Pirimidina: Perbedaan antara revisi

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Physical properties are shown in the data box. A more extensive discussion, including spectra, can be found in Brown ''et al.''<ref name="BrownPyrimidines1994p242">{{cite book |author=Brown, D. J.; Evans, R.F.; Cowden, W. B.; Fenn, M. D. |title=The Pyrimidines |publisher=John Wiley & Sons |location=New York |year=1994 |pages=242–244 |isbn=0-471-50656-7|url= |accessdate=}}</ref>▼

Per the classification by Albert<ref name="Albert1968p56">{{cite book |author=Albert, Adrien |title=Heterocyclic Chemistry, an Introduction |publisher=Athlone Press |location=London |year=1968 |pages=56–62 |isbn= |url= |accessdate=}}</ref> six-membered heterocyclics can be described as π-deficient. Substitution by electronegative groups or additional nitrogen atoms in the ring significantly increase the π-deficiency. These effects also decrease the basicity.<ref name="Albert1968p437">{{cite book |author=Albert, Adrien |title=Heterocyclic Chemistry, an Introduction |publisher=Athlone Press |location=London |year=1968 |pages=437–439 |isbn= |url= |accessdate=}}</ref> ▼

Like pyridines, in pyrimidines the π-electron density is decreased to an even greater extent. Therefore [[electrophilic aromatic substitution]] is more difficult while [[nucleophilic aromatic substitution]] is facilitated. An example of the last reaction type is the displacement of the [[amino]] group in 2-aminopyrimidine by chlorine<ref>[[Organic Syntheses]], Coll. Vol. 4, p.182 (1963); Vol. 35, p.34 (1955) [http://www.orgsynth.org/orgsyn/pdfs/CV4P0182.pdf Link]</ref> and its reverse.<ref>[[Organic Syntheses]], Coll. Vol. 4, p.336 (1963); Vol. 35, p.58 (1955) [http://www.orgsynth.org/orgsyn/pdfs/CV4P0336.pdf Link]</ref> ▼

Electron lone pair availability ([[basicity]]) is decreased compared to pyridine. Compared to pyridine, [[N-alkylation]] and [[N-oxidation]] are more difficult. The [[pKa]] value for protonated pyrimidine is 1.23 compared to 5.30 for pyridine. Protonation and other electrophilic additions will occur at only one nitrogen due to further deactivation by the second nitrogen.<ref name="JouleMills5thp250">{{cite book |author=Joule, J. A.; Mills, K. |publisher=Wiley |location=Oxford |year=2010 |pages=250 |isbn=978-1-405-133300-5 }}</ref> The 2-, 4-, and 6- positions on the pyrimidine ring are electron deficient analogous to those in pyridine and nitro- and dinitrobenzene. The 5-position is less electron deficient and substitutents there are quite stable. However, electrophilic substitution is relatively facile at the 5-position, including [[nitration]] and halogenation.<ref name="BrownPyrimidines1994p4">{{cite book |author=Brown, D. J.; Evans, R.F.; Cowden, W. B.; Fenn, M. D. |title=The Pyrimidines |publisher=John Wiley & Sons |location=New York |year=1994 |pages=4–8 |isbn=0-471-50656-7|url= |accessdate=}}</ref>▼