Structure, electronic effects, bonding and moles

What is the pKa value?

Negative log₁₀ of Ka

The smaller the value of pKa…

…the stronger (more likely to lose H+) the conjugate acid is in the equilibrium, and the weaker (less likely to gain H+) is the conjugate base.

Does the pKa value refer to an equilibrium or a substance?

Equilibrium

For acidic compounds, the pKa for dissociation into a proton and the conjugate base will depend on factors which what?

Lower the energy of the anionic conjugate base, for example, factors which “spread out” the negative charge over several atoms. 

Draw the equilibria for the ionisation of this acid

Draw the equilibria for the ionisation of this acid

Draw the equilibria for the ionisation of this acid

Explain the differences between the pKa values

• In acetic acid, the energy of the COO- anion is lowered by the delocalisation of the negative charge across both oxygens

• In D-lactic acid, the energy of the COO- anion is lowered by delocalisation of the charge across both oxygens and by the presence of an inductively electron withdrawing oxygen in the OH

• In trifluoroacetic acid, the energy of the COO- anion is lowered by delocalisation of the charge across both oxygens and by the presence of three inductively electron withdrawing fluorine atoms

Draw the equilibria for the protonation of this amine base

The pKa of bases refers to the pKa of the conjugate acid. The conjugate acid is NH3. pKa is always acid → base, so the LHS has to have the NH3 one, rather than the base.

Draw the equilibria for the protonation of this amine base

Draw the equilibria for the protonation of this amine base

Explain the differences between the pKa values

• In 2,2,2-trifluoroethylamine, the energy of the cation (conj acid) is raised by the inductive effect of the 3 F withdrawing electron density from the cationic nitrogen

• The energy of the cation (conj acid) is raised by the inductive effect of the oxygen withdrawing electron density from the cationic nitrogen