10.1 · Chemistry & society
Natural and synthetic chemicals
A molecule's properties come from its structure, not its source.
By the end you should be able to:
- Evaluate claims about natural and synthetic chemicals using molecular evidence
Key idea
Structure, not source
A molecule's properties are set by its atoms, how they are connected and how they are arranged in 3-D, not by where it came from. Synthetic ascorbic acid () is the same molecule as the vitamin C in an orange, and the body cannot tell them apart. The same holds for vanillin, caffeine or salicylic acid made in a lab.
Real differences between a "natural" and a "synthetic" product come from what else is in it. An orange also contains fibre, sugars and many other compounds; a synthetic batch may contain traces of reagents or solvents. Judge each component by its structure and its amount.
Key idea
The dose makes the poison
Every substance is toxic at a high enough dose and harmless at a low enough one (Paracelsus). Acute toxicity is compared using the LD₅₀: the dose, in mg per kg of body mass, that kills half of a test population. A lower LD₅₀ means more toxic.
| Substance | Where it comes from | Approximate LD₅₀ (mg/kg) |
|---|---|---|
| vitamin C | fruit, or synthesis | 11,900 |
| ethanol | fermentation | 7,060 |
| table salt | sea water, mines | 3,000 |
| caffeine | coffee, tea | 192 |
| nicotine | tobacco | 50 |
| botulinum toxin | a bacterium | about 0.000001 |
Values are oral doses in rats, except botulinum toxin (an estimated human lethal dose). The most toxic substance known is entirely natural.
Key idea
Natural toxins and the appeal to nature
The appeal to nature ("natural means safe, synthetic means harmful") fails in both directions.
- Natural toxins are everywhere: ricin (castor beans), amatoxins (death cap mushrooms), tetrodotoxin (pufferfish), solanine (green potatoes), amygdalin in apricot kernels (which releases HCN), aflatoxins (moulds on grain and nuts).
- Synthetic molecules include most medicines, many designed to improve on a natural lead: aspirin was made from salicylic acid (from willow bark) to reduce stomach irritation.
- "Chemical-free" is meaningless: water, air and food are all chemicals.
Method
Evaluating a natural-versus-synthetic claim
- Identify the specific molecule or molecules the claim is about.
- Compare structures: identical molecules (including their 3-D arrangement) have identical properties, whatever their source.
- Ask about dose and exposure: how much, how often and by what route.
- Consider the other components of each product: impurities, additives and other natural compounds.
- Look for evidence from controlled studies, not for the word "natural" or "synthetic" on the label.
Common mistake
Natural-versus-synthetic traps
- Wrong: natural vitamin C is healthier than synthetic vitamin C. Right: it is the same molecule; any extra benefit of eating an orange comes from the other things in it.
- Wrong: a substance is either toxic or safe. Right: toxicity depends on dose. Water and salt can kill at very high doses, and a potent poison such as botulinum toxin is used safely as a medicine (Botox) in tiny doses.
- Wrong: two samples with the same molecular formula are the same compound. Right: isomers share a formula but differ in structure (ethanol and dimethyl ether are both ), and even mirror-image forms of a molecule can act differently in the body. Compare full structures, not formulas.
Worked example
Worked example: vanillin and caffeine
Claim: "Our vanilla extract is natural, so its vanillin is better than synthetic vanillin." Vanillin () has the same structure from either source, so it has the same taste, smell and toxicity. Real vanilla extract does taste different, because it also contains many other flavour compounds: a difference in the mixture, not in the vanillin.
Dose: the LD₅₀ of caffeine in rats is 192 mg/kg. Scaled to a 60 kg person, that is 192 × 60 = 11,520 mg, or about 11.5 g: roughly 120 cups of coffee at about 95 mg per cup. Normal intake is far below a lethal dose, though animal LD₅₀ values only roughly predict human toxicity.
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