Nice little easter egg.
The capsule is in fact clobenzorex and it does technically contain two hydrochloride moieies (sort of) so 'phenyldihydrobenzorex' isn't technically correct but is quite possibly how someone with at least some knowledge of chemistry might try to describe it. I would give an O-level student 3 out of 5 as they note the two benzene rings, the benzylic moiety and what might appear to be two hydrogen-chlorine bonds.
I would credit them with having made an informed guess because effort was made to overcome gaps in their knowledge.
I suppose one might term it a farce-à-clef in that the script, set and acting all conjoin to make it a fantastic scene, BUT the author went that extra mile to nod to those few people who would recognize the capsule and understand that Danny the Dealer is playing with Withnail's expectations... since it seems he was their drug dealer, he of all people would KNOW the pair were about to crash so implying that he was offering a powerful sedative was to lure Withnail.
The joke of course being that it's a STIMULANT so rewind the clip and it opens with I (Bruce Robinson) talking about how chronic stimulant abuse strips away a person's nervous system until the inevibable crash.
FYI I was on nodding terms with a small group of Gabber fans from Leiden University. They all consumed far, far to many illicit amphetamine-type stimulants. But their idea of a joke was if one of them fell asleep the others would use a fine brush to paint a layer of their 'whizz-de-jour' onto the lips of their slumbering companion. Then scream... Think about it, when startled awake, if a person feels something is on their lips, the instinct is to abstract whatever it is using the tongue. I think it a very similar and very cruel trick. I mention it just to show that such things really do happen.
Worse still, if they judged (speed freaks being renowned for judgment) a member was consuming to much of their yaba they would spike them with acid! For fun!!