Breaking Bad : Season 3 : Episode 10 : “Fly”
Walt becomes obsessed with removing a fly from the Chemistrycave – or is it a sign of something deeper? In this post, I’ll be talking about chemical yield.
The piece of equipment that Jesse is cleaning looks like an immersion heater (not unlike the one in your kettle), used to keep a large reaction vessel at an appropriate temperature.
Walt may be hoping that the increased air pressure in the lab will make it harder for the fly to move – air at higher pressure is of course denser. However, it’ll also make it more difficult to swing a swatter (which looks like it’s made from two scrubbing brushes stuck into some piping).
Jesse tries to add sodium hydroxide (NaOH) to the reaction vessel, presumably to increase the pH and stop the reaction at a certain point (given how concerned he is about waiting too long). He doesn’t seem too concerned about how much to put in (i.e. he hauls a sack over without bothering to weigh anything out), so it’s probably a quenching work-up step to get rid of any unreacted reagents.
Before Walt’s fear of contamination gets the better of him, he’s scribbling down numbers and muttering about the yield being too low (which is how he figures out that Jesse has been skimming). How does he know that the container is half a pound too light? Like any good scientist, Walt is continually refining and improving his process, which means always recording how much product is made from a given set of reaction conditions. Although he talks about the yield in terms of weight (pounds), the term is more often used to describe the relative molar yield, which is more informative as it tells us more about the reaction.
Let’s say for the sake of argument that Walt and Jesse’s process takes 225 lb of phenylacetone and produces 200 lb of methamphetamine. The process is around 89 % efficient, right? Well, actually it’s only around 80 % efficient (which is still pretty good for lab-scale organic chemistry) because when we talk yield we have to talk in terms of moles.
For background information on this topic, see the primer on molar quantities.
|Molar mass / g mol-1||134.18||149.233|
|Weight / lb||225||200|
|Mass / kg||102.27||90.91|
|Amount / mol||762.19||609.18|
What this table highlights is that a molecule of phenylacetone is lighter than a molecule of methamphetamine. If our reaction was 100 % efficient, one mole of phenylacetone (weighing 134.18 g) would give us one mole of methamphetamine (weighing 149.233 g). Here, 762.19 moles of phenylacetone (weighing 225 lb) gives us only 609.18 moles of methamphetamine (weighing 200 lb) and the process is therefore around 80 % efficient.
By considering the yield and efficiency in terms of moles, we always know how many molecules of reagent we have in our reaction – and molecules, as we know, are what’s important. If Jesse slipped a little when adding phenylacetone and accidentally put in 226 lb, we would still be able to figure out the yield and efficiency once we weighed the methamphetamine. For well-understood reactions, we can also figure out a projected yield based on the amount of reactants put in. If Walt knows that 225 lb of phenylacetone went into the vessel and that the process is 80 % efficient then he also knows that 200 lb of methamphetamine should come out. If he gets 199 lb, then something is wrong.
In real organic laboratories, chemical yields are notoriously fickle and vary wildly depending on the person using the equipment, the chemical supplier, the time of day, the ambient temperature, the direction of the wind and so on. Well-refined industrial processes like the one Walt and Jesse are using should be more consistent, which is why Walt struggles so much with the unexpected drop in output. A one-time slight drop in yield should not be a cause for major concern – perhaps a number got recorded wrong or the thermometer was a little off or something – but a regular fall in efficiency has to be investigated in case equipment is malfunctioning or an employee is helping himself to extra product.
Elements in the credits
|StewArt A. Lyons||Argon|