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.