Answers to Concept Checks
4.1
An acid–base reaction will proceed when the acid and base react to form conjugate products
that are weaker than the reactants.
Amphoteric species can act as either an acid or a base. Water, bicarbonate, and dihydrogen
phosphate are common amphoteric species in biological systems.
pKa = –log Ka, where Ka is the equilibrium constant for the dissociation of an acid. pKa indicates
acid strength: a stronger acid has a lower (or even negative) pKa.
Alcohols, aldehydes and ketones, carboxylic acids, and most carboxylic acid derivatives act as
acids. Amines and amides act as bases.4.2
Nucleophilicity and electrophilicity are based on relative rates of reactions and are therefore
kinetic properties. Acidity and basicity are measured by the position of equilibrium in a
protonation or deprotonation reaction and are therefore thermodynamic properties.
The four main determinants of nucleophilicity are charge (more negative = better nucleophile),
electronegativity (more electronegative = worse nucleophile), steric hindrance (larger = worse
nucleophile), and the solvent (protic solvents can protonate or hydrogen bond with the
nucleophile, decreasing its reactivity).
A substitution reaction will proceed when the nucleophile is a stronger base (more reactive)
than the leaving group.
Greater positive charge increases electrophilicity, and better leaving groups increase
electrophilicity by making the reaction more likely to proceed.
Good leaving groups can stabilize the extra electrons that result from heterolysis. Weak bases
(the conjugate bases of strong acids) are good leaving groups. Resonance stabilization and
inductive effects from electron-withdrawing groups also improve leaving group ability.
4.3
Good oxidizing agents have a high affinity for electrons or have very high oxidation states.
Examples include O 2 , O 3 , Cl 2 , permanganate ( ), chromate ( ), dichromate (
), and pyridinium chlorochromate. These compounds often contain a metal and a large number