Quick questions on Designing a chemical synthesis explained: HSC Chemistry Module 8

Is the by-product valuable (then sell it), inert (then dump it), or hazardous (then treat it)? Atom economy is the percentage by mass of reactant atoms that end up in the product. 5.

When choosing a synthesis, a chemist works through six considerations:

For a multi-step synthesis, the supply of the rarest intermediate dominates. A natural product synthesis using a rare plant extract may be prohibitively expensive at scale.

Conditions set the scale of the engineering. The Haber process (450 degrees C, 200 atm) needs steel reactors with thick walls and energy-intensive compressors. Aspirin synthesis (70 degrees C, atmospheric pressure) runs in ordinary glass-lined reactors.

Yield is the actual mass divided by the theoretical mass, expressed as a percentage:

Atom economy is the percentage of the total mass of reactants that ends up in the desired product:

The twelve principles of green chemistry (Anastas and Warner, 1998) provide the framework. The four most relevant at HSC are:

Plants are designed for a specific scale. A penicillin plant making 100 tonnes per year of an active pharmaceutical ingredient looks very different from a polyethylene plant making 500,000 tonnes per year. Capital cost typically scales as the 0.6 power of capacity ("six-tenths rule"), and operating cost is dominated by feedstock for bulk chemicals or by labour and purification for pharmaceuticals.

Ethanol can be made two ways. Both give the same product, but the trade-offs differ.

The standard HSC synthesis: salicylic acid plus ethanoic anhydride, catalysed by a few drops of concentrated $H_2SO_4$:

"Yield is important. Conditions are important. Cost is important."

Yield measures how much of the theoretical product you actually got. Atom economy measures how much of the input mass is built into the product even before yield is considered. Both can be improved independently.

A modern HSC answer is expected to integrate green chemistry, not bolt it on.

Every reaction has at least one. State what it is, where it goes, and whether it is a problem.

Not always. Some reactions need it, and at industrial scale heat is often recovered through heat exchangers. Specifics matter.