The driving force for this rearrangement step is believed to be the relative stability of the resultant oxonium ion.
Although the initial carbocation is already tertiary, the oxygen can stabilize the positive charge much more favorably due to the complete octet configuration at all centers.
In this case although both choices are tertiary, the phenyl groups result in significantly higher stabilization of the positive charge through resonance.
[6] Fittig wrongly assumed a molecular formula of (C3H3O)n for acetone, the result of a long-standing atomic weight debate finally settled at the Karlsruhe Congress in 1860.
He also wrongly believed acetone to be an alcohol which he hoped to prove by forming a metal alkoxide salt.
Finally Butlerov in 1873 came up with the correct structures after he independently synthesised the compound trimethylacetic (pivalic) acid which Friedel had obtained earlier by oxidizing with a dichromate.
[8] Some of the problems during the determination of the structure are because carbon skeletal rearrangements were unknown at that time and therefore the new concept had to be found.