Organic reactivity principle: The reactions of molecules are most directly the reactions of which structural features?

Introduction / Context:
In organic and biochemical reactions, functional groups dictate how molecules behave. Recognizing reactive sites allows prediction of mechanisms and products without memorizing every individual molecule’s behavior.

Given Data / Assumptions:

• Standard organic functional groups: hydroxyl, carbonyl, carboxyl, amino, phosphate, thiol, etc.
• Solvent: typically water in biochemistry.
• Normal temperature/pressure for biological systems.

Concept / Approach:
Functional groups carry characteristic electron distributions and polarities. They undergo well-defined reaction types (for example, nucleophilic acyl substitution at carboxyls, addition to carbonyls, phosphorylation at phosphates). Therefore, when two molecules react, it is their functional groups that engage first and determine the pathway.

Step-by-Step Solution:
Identify the functional groups on each reactant.Map each group to typical reactivity patterns (acid/base, nucleophile/electrophile).Predict outcomes based on those group-specific mechanisms.

Verification / Alternative check:
Peptide bond formation occurs between specific functional groups (carboxyl and amino) via condensation, illustrating the rule.

Why Other Options Are Wrong:

• Independent of functional groups: Contradicts core principles of organic chemistry.
• Only enzyme-catalyzed sites: Enzymes accelerate reactions but do not create entirely new chemistry unrelated to the functional groups.
• Crystal lattice defects: Irrelevant to molecules in solution.

Common Pitfalls:
Overlooking solvent and pH effects that modulate functional group reactivity.

Final Answer:
Functional groups.