The Dictionary of Organic Compounds(https://franklin.library.upenn.edu/catalog/FRANKLIN_9919410723503681) is the first stop when looking for information such as melting point, boiling point, molecular weight, and certain hazard information. It is located offsite in LIBRA and is searchable by name, CAS number, and molecular formula.
Handbook of Data on Organic Compounds(https://franklin.library.upenn.edu/catalog/FRANKLIN_9918655663503681) also contains useful information on organic compounds. This source generally has more physical property information than The Dictionary of Organic Compounds. It is searchable by name, CAS number, and molecular formula. It is located in the Chemistry Library reading room.
Names, Synonyms, and Structures of Organic Compounds(https://franklin.library.upenn.edu/catalog/FRANKLIN_9918825833503681) If you have a compound's formula or CAS number and want to know what it is called or if you have a compound name and want some synonyms, this is the source for you. It is arranged numerically by CAS number and has name and formula indices. It is located in the Chemistry library reading room.
Handbook of Physical Properties of Organic Compounds(https://franklin.library.upenn.edu/catalog/FRANKLIN_9922035543503681) is another good source of physical properties. It is more difficult to navigate than the other two sources, and it is arranged by CAS number with formula and name indices in the back of the book. This single volume is located in LIBRA.
The Merck Index(https://catalog.hathitrust.org/Record/009894947") contains useful information on biologicals, pharmaceuticals, and other similar organic compounds. The most recent edition is kept behind the Chemistry library circulation desk, and others are in the reading room. It is also available online though the HathiTrust Digital Library