The reaction of an aromatic heterocycle with a proton is called a protonation. One of articles about this theory is 《Amino acids. II. Connection between structure and properties of aliphatic amino acids》. Authors are Remizov, A. L..The article about the compound:1-Piperidineacetic Acidcas:3235-67-4,SMILESS:OC(=O)CN1CCCCC1).Quality Control of 1-Piperidineacetic Acid. Through the article, more information about this compound (cas:3235-67-4) is conveyed.
The following pKa1 and pKa2 at 25° were determined conventionally from half-neutralization data for RR’NCH2CO2H (RR’N given): H, H, 2.33 and 9.71; Me, Me, 2.06 and 9.88; Et, Et, 1.97 and 10.51; piperidino, 1.98 and 10.2; morpholino, 1.91 and 7.53; C6H11, Me, 1.96 and 10.71; MeOCH2CH2, MeOCH2CH2, 1.89 and 8.3; HOCH2CH2, HOCH2CH2, 1.81 and 8.27. Also were determined ionization constants of Et2NCH2CO2Et (10-7.82) and Et ester of morpholinoacetic acid (10-5.03). These data indicated that these tertiary amino acids and esters exist in aqueous solution almost completely as zwitterions. The phys. properties of these acids are explained also by tridimensional H bonding at all active H atoms in primary amino acids and by the lack of such cross-linking by H bonds in tertiary amino acids such as those above. Morpholinoacetic acid-HCl in EtOH and dry HCl gave the Et ester-HCl, decomposed 176°, in 91% yield, after 1 hr. on a steam bath. Similarly was obtained N,N-diethylglycine Et ester, b17 72°; the HCl salt failed to crystallize.
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Reference:
Isoxazole – Wikipedia,
Isoxazole | C3H3NO – PubChem