e-EROS Encyclopedia of Reagents for Organic Synthesis
Allows for general browsing and searching of the text, as well as searching by chemical structure, substructure, and reaction. e-EROS includes a database of reactions and of the most frequently consulted reagents and catalysts.
Encyclopedia of Supramolecular Chemistry
Covers the fundamentals of supramolecular chemistry and includes articles on the latest supramolecular advancements and methods in the areas of chemistry, biochemistry, biology, environmental and materials science and engineering, physics, computer science, and applied mathematics. Browse or search by text.
IUPAC Compendium of Chemical Terminology- the Gold Book
Contains "terminology definitions from IUPAC recommendations already published in Pure and Applied Chemistry and in the other Colour Books". Searchable by term and structure using Marvin Sketch structure editor. Also browsable by alphabetical index, and contains constants, units, and equations for math and physics as well as chemistry. It is freely available online.
The Merck Index
Encyclopedia of chemicals, drugs, and biologicals that contains more than 10,000 monographs. Each is a concise description of a single substance or a small group of closely related compounds. Search by text, property, or structure.