Am I right in thinking a number of legal highs are based on illegal counterparts, by modifying the molecule a small amount.
Is it possible to predict how close the effect to the illegal one, will be.
[Edit] Also I'm wondering how is it possible that chemists like Shulgin, have an ability to create such a large number of molecules, with psychoactive effects.
Nope, most 'legal highs' are manufactured to behave like legal alternatives, but are chemically different. FAA: "the chemical structure of which is substantially similar to the chemical structure of a controlled substance in schedule I or II" is illegal. Designer drugs have to be made chemically substantially different.
The key here is "if intended for human consumption." This is why legal highs are frequently sold as "plant food" or "bath salts." Some vendors are more careful than others, in the height of the first "wave" of designer drugs around ~2004, some vendors were as brazen as to offer "sample packs" and the like. And then Web Tryp [1] (my favorite codename ever) happened.
Many designer drugs are close analogs of schedule I substances.
That's a US-specific law though. The article is mostly about the UK market, and in the UK the law bans specific chemicals, and attempts to address families of chemicals. It invariably falls short and molecule-tweaks can skirt it.
The drugs mentioned in there - Benzo Fury (I hate that name, let's call it 6-APB because that's the molecule on the wrapper) is extremely close to MDA, aMT is a tryptamine which escaped the generic tryptamine regulations and the NBOMes are a twist on the generic 2C-x structure.
You can make educated guesses about the effects, but at the end there is no substitute for genuine data. You'd think that a good enterprising chemist makes field trials and pays close attention to user reports: "Hey, I love the pills with the skull on, but those with the rabbit just give me palpitations."
Is it possible to predict how close the effect to the illegal one, will be.
[Edit] Also I'm wondering how is it possible that chemists like Shulgin, have an ability to create such a large number of molecules, with psychoactive effects.