Hints for identifying unknowns

Search Properties of Organic Compounds and The Combined Chemical Dictionary to find compounds whose physical and chemical properties are consistent with those you determined for your unknown. Use your spectral information to narrow this list of candidates. Warning: Do not be misled by published spectral data; your IR and NMR spectral data will not be identical to literature values obtained in different solvents or physical states or under different instrumental conditions. Note the CAS Registry Number of your suspected unknown for use later in searching. Once you think you have identified your unknown, you need to make a known solid derivative to clinch its identity.

First, determine what derivatives are known for your suspected unknown. Derivatives of Chem 130 unknowns can be found in The Combined Chemical Dictionary or in Beilstein Crossfire (use the CAS Registry Number to access Beilstein). See below Hints for searching by compound name. A compound listing often includes derivatives, maybe not specified as “derivatives”, but as compounds you should recognize that you can make from your unknown (see your Course Reader for common derivatives of different chemical classes). For some classes of compounds, you will probably have to search using a specific derivative as parent compound. See below Hints for searching for derivatives. If your unknown is multifunctional (e.g., p-aminobenzaldehyde), consider derivatives of all functional groups. Derivatization of one group may be easier or yield a known compound.

Second, decide on the derivative to be made (make sure your derivative has a known melting point) and the procedure to be used. Generic preparative procedures for derivatives of Chem 130 unknowns are included in Experiments and Techniques in Organic Chemistry, by Pasto, Johnson and Miller. You may want to check additional resources for a procedure for making your specific derivative. If you select a procedure other than the one in Pasto referenced under Derivatives below (see Lecture Notes, p.9), be sure to check the Reserve Shelf in Swain Library for the list of reagents and chemicals available for use in Chem. 130/132.

Chem 132 students typically will find it necessary to search many more references, and primary sources, to identify their unknowns, derivatives, and preparative procedures.

Hints for searching by compound name

Ether groups sometimes are listed as the alkyl or aryl ether derivative of the parent compound (e.g., o-dimethoxybenzene is the dimethyl ether derivative of o-dihydroxybenzene).

An N-alkyl substituent on an amine is sometimes listed as the alkylated derivative of the amine (e.g., N-isopropylbenzylamine is the isopropylated derivative of benzylamine).

Esters are sometimes listed as ester derivatives of the corresponding alcohol or carboxylic acid (e.g., n-propyl acetate could be the acetate derivative of n-propyl alcohol or the n-propyl derivative of acetic acid).

Nitriles are sometimes listed as cyano derivatives of the corresponding acids (e.g., acetonitrile is the cyano derivative of acetic acid).

A few aromatic substituted acetanilides are listed as the acetylated derivative of the parent aromatic amine (e.g., acetanilide is the acetylated derivative of aniline).

Hints for searching for derivatives

Typically, no specific derivatives are listed for amides, esters, and nitriles. Instead, search for their hydrolysis products as parent compound. (For Chem 130/132, only one of the hydrolysis products has to be isolated for identification.) Remember, a liquid hydrolysis product still needs to be converted into a solid derivative for identification.

Typically, few/no derivatives of “aromatics” are listed as such. Instead, search for a corresponding oxidation (alkyl side chain), substitution (bromination, nitration, sulfonation) or reduction product (nitro) as parent compound.