'''Tautomeri''' er et specialtilfælde af strukturel isomeri. En '''tautomer''' er sin egen [[Isomeri|isomer]] af en [[organisk forbindelse]], som har den egenskab at den hurtigt kan ændre sin isomeriske form ved en [[kemisk reaktion]] som kaldes '''tautomerisering'''.
Sædvanligvis forekommer dette som migrering af [[hydrogen]]atomer ([[protoner]]) med et ombytning af en [[enkeltbinding]] med en [[dobbeltbinding]]. I opløsninger hvor tautomeri er mulig, vil en [[kemisk ligevægt]] mellem de to tautomerene opnås. Forholdet mellom tautomerene kommer an på mange faktorer, inklusiv [[temperatur]], [[opløsningsmiddel]] og [[pH]]. '''Tautomeri''' er et specialtilfælde af strukturel isomeri. Tautomeri kan [[katalyse]]res med [[syre]]r og [[Base (kemi)|base]]r.<ref>[https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2776169/ J Comput Aided Mol Des. 2009 Oct; 23(10): 693–704, ncbi.nlm.nih.gov: Let’s not forget tautomers] Citat: "...A compound exhibits tautomerism if it can be represented by two structures that are related by an intramolecular movement of hydrogen from one atom to another...Frequency of molecules that can tautomerize. A summary of one program’s enumeration of tautomers [31] of marketed drugs [32] is shown in Fig. 13. Of the 1,791 compounds, 1,334 or 74% exist as only one tautomer—put another way, 26% exist as an average of three tautomers...Although many workers have investigated the relative stabilities of tautomers in different liquid phases, because of the difficulty of measuring the equilibrium constants there is no publically available comprehensive database of this data. This lack hinders the development of empirical methods to predict the ratios of tautomers of a molecule..."</ref>
