tautomerism and functional group isomerism

Please follow the definition of tautomerism and functional group isomerism.

→Tautomers are organic compounds that are interconvertible(可互相轉換) by a chemical reaction called tautomerization.

→Functional group isomers in which one functional group is split up into different ones.

ethenol and ethanal are different groups so that we define them are alkanol and alkanal respectively. It is because they can not interconvert through a chemical reaction.

What is a pair of tautomers???
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Tautomerism is basically due to reactions involving a loss of proton, followed by protonation again by acid. Eg. Imidazole - the proton can be at C4 or 5 positions because the proton can be deprotonated to give the anion, then a proton can be put back onto either C4 or C5 due to the lone pair on nitrogen and the -ve charge - and resonance.
Functional group isomers are compounds with the same number or carbons and hydrogens (and other elements such as oxygen, sulfur etc etc etc) but different functional groups. Eg. 1-hexene and cyclohexane, where 1-hexene has a double bond but the formula is the same as cyclohexane.
Hope that helps.