Obviously Listerine burns because you’ve been leaving it out in the Phoenix sun. Putting any liquid heated to just short of the boiling point in your mouth is going to burn.
Actually it would be more appropriate to say Listerine burns, not because of the alcohol, but because of the alcohols. The active ingredients in Listerine are Methyl Salicylate, Thymol, Eucalyptol, Menthol and Ethanol. Four out of five of these are alcohols (esters).
What probably doesn’t burn would be the menthol – it’s a cool minty flavor but has anesthetic and mild antiseptic properties, the eucalyptol – also a minty flavoring but more medicine like, also an anesthetic/mild antiseptic and Thymol – a more herby flavor and a good antiseptic, but with anesthetic properties (also an ingredient used in ancient Egypt to help preserve mummies). These three esters would have more of a numbing effect than a burning effect.
The burning is more likely due to the ethanol and the methyl salicylate. Ethanol acts as a solvent for the botanical oils in the previous three ingredients, dissolving them in a solution of 21.6% – 26.9%. The ethanol also dissolves some of the stuff in your mouth. Ethanol breaks down lipids (fatty molecules) and could be causing some of the inner cells of your mouth to break down, essentially stripping some of your cells like stripping paint off of a wall. Ethanol in high enough concentrations could be an antiseptic, but at the concentrations used in Listerine would probably be ineffective.
Methyl Salicylate is an ester of salicylic acid and is a very effective antiseptic. Salicylic acid is used in cosmetics as a debriding agent. It’s in dandruff shampoos to remove dead skin, face scrubs to exfoliate, etc. Salicylic acid is also more commonly known as Aspirin. Methyl Salicylate, as well as some of the other active ingredients in Listerine, is also an important ingredient in topical analgesics like Icy Hot or Tiger Balm. These also have a cooling, numbing effect while the Salicylic acid penetrates the skin to reduce inflamation. Methyl Salicylate is also known as Oil of Wintergreen, a popular flavoring. Wintergreen oil can be used to kill a variety of skin and nail fungus, the wiki page mentions using it as a treatment for toe nail fungus by applying it daily for three months.
Between the ethanol breaking down cells and the methyl salicylate debriding dead tissues I would expect Listerine to burn a lot more than it does. Fortunatley the other three esters are there to numb things up a bit. Adding water, even a little bit, changes the concentration of the alcohol enough to release more of the oils which would anestisize things further, but would reduce the efficacy of these oils in killing “the germs that cause bad breath.” Since Listerine is an oral hygene product and not a cocktail mixer, and ethanol can cause more damage to the soft skin cells than desired at higher concentrations, I would think that Listerine only uses as much ethanol as needed to hold the oils in place to make them effective. A tiny bit of water would put this out of balance resulting in an oral rinse that is essentially bad tasting Aspirin water.
Methyl salicylate - http://en.wikipedia.org/wiki/Methyl_salicylate
Salicylic acid - http://en.wikipedia.org/wiki/Salicylic_acid
Thymol - http://en.wikipedia.org/wiki/Thymol
Listerine - http://en.wikipedia.org/wiki/Listerine