NMR Study of the Protolysis Kinetics in Simple Amino Acids. I. Sarcosine Hydrochloride and its Ester

The mean lifetime of the amino group τ of acidic aqueous solutions of sarcosine hydrochloride and sarcosine methylester hydrochloride were measured using the NMR technique at 21±1°C. The experimental results show: (1) 1/τ is first order in respect to both sarcosine and its ester concentration. (2) 1/τ depends strongly on 1/a_H⁺ in very acidic solutions and becomes almost independent in dilute acid solution. The interpretation is given in terms of an exchange of protons between the amino acid or its ester with a water molecule which is hydrogen bonded to the amino group. The exchange mechanism involves two steps: [Complex chemical formula] [Complex chemical formula] Values of the rate constants for sarcosine hydrochloride are: $$k_{\mathrm{I}}^{\prime }\text{=110(}{\sec }^{\text{−1}}{\mathrm{),\hspace{0.17em}\hspace{0.17em}k}}_{-\mathrm{I}}^{\prime }{\text{=3.2×10}}^{10}({\sec }^{\text{−1}}{\mathrm{M),\hspace{0.17em}\hspace{0.17em}k}}_{\mathrm{H}}{\text{=8.3×10}}^9({\sec }^{\text{−1}}\mathrm{);}$$ and for sarcosine methyl ester are: $$k_{\mathrm{I}}^{\prime }\text{=120(}{\sec }^{\text{−1}}{\mathrm{),\hspace{0.17em}\hspace{0.17em}k}}_{-\mathrm{I}}^{\prime }{\text{=3.2×10}}^{10}({\sec }^{\text{−1}}{\mathrm{M),\hspace{0.17em}\hspace{0.17em}k}}_{\mathrm{H}}{\text{=1.9×10}}^{10}({\sec }^{\text{−1}}\mathrm{).}$$