Which option is more environmentally friendly and why: a reaction that requires the use of an organic solvent or one that uses water as a solvent?
This is an easy one to answer. A reaction that utilizes water as the solvent has many advantages over one that uses an organic solvent. One is cost. Water is much cheaper than any organic solvent. Another is safety. Organic solvents are flammable and many have other health hazard concerns, especially in bulk quantities. Finally, another advantage is environmental. Water at the end of a reaction can be neutralized, separated from any organics present, and introduced into the wastewater stream like normal sewage or wastewater from a house or business. Organic solvent must be collected and treated as hazardous waste. It will have to be shipped off to a waste handling facility where it will ultimately be incinerated and converted to waste gasses including CO2. So water is always the solvent of choice if at all possible, expecially at bulk scale.