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Pyrimidines [1,3-Diazine] are six-membered heteroaromatic compounds containing two nitrogen atoms at the 1 and 3 positions of the ring. It is isomeric with two other forms of diazine, namely pyridazine and pyrazine. Pyrimidines are less basic in nature and relatively unreactive towards electrophilic aromatic substitutions as the ring pi electrons become less energetic. In the pyrimidine ring, the 2-, 4-, and 6- positions are electron deficient and the 5-position is relatively less electron deficient. Hence, electrophilic substitution through nitration and halogenation is relatively facile at the 5-position of the ring. In contrast to pyridine, the protonated pyrimidine pKa value is 1.23 compared to pyridine which is reported as a 5.30 (pKa value).
In general, pyrimidines serve as a form of energy for cells, and are essential for the production of DNA and RNA, proteins, starch, regulations of enzymes, cell signalling etc. In nucleic acids, three types of pyrimidine based nucleobases are present and named as cytosine (C), thymine (T), and uracil (U). Pyrimidine can be photolytically converted into uracil under ultraviolet light. The pyrimidine ring system can be viewed in many natural occurring compounds as a substituted pyrimidine and fused with other rings and derivatives. Pyrimidines have also been found in many synthetic compounds, such as bariturates.