Question

Below are the observed rate constants for this reaction at varying pH values 4 Table 1. Observed rate constants (x 103) for alanine reaction of 0.5 m alanine solutions at different pH values Initial pH Temp2.3 9.5 290 3002.04 310 320 9.92 330 21.07 1.11 2.24 4.72 909 18.03 37.0 2.87 6.02 9.92 20.3 5.2 QUESTION 2: What conclusions can you draw from this table? Explain how the data support your conclusion (hint remember that ala nine occurs in different forms depending on pH)

Answer 1

The isoelectronic point for alanine is near 6. Hence at pH value of 6, it remains in zwitterion form, which will be neutral due to presence of both positive and negative charge.

At pH of 2.3, the -COOH group of alanine will remain protonated and at pH 9.5, the -NH₂ group will not remain as -NH₃⁺.

From the table, we see that when the pH is 6, the reaction takes place even at the lowest temperature considered which is 280^oC. At other pH values, the reaction happens only at slightly higher temperatures. Even at these higher temperatures, the reaction rate will be higher for alanine at pH 6 as can be seen from the higher rate constants.

At other pH values, the rates are nearly same at all temperatures. From these observations, we can conclude that the most suitable state for the reaction to happen with alanine is its zwitterion form which will be present at pH 6.