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So apparently, e-cigarettes produce formaldehyde/acetaldehyde because they vaporize propylene glycol/glycerin. What is the chemical reaction through which propylene glycol/glycerin forms formaldehyde/acetaldehyde? Why does propylene glycol form more carbonylated products than glycerol?

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I think the reaction is essentially an incomplete combustion reaction.

In a vaporizer, the idea is to get the propylene glycol, glycerine, and and "flavor" molecules into the vapor phase (or into aerosol droplets) without chemically degrading them.

However, the way this is accomplished is with very high-temperature resistor coils - the idea is that if you have a small amount of heat in a confined space, and the liquid is distributed throughout an absorbent (like cotton, for example), then the heat transfer will happen very quickly and a large amount of liquid will be vaporized.

The problem is that when you have a very high temperature heat source, and there is lots of oxygen in comparison to the liquid, there is a good chance that at least some of the liquid will reach the combustion temperature instead of just vaporizing.

What happens is that the hydrocarbons in the propylene glycol and glycerin molecules are partially oxidized - oxygen reacts with them to "steal" electrons. If the combustion was complete, you would wind up with just carbon dioxide and water. When combustion is incomplete, you can get any number of compounds, two of which happen to be formaldehyde and acetaldehyde. Carbon monoxide is another one I would be concerned about.

To see how this works, take a look at proplyene glycol (all pictures are from wikipedia):

Propylene glycol

and glycerine:

Glycerol

Notice how they both have three carbon atoms in the "backbone" and either one or none $\ce{O-H}$ groups (called hydroxyls or alcohols) attached to each one.

Oxygen can react with any one of those carbons and will try to form $\ce{CO2}$ and water, the most thermodynamically stable products that are possible:

Carbon dioxide Water

If there is plenty of oxygen, it will. However, if there is not enough, you will get some intermediate species - formaldehyde or acetaldehyde, for example.

Let's look at formaldehyde:

Formaldehyde

In this case, oxygen has reacted with the the other two carbons on the backbone, leaving one with a hydroxyl. The hydrogen on the oxygen goes somewhere (either water or one of the other pieces of the original molecule) and the leftover electrons end up in the oxygen-carbon bond, forming a double bond. This molecule is more thermodynamically stable than proplyene glycol or glycerol, but less stable than carbon dioxide.

Now let's look at acetaldehyde:

Acetaldehyde

Notice the similarity - it's almost the same, there is just a $\ce{CH3}$ (methyl) group in place of one of the hydrogens. You could get this by reacting the first carbon on propylene glycol with oxygen, and then removing the hydrogen from the hydroxyl on the second carbon. This is again more stable than propylene glycol, but less stable than carbon dioxide - it is also an intermediate.

Why does propylene glycol form more carbonylated products than glycerol?

This is a guess - but I think it likely has to do with the reaction kinetics. The reason I think this is true is that all of the intermediate products are less thermodynamically stable than carbon dioxide and water. Therefore, if a propylene glycol (PG) gives more of a given intermediate product than glycerine does, that product is "easier" to make for PG. This would happen if the activation energy for forming it is lower, or if the activation energy to go from the intermediate to carbon dioxide and water is higher compared to the intermediates formed by glycerine.

Note: Many people in other answers are pointing out that there are other mechanisms that could lead to these products, and that combustion is not likely at these temperatures. I think that given the mechanics of heat transfer we can't just rule it out, but I don't have any evidence for that. My answer is meant to show you how the reactant molecules contain the same structures as the products - I wouldn't put much faith in my mechanism (or anybody else's) without some real evidence.

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What is the chemical reaction through which propylene glycol/glycerin forms formaldehyde/acetaldehyde?

  1. Propylene glycol is NOT the same as glycerin. Propylene glycol does not decompose on heating, it simply boils. In theory, under certain conditions (pressure/encapsulation/catalysts) it could dehydrate to either acetone or Propionaldehyde, probably the former.

As for glycerine (= glycerol = VG = animal glycerine), that decomposes below its boiling point by a simple dehydration mechanism, to acrolein and water.

$$\ce{C3H8O3$\xrightarrow{\Delta}$C3H4O +2H2O}$$

Acrolein could further degrade by a retro aldol condensation to acetaldehyde and formaldehyde. This could be catalysed by acids/bases (both of which are present in eliquids) or even by hot metal catalysts (such as the heating coil):

$$\ce{C3H4O\xrightarrow{H^+}H3CCHO +HCHO}$$

or

$$\ce{C3H4O\xrightarrow{hot~metal}H3CCHO +HCHO}$$

These are well known reactions to any organic chemist, and are far more likely to happen than any vague/spurious 'oxidation' reactions as suggested by a few.

  1. Why does propylene glycol form more carbonylated products than glycerol? Because it has a dual mode of dehydration. Really, this is a 'Have you stopped beating your wife?' question: PG and glycerol can form multiple carbonyl compounds, depending on what further side reactions happen:

Formaldehyde is stable, other than the possibility of oxidation to formic acid or carbon dioxide. Acetalydehyde can dimerize to crotonaldehyde. Acrolein can both dimerize, polymerise and undergo addition reactions, in all cases forming a variety of carbonyl compounds

  1. One of the most bizarre things about some studies (including those of Dr Fasolinos), is that while formaldehyde and acetaldehyde are observed, there is insignificant acrolein. And he makes little comment about the latter! Don't you wonder where it's all going? I'd suggest that the bulk of the acrolein is decomposing to those two other carbonyl compounds.

  2. Most aldehydes undergo oxidation over time, but this is not combustion taking place.

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protected by orthocresol May 16 '17 at 0:28

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