> ... Methamphetamine has a chiral center, so you can produce the pure (D) isomer, the pure (L), the exact 50/50 racemic mixture, or anything in between. (L) methamphetamine is a pretty good decongestant, but it is otherwise unenjoyable and has no market as a drug of abuse. The classic P2P reductive amination will of course give you the racemate; there’s nothing chiral about the synthesis at all. At the other end of the scale, reduction of pseudoephedrine starts with a pure chiral material, and you get only (D) methamphetamine as a product.

What he's talking about is molecular mirror images. Our bodies can tell the difference between the (L) and the (D) form. The former drains your sinuses and the latter gets you high.

This is a tangential plot point in Breaking Bad. In the episode "Boxcutter," Walter White asks: "If our reduction is not stereospecific, then how can our product be enantiomerically pure?"

There's a lot to unpack there, but it implies that Walt is isolating (D) methamphetamine exclusively. It also implies that this is occurring through a selective reductive procedure.

The first few seasons established that the synthesis relies on methylamine through a reductive procedure called P2P. But that procedure produces (D) and (L) forms in equal quantities.

What Walt is revealing in that one sentence is that he's developed a way to do the reductive amination step in the P2P cook such that just the (D) form is isolated. His use of the term "stereospecific," implies that the selectivity is close to 100% in the bond-forming step itself. The process therefore doesn't just produce highly pure product, but does so very efficiently.

At the time period of the show, such a reaction would have been very close to cutting edge research.