# 6.S: Acid-base Reactions (Summary)


Before you move on to the next chapter, you should:

• Know the Bronsted-Lowry definition of acidity and basicity: a Bronsted acid is a proton donor, a Bronsted base is a proton acceptor.
• Know the Lewis definition of acidity and basicity: a Lewis acid is an electron acceptor, a Lewis base is an electron donor.
• Understand that the Lewis definition is broader: all Bronsted acids are also Lewis acids, but not all Lewis acids are also Bronsted acids.
• Be able to draw a curved arrow mechanism for both Bronsted and Lewis acid-base reactions.
• Know the expressions for $$K_a$$ and $$pK_a$$.
• Commit to memory the approximate pKa values for the following functional groups:
• $$H_3O^+$$, protonated alcohol, protonated carbonyl (~ 0)
• carboxylic acids (~ 5)
• imines (~ 7)
• protonated amines, phenols, thiols (~ 10)
• water, alcohols (~ 15)
• $$\alpha$$-carbon acids (~ 20)
• Be able to use $$pK_a$$ values to compare acidity: a lower $$pK_a$$ corresponds to a stronger acid.
• Know that:
• For a given pair of acids, the stronger acid will have the weaker conjugate base.
• For a given pair of basic compounds, the stronger base will have the weaker conjugate acid.
• Be able to identify the most acidic/basic groups on a polyfunctional molecule.
• Be able to calculate the equilibrium constant of an acid base equation from the $$pK_a$$ values of the acids on either side of the equation.
• Be able to use the Henderson-Hasselbalch equation to determine the protonation state/charge of an organic compound in an aqueous buffer of a given $$pH$$.
• Understand the idea that the best way to compare the strength of two acids is to compare the stability of their conjugate bases: the more stable (weaker) the conjugate base, the stronger the acid.
• Be able to compare the acidity or basicity of compounds based on periodic trends:
• acidity increases left to right on the table, so alcohols are more acidic than amines
• acidity increases top to bottom on the table, so a thiol is more acidic than an alcohol.
• Be able to compare the acidity or basicity of compounds based on protonation state: $$H_3O^+$$ is more acidic than $$H_2O$$, $$NH_4^+$$ is more acidic than $$NH_3$$.
• Understand how the inductive effect exerted by electronegative groups influences acidity.
• Understand how resonance delocalization of electron density influences acidity.
• Be able to explain/predict how orbital hybridization affects the relative acidity of terminal alkynes, alkenes, and alkanes.
• Be able to explain why phenols are more acidic than alcohols, and how electron-withdrawing or donating groups influence the acidity of phenols.
• Be able to identify the relative basicity of a nitrogen-containg group in a compound, based on whether it is an amine, amide, imine, aniline, or 'pyrrole-like'.
• Be able to identify -carbon(s) on a carbonyl compound, and explain why protons are weakly acidic. You should be able to draw the enolate conjugate base of a carbonyl compound.
• Be able to identify tautomeric relationships, specifically keto-enol and imine-enamine tautomers.
• Understand what a polyprotic acid is, what is meant by multiple pKa values, and why these values get progressively higher.