An aromatic ring can assist in the formation of a carbocationic intermediate called a phenonium ion  by delocalising the positive charge.
When the following tosylate reacts with acetic acid in solvolysis then rather than a simple S2 reaction forming B, a 48:48:4 mixture of A, B (which are enantiomers) and C+D was obtained  . 
The mechanism which forms A and B is shown below.