Aspartame degrades over time and in high temperatures to form methanol.  
Methanol would then be metabolized in the body to formaldehyde.  I don't know 
what the kinetics are for methanol formation in Diet Coke in a hot car but 
theoretically it would be possible in a couple hours.  I do know that diet 
sodas are not stored unrefrigerated during distribution and that the makers of 
sugar-free hot cocoa mixes recommend that consumers do not premix the cocoa 
with the hot water and then store in a thermos for consumption later in the 
day.  Also, I suspect that PepSi's recent freshness dating thing for Diet 
PepSi is due to the inevitable breakdown of aspartame over time.  Even 
non-toxic levels of aspartame degradation would probably cause the product to 
taste bad.


Someone who really knows the chemistry should really go through this,
but here is my reasoning:

  1)  The overall amount of aspartame in (say) a liter of Diet Coke
probably corresponds to a few tablespoons, similar to the sugar
content of the non-diet version.

  2)  When it breaks up, my bet is that each molecule of aspartame
breaks up into its two amino acids and one molecule of methanol.
The methanol is a rather smaller molecule than the amino acids, so
it'll be a comparatively small actual amount.

  3)  Sonovagun!  My dictionary has all the chemical formulas in it!
I don't remember the "other" breakdown product (other than
phenylalanine) so I'm listing the two likely suspects:

	Compound	formula		      C   H   N   O
	aspartame	C14H18N2O5	   => 14  18  2   5
	pheynylalanine  C6H5CH2CH(NH2)COOH => 9   13  1   2
	aspartic acid	HOOCCH(NH2)CH2COOH => 4   7   1   4
	asparagine	COCH2CH(NH2)COOH   => 4   6   1   3
	methyl alcohol	CH3OH		   => 1   4   0   1
	water		H2O			  2       1

Hmm... I can't seem to make both the hydrogens and the oxygens come 
out right there... Ignoring that and assuming one mole methanol per
mole of aspartame, I guesstimate that aspartame (M.W. 294) is about
10% methanol (M.W. 32)  (M.W. == Molecular weight.).  So in that
liter of Diet Coke, they guy could get maybe a few grams of methanol.
Now methanol, a.k.a. wood alcohol, is not good for you but I doubt
such a small amount would be a serious danger to an adult, 
especially with a liter of water and sundry other chemicals.  After
all, people do often survive drinking Sterno.  (They also sometimes
die from it, so don't do it!)


>Someone who really knows the chemistry should really go through this,
>but here is my reasoning:

>  1)  The overall amount of aspartame in (say) a liter of Diet Coke
>probably corresponds to a few tablespoons, similar to the sugar
>content of the non-diet version.

>  2)  When it breaks up, my bet is that each molecule of aspartame
>breaks up into its two amino acids and one molecule of methanol.
>The methanol is a rather smaller molecule than the amino acids, so
>it'll be a comparatively small actual amount.

>  3)  Sonovagun!  My dictionary has all the chemical formulas in it!
>I don't remember the "other" breakdown product (other than
>phenylalanine) so I'm listing the two likely suspects:


>Ignoring that and assuming one mole methanol per
>mole of aspartame, I guesstimate that aspartame (M.W. 294) is about
>10% methanol (M.W. 32)  (M.W. == Molecular weight.).  So in that
>liter of Diet Coke, they guy could get maybe a few grams of methanol.
>Now methanol, a.k.a. wood alcohol, is not good for you but I doubt
>such a small amount would be a serious danger to an adult, 
>especially with a liter of water and sundry other chemicals.  After
>all, people do often survive drinking Sterno.  (They also sometimes
>die from it, so don't do it!)


The Merck, 11th Ed., says that death has been reported after ingestion of less 
than 30 mL although the usual fatal dose is 100-250 mL, administered acutely.  
Other acute problems are headache, vision problems or complete blindness, 
convulsions, respiratory failure, etc.  Chronic administration (lower dose, 
longer time of exposure) can result in vision problems since the retina is a 
target tissue for toxicity.  Since the density of methanol is 0.79 g/mL, a few 
grams would be about a few mL.  Thus,  it would be possible for certain, 
probably sensitive,  individuals to suffer from methanol toxicity from 
ingestion of degraded diet sodas in large amounts or through chronic ingestion 
of such products, although death would seem an unlikely endpoint.


>>Someone who really knows the chemistry should really go through this,
>>but here is my reasoning:

>>  1)  The overall amount of aspartame in (say) a liter of Diet Coke
>>probably corresponds to a few tablespoons, similar to the sugar
>>content of the non-diet version.

>>10% methanol (M.W. 32)  (M.W. == Molecular weight.).  So in that
>>liter of Diet Coke, they guy could get maybe a few grams of methanol.
>>Now methanol, a.k.a. wood alcohol, is not good for you but I doubt

>The Merck, 11th Ed., says that death has been reported after
ingestion of less  [than 30 mL.]

>grams would be about a few mL.  Thus,  it would be possible for certain, 
>probably sensitive,  individuals to suffer from methanol toxicity from 
>ingestion of degraded diet sodas in large amounts or through chronic ingestion 
>of such products, although death would seem an unlikely endpoint.


As I understand it, the only monkey wrench that will prevent this
sudden onslaught of blind diet-cola drinkers is the fact that
Aspartame, like most other artificial sweeteners, is _thousands_ of
times sweeter than sugar. You're probably getting a dosage measured in
milligrams in a typical 355 mL serving of delicious, refreshing pop.

As for Pepsi's freshness, I side with Coke, here. With the possible
exception of diet sodas, what is going to break down in a hermetically
sealed aluminium can? The sugar? Caramel, perhaps? Maybe the water? I
doubt it, and that's all there really is in cola.