[Shadow]

But another molecule will come to form the acetal. What reaction is it then?

[rwiew]

Yeah, and another molecule attacks a C=O(+)R intermediate right? This has two canonical forms - + on C (carbocation), + on R (oxygen cation). So it is a cation. Pure S_N1 mechanism.

[Irlanur]

The molecularity of a reaction has nothing to do with its rate equation. the first step in Sn1 is unimolecular, the second step is usually bimolecular, if it's intramolecular then its unimolecular. u cannot define the molecularity if you look at different elementary reactions (as in most mechanisms).

[Shadow]

Good, understood.