Organic Compounds - AP Chemistry

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Question

What is the most acidic functional group on the amino acid cysteine? Cysteine has the chemical formula C_3H_7NO_2S.

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Answer

We know that by definition, all amino acids contain a carboxylic acid functional group (CO_2H), and an amino functional group (NH_2). Therefore the additional functional group can only contain sulfur and hydrogen.

The amino functional group is slightly basic, and the carboylic acid functional group is slightly acidic. The -SH functional group will also be acidic, but because sulfur and oxygen are in the same group on the periodic table, the -SH group will have similar acidic properties to an -OH group, and alcohols are less acidic than carboxylic acids.

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