Practical investigations

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I am doing an investigation into the effects of 1 month’s oxidation on wine and in one of the other questions you mentioned that:
"Others will cause the pH to increase: e.g. the oxidation of ethanol (CH₃CH₂OH) to ethanal (CH₃CHO) causes “free” (acidic) sulphur dioxide to be removed from the system by becoming bound to the product, which is, of course, an aldehyde; the conversion of (acidic) phenols to quinones; and perhaps the escape of (acidic) SO₂ to the atmosphere, particularly if the wine has been splashed about".
I was wondering whether there was an equation or structural equation to demonstrate the bonding of the aldehyde (ethanal) to the SO₂. I presume it would be an addition reaction over the double bond but I am unsure of how this would be shown in structural formula and the reaction mechanism (showing where the electrons go during the reaction?). I am also wondering about the conversion of phenols to quinones.

Igloo writes ...
 
As you suspect the reaction of sulphur dioxide with aldehydes involves an addition reaction over the carbonyl double bond.
 
First sulphur dioxide reversibly interacts with water to form hydrogen ions and hydrogensulphite (hydrogensulphate(IV)) ions:
 
SO₂(aq) + H₂O(l) <---> H⁺(aq) + HSO₃^-(aq)
 
In the first stage of the addition process these HSO₃^- ions act as nucleophiles and attack the electron deficient carbonyl carbon atom, with the double bond being broken and the carbonyl oxygen atom being converted into -O^-. Proton transfer occurs subsequently and the final product is often known as a “bisulphite addition compound”:
 
CH₃CHO(aq) + HSO₃^-(aq) <---> CH₃CH(O^-)SO₃H
 
CH₃CH(O^-)SO₃H <---> CH₃CH(OH)SO₃^-
 
Though these processes are reversible, the “bisulphite” addition ions are quite stable and hold the “free” sulphur dioxide until the process is reversed in some way.
 
I use the word “bisulphite” since this is how you are likely to find out more information if you wish to research this process in more detail on the internet. An alternative name is “bisulfite”. However, these compounds should in modern nomenclature be referred to as hydrogensulphate(IV) addition compounds – a bit of a mouthful!
 
As for the conversion of phenols to quinones, perhaps I can give you the simplest of all, i.e. the oxidation of phenol to quinone itself:
C₆H₅OH --- oxidation --> C₆H₄O₂
 
Quinone is a sort of double ketone based on a six C-atom ring, with two C=O groups on opposite sides of the ring, which is why its name ends in “one”. I haven’t the facility to show the structural formula here, but if you go to a website such as www.wikipedia.com and use “quinone” as your search phrase, I am sure that you will find a structure for it. All quinones have this very recognisable structure, and are derived from phenolic oxidations. However, I cannot help with a mechanism. Perhaps you will find this on Google or Wikipedia.

Risk assessment
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updated: 01 May 2007