How is pka related to the strength of an amino acid

The most common acid we will talk about in BIS2A is the carboxylic acid functional group. Note that the key difference in the figure below between a strong acid or base and a weak acid or base is the single arrow strong versus a double arrow weak.

In the case of the single arrow you can interpret that by imagining that nearly all reactants have been converted into products. Moreover, it is difficult for the reaction to reverse backwards to a state where the protons are again associated with the molecule there were associated with before. In the case of a weak acid or base, the double-sided arrow can be interpreted by picturing a reaction in which:. Figure 1. An example of strong acids and strong bases in their protonation and deprotonation states.

The value of their pKa is shown on the left. Attribution: Marc T. Electronegativity plays a role in the strength of an acid. If we consider the hydroxyl group as an example, the greater electronegativity of the atom or atoms indicated R attached to the hydroxyl group in the acid R-O-H results in a weaker H-O bond, which is thus more readily ionized. This means that the pull on the electrons away from the hydrogen atom gets greater when the oxygen atom attached to the hydrogen atom is also attached to another electronegative atom.

An example of this is HOCL. The electronegative Cl polarizes the H-O bond, weakening it and facilitating the ionization of the hydrogen. If we compare this to a weak acid where the oxygen is bound to a carbon atom as in carboxylic acids the oxygen is bound to the hydrogen and carbon atom. In this case, the oxygen is not bound to another electronegative atom.

Thus the H-O bond is not further destabilized and the acid is considered a weak acid it does not give up the proton as easily as a strong acid. Figure 2. The strength of the acid can be determined by the electronegativity of the atom the oxygen is bound to. For example, the weak acid Acetic Acid, the oxygen is bound to carbon, an atom with low electronegativity.

In the strong acid, Hypochlorous acid, the oxygen atom is bound to an even more electronegative Chloride atom. Attribution: Erin Easlon. In Bis2A you are going to be asked to relate pH and pKa to each other when discussing the protonation state of an acid or base, for example, in amino acids. How can we use the information given in this module to answer the question: Will the functional groups on the amino acid Glutamate be protonated or deprotonated at a pH of 2, at a pH of 8, at a pH of 11?

In order to start answering this question we need to create a relationship between pH and pKa. Carboxylic acids containing -COOH , such as acetic and lactic acids, normally have a Ka constant of about 10 -3 to 10 Consequently, expressing acidity in terms of the Ka constant alone can be inconvenient and not very intuitive. Therefore, pKa was introduced as an index to express the acidity of weak acids, where pKa is defined as follows.

In addition, the smaller the pKa value, the stronger the acid. For example, the pKa value of lactic acid is about 3. Another important point is the relationship between pH and the pKa of an acid. This relationship is described by the following equation. If the pH changes by 1 near the pKa value, the dissociation status of the acid changes by an extremely large amount.

In the case of acetic acid, for example, if the solution's pH changes near 4. When the pH is 3. Conversely, to change the pH level near the pKa value of an acid, the dissociation status of the acid must be changed significantly, which requires using an extremely large amount of acid or base.

The ability of a substance to maintain the pH of such solutions is referred to as its buffer capacity, where the closer the pKa and pH are, the higher the buffer capacity.