The E-factor describes the waste generated by a preparative experiment. The E-factor compares the amount of product actually obtained with the amount of waste that must be discarded. E-factors are expressed as a mass ratio:
E-factor = total mass discarded / mass product
A larger E-factor obviously represents a more wasteful process. Although the ideal E-factor, zero, is extremely hard to reach, it has been estimated that E-factors in the oil refining industry are less than one tenth (< 0.1), which is pretty impressive. E-factors for structurally complex chemicals, like pharmaceuticals, tend to be much higher (25-100). This is probably not surprising since more processing steps are required to make a pharmaceutical.
Unfortunately, it is difficult to calculate accurate E-factors because it is very hard to identify all of the waste generated in any given experiment. In addition, we do not ask you to weigh the material that you throw away. However, you can estimate the combined mass of the material that must be discarded as follows:
- Assume that everything you use (consume) during an experiment eventually is discarded (except for the product). Therefore,
total mass discarded = total mass consumed – mass product
- Chem 201/2 assumptions: When estimating total mass consumed, we will ask you to make several simplifying assumptions: [NOTE]These assumptions will tend to reduce your waste estimates and cannot be defended, but they make the E-factor calculation much easier.
- Ignore water wherever it gets used (to cool a reflux condenser, to wash a product mixture, as the solvent for an aqueous reagent, and so on)
- Ignore fuel that is consumed to produce energy
- Ignore coolants (ice, dry ice)
- Ignore items like filter paper, rubber tubing, pH paper, disposable pipets, broken glassware, and the like
- Ignore items that are used to monitor the reaction (TLC plates, etc.) or characterize the product (NMR solvent, etc.)
- Include the solutes in aqueous reagents, but not the weight of the water itself (see above), e.g., include the weight of the NaCl that is dissolved in a saturated brine solution
- Include dry ice if it is used as a chemical reagent
- Include all solvents (but not water; see above)
- Include your starting materials and reagents
- Include silica gel used for preparative columns
Remember, after you total up everything you have consumed, subtract the mass of your product from the total (your product is not waste).
Also, do not worry about having accurate masses. You can estimate volumes of solvents and reagents, and convert these to masses either by using the liquid’s or solution’s density if known, or by assuming a realistic density (1 g/mL).