2019 A Level H2 Chemistry Paper 1 Question 25 - Deduce Major Species for Glutamic Acid at pH 7

Here's 2019 A Level Chemistry Paper 1 Question 25.

We have to deduce the major species for glutamic acid at pH 7.

First we need to deduce the pK_a values for the functional groups present in glutamic acid.

alpha acid - pK_a 2.1
R group acid - pK_a 4.1
alpha amine - pK_a 9.5

Next we have to figure out how each of these functional groups would respond at pH 7.

alpha acid - since pH 7 > pK_a 2.1, the solution is alkaline with respect to alpha acid, so alpha acid will react with alkaline solution to form -COO^- group

R group acid - since pH 7 > pK_a 4.1, the solution is alkaline with respect to R group acid, so R group acid will react with alkaline solution to form -COO^- group

alpha amine - since pH 7 < pK_a 9.5, the solution is acidic with respect to alpha amine, so alpha amine will react with acidic solution to form -NH₃⁺ group

So we can put everything together to deduce the major species at pH 7.

Comparing the options, we can deduce the answer to this question is option B.

Topic: Nitrogen Compounds and Proteins, Organic Chemistry, A Level Chemistry, Singapore

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