All A are not B?

(1) All apples are not oranges.

or

(2) All apples are not an orange.

Both make sense to me:

In (1), there are many apples so they all correspond to different oranges.

In (2), though there are many apples, each of them regarded individually, are not equal to a single orange.

So which one is correct?

• The first is idiomatic and the second is not. By the way, language is not logic, and trying to analyse it as if it were does not always yield meaningful results. Oct 25, 2015 at 21:35
• What @Colin said. Although OP's #2 strikes me as totally unacceptable, I don't see how it could be said to violate any "rule" that wouldn't also debar Not all homes are a paradise (which I have no problem with at all). But in both cases, Not every X is a Y would be far more likely. Oct 25, 2015 at 22:40
• Well, paradise isn't often seen in the plural, so that could be why it sounds more acceptable to you Oct 26, 2015 at 14:15

English is full of weird syntax. Your sentences with apples and oranges are so contrived as to be mind numbing. Consider instead:

All men are not rich.

absolutely ok.

All men are not an accountant.

This construction is just wrong. "All" implies more than one, more specifically a large set. However "an" implies one. So the sentence should be

All men are not accountants.

I just can't think of any sentence where the phrase "All men are not an ..." makes sense.

• Being mathematically inclined the "all men are not..." constructions tend to irritate me since there's a logical difference between "all are not" and "not all are". Usage doesn't always follow what would be most sensible. Dec 10, 2015 at 3:28

The word each is what you need to express "though there are many apples, each of them regarded individually, are not equal to a single orange.":

Each apple is not an orange.

X {form of to be} Y means you can substitute Y freely for X (because X is Y), but when you change the plurality, you can't do that, so it fails to work most of the time unless you are specifically comparing the two concepts "all apples" and "an orange" which would be rare outside of a teaching or philosophical context.