Now that the bisphenol A is a salt, it can go to work on the phosgene. (Remember phosgene?) You see that oxygen in the bisphenol A salt now has a negative charge on it. This means it can donate a pair of electrons to the carbon atom in the phosgene. Remember, that carbon is lacking electron density because it's next to that electronegative oxygen. When that carbon gets a new pair of electrons from the bisphenol A salt, it lets go of one of the pairs it had been sharing unequally with the carbonyl oxygen. This pair rests itself on that oxygen giving it a negative charge.