From my book:
An antioxidant is a substance that prevents oxidation by reacting with an oxidizing agent
If that's the case, why do we distinguish between antioxidants? Are all compounds containing hydroxyl group not capable of donating protons to oxygen-containing free radicals?
Your question is a precise equivalent of "How are red-haired men different from tall men". They are different in that they have red hair; also, not all of them are tall. That's about it.
Same thing here. Donating a proton has nothing to do with antioxidant properties. Indeed, among the volumes of forgotten lore of inorganic chemistry one can find the notion of acids; these are distinguished by their ability to donate protons. Are they antioxidants? No, and often quite the contrary.
Antioxidants are compounds that oxidize easily, and oxidation has to do with transfer of electrons, not protons. That they all seem to contain -OH groups is just a feature of our biology, which is a consequence of our chemistry, which in turn is centered around water. Look at other compounds in our bodies, most of them (except fats) have -OH groups too. There is no fundamental chemical reason behind that.
So it goes.
And I'm sorry if it was a stupid question, what I wanted to ask was what separates antioxidants from other compounds that can oxidize. I believe you also asked about which book I got the quote from and it was from [Fundamentals of General, Organic, and Biological Chemistry with MasteringChemistry, SI Edition] by McMurry.
– Patrick Mar 25 '22 at 11:36Reducing compounds are like weapons - some can be as weak as a table knife, some as strong as a nuclear bomb (figurativelly).
As antioxidants
Antioxidants are compounds that inhibit oxidation, a chemical reaction that can produce free radicals and chain reactions that may damage the cells of organisms. Antioxidants such as thiols or ascorbic acid (vitamin C) may act to inhibit these reactions. To balance oxidative stress, plants and animals maintain complex systems of overlapping antioxidants, such as glutathione.
are usually considered such reducing compounds that are neither too strong neither too weak, reducing just too strong oxidants, including oxidizing radicals and dissolved oxygen. There is additional requirement to be chemically and/or biologically compatible with the material protected against oxidation.
Water does contain $\ce{-OH}$ bond and is not considered as antioxidant, reducing just extremely strong oxidants like e.g. fluorine. Similarly ethanol is very weak antioxidant.
Reducing abilities of $\ce{-OH}$ group is very different across compounds. Typical antioxidants are polyphenols like epigallocatechin gallate, one of green tea infusion components. Other ones are e.g. pyrogallol(used in gas analysis for oxygen absoption) or ascorbic acid, also known as vitamin C, or tocoferol - vitamin E.
