for a reason. Even if we're doing something to avoid a consequence, we still tend to phrase it without negation (as in this sentence). Thus, in the (slightly) unusual construction where the reason for the action is expressed in the negative, the addition of "so as to" or "in order to" makes it easier to understand without jumping through mental hoops.