If you caught the news recently, a short list of soft drinks either (a) contain or (b) can form benzene. One story I read sounded as-if the water in the soft drink accidentally contained high levels of benzene. However, a TV report sounded as if (IIRC) the “benzoate” ingredient can readily form benzene, is that correct?
The incident is linked to drinks with ascorbic acid. And, most curiously, one drink comes in a powder! I ask because I do drink Crystal Lite’s orange flavor - a sugar free powered drink mix.
Sodium benzoate is a chemical preservative added to food to prevent spoilage caused by microorganisms. It is only effective in an acidic solution.
Ascorbic acid (vitamin C) can react with sodium benzoate to form benzene, which is a known carcinogen (in the laboratory it has been found to cause cancer in rats). The FDA are apparently examining this.
Sodium benzoate and benzoic acid are absorbed readily through the gut into the bloodstream. They are converted in the liver to hippuric acid which is then eliminated from the body in the urine. In rats the oral LD50, the dose at which 50% die, is >1940mg/kg, cats are a little more sensitive with an oral LD50 of 450mg/kg. Therefore it’s considered to have a low toxicity.
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The maximum allowable level for benzene in drinking water is 5ppb (5 parts per billion).
Benzene is a known carcinogen (it causes cancer in laboratory animals) and so the levels of benzene in some of the drinks, whilst not thought to be hazardous, is causing concern.
The major problem is that sodium benzoate and benzoic acid are found naturally in plants. So, whilst they could eliminate the use of these compounds as artificial preservatives they won’t be able to eliminate them entirely from food.
Yes. You mix sodium benzoate and a proton from the acid, wave a stick over it, and voila, you have benzene, in concentrations far above those that will give you cancer.
It’s just a standard decarboxylation. You can check this out by refluxing a carboxylic acid in conc H2SO4. This would be a bit faster than anything that might occur in a soda bottle. I imagine that the barrier is pretty high, but it’s thermodynamically downhill. As for the exact mechanism, I haven’t seen any computational studies on decarboxylation (not that I’ve looked,) but one could imagine something that looks a bit like a 4-electron sigma-bond metathesis.
Unfortunately for us, I was researching decarboxylating an acid. It turns out that for a carboxylate anion, it goes readily via the S[sub]E[/sub]1 mechanism. (Thank you, Jerry March!)
Fair enough. I just couldn’t come up with a way to get over the energy barrier in a bottle of a drink at room temperature using only Vitamin C. All I could think of was what you said, decarboxylation using heat to give CO2 and benzene.
And chaoticbear, you’re kidding, right? Mix them and now you’ve got the benzoic acid, depending on the pKas, but you’ve still got to get it off the ring. And then there’s the question of how much sodium benzoate was in the bottle to begin with.
Well, you know what? This explains my B in advanced ochem. I didn’t even think about that. But would it be a terrible stretch of the imagination to view that as an equillibrium, and when the acid was deprotonated, hit it with the heat to decarboxylize it? I think that this version of the answer is more realistic than the one alluded to with the conc H[sub]2[/sub]SO[sub]4[/sub].
But thank you for giving me enough credit to think that I would have caught my mistake with the COO- + proton -> COOH.
Hey, sorry. I agree you can decarboxylize it with heat. But I thought most decarboxylations took some heating, more than a bottle would get just sitting around. Unless a little bit can go spontaneously and then LeChatlier’s principle kicks in.
that 1. it was that labile and 2. the concentrations really can be that high. I couldn’t decide if you were poking fun at scaremongerers or saying it really is just that simple.
Well, I’m out of this thread before I make any more of an ass of myself.
Oh, when I said the part about the stick and such, it was poking fun at scaremongers, because I hadn’t found any examples of such a reaction yet, and I just assumed that the people running the website were talking a bunch of fake science to get people to trust them.
I think you guys are all wrong on this decomposition mechanism. In order to decarboxylate via an SE1 mechanism all you would need is the anion plus some heat. Keeping in mind that the solution is acidic for most drinks and that the product of the SE1 mechanism is extremely high energy phenyl carbanion I don’t see this mechanism taking place at ordinary temperatures.
Additionally, it should be noted that the reports strongly suggest that ascorbic acid is a necesary component of this degredation. Additionally its been suggested that light may help initiate the reaction. This all reaks of a radical process. Light/heat creates radicals in small concentrations. Ascorbic acid(being an anti-oxidant) has a really stable radical so all these random radicals end up with the ascorbic acid. Ordinarily ascorbic acid would lose another electron to a second radical to become dehydroascorbic acid, but since these radicals are in dilute concentration the singly oxidized ascorbic acid may hang around as a stable radical for a while.
If instead of quencing this radical with another radical, it were to pull a hydrogen atom off of benzoic acid to leave the carboxyl radical, I suspect that its all down-hill to CO2 and the phenyl radical. Phenyl radical picks up another hydrogen atom to become benzene and creates another radical.
However, I am not going to bother to go look at the primary sources that it cites at this time of night.