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Contents 1. Introduction 3217 2. Addition of a Single Nucleophile to OPA 3218 2.1. Addition of Water and Hydroxide Ions 3218 2.2. Addition of Alcohols 3219 2.3. Addition of Ammonia and Primary Amines 3219 2.4. Reactions with Hydrazine 3219 2.5. Reactions with Amino Acids 3220 2.6. Reactions with Amides, Urea, and Thiourea 3221 2.7. Reactions with Carbanions 3221 2.8. Reactions with Thiols and Other Sulfur Compounds3222 3. Reactions in the Presence of Two Nucleophiles 3223 3.1. Reagents 3224 3.1.1. Role of the Carbonyl Compound 3224 3.1.2. Stability of the Mixed Reagent 3224 3.1.3. Structure of the Thiol 3224 3.1.4. Structures of Amino Acids and Amines 3225 3.2. Sequence of Addition 3225 3.3. Reaction Medium 3225 3.4. Kinetics of the Formation of the Primary Product 3227 3.5. Consecutive Reactions of the Isoindole DerivativesStability of the Product of the Analytical Reaction 3228 3.6. Proposed Reaction Schemes 3232 3.6.1. Scheme Proposed by Simmons and Johnson 3232 3.6.2. Scheme Proposed by Wong, Sternson, and Schowen 3232 3.6.3. Scheme Proposed by Sternson, Stobaugh, and Repta 3233 3.6.4. Scheme Proposed by Gladilovich, Kartsova, and Zakharova 3233 3.6.5. Scheme of the Degradation of the Derivative of 2-Mercaptoethanol 3233 3.6.6.Scheme for Autoxidation of the Isoindole 3234 3.6.7. Conclusions Regarding Reaction Scheme 3234 3.7. Techniques Used 3234 3.8. Other Uses of the Reagent Formed in Reaction of OPA with Thiols 3235 3.9. Reagents Prepared by Reaction of Orthophthalaldehyde with Nucleophiles Other than Thiols 3235 3.10. Some Biochemical Applications 3236 4. Conclusions 3237 5. Acknowledgment 3237 6. References 3237

ABSTRrlCTComparing reduction potentials of aromatic nitro compounds with those of corresponding nitroso compounds, by studying shifts o f half-wave and peak potentials with pH and by following the decrease of limiting current in buffered 35% DMF-water mixtures, the sequence of protons and electrons in the fourelectron reduction step was proved to be: H ', e, H ', c, Ze, 2H+. Arylhydroxylamines formed in the fourelectron reduction of nitro compounds can be protonated and further reduced, they can undergo dehydration (provided that the dehydration product is stable), they can undergo chemical reactions with intermediates of the reduction of the nitro group, like ArNO, they can be electrooxidized to nitrosobenzenes (which can undergo chemical reactions) or they can react with carbony1 groups when present. Stabilization of the dehydrated form can occur in quinonemethide (nifedipine) o r in some quinone-like heterocyclic derivatives. Side reactions of ArNHOH and ArNO are demonstrated by smaller peaks in CV and by controlled potential electrolysis. Reactions of ArNHOH with carbony1 groups were observed by formation of new waves of nitrones in the reduction of nitrobenzenaldehydes.

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