Final answer:
Weaker bases lead to more 1,4-addition in reactions with vinyl ketones because they have lower nucleophilicity and are better leaving groups, as indicated by their lower base ionization constant (Kb) values.
Step-by-step explanation:
Weak bases tend to favor 1,4-addition (also known as conjugate addition) in reactions with vinyl ketones. The strength of a base is indicated by its base ionization constant (Kb), where a lower Kb value suggests a weaker base. Weak bases are less likely to engage in direct attack on the carbonyl carbon (1,2-addition) because they are less nucleophilic. Instead, they are more likely to add to the beta carbon, leading to 1,4-addition products. This tendency is due to the fact that weaker bases prefer to react with less electrophilic centers and are also better leaving groups in elimination reactions.