Organic Chemistry

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13

Mass Spectrometry and

Infrared Spectroscopy

2.5 2.6 2.7 2.8 2.9 3 3.5 4 4.5

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I

t is essential for chemists

to be able to determine

the structures of the com-

pounds with which they work.

For example, you learned that

an aldehyde is formed when

a terminal alkyne undergoes

hydroboration–oxidation (Sec-

tion 6.7). But how was it de-

termined that the product of that reaction is actually an aldehyde?

Scientists search the world for new compounds with physiological activity. If a

promising compound is found, its structure needs to be determined. Without knowing

its structure, chemists cannot design ways to synthesize the compound, nor can they

undertake studies to provide insights into its biological behavior.

Before the structure of a compound can be determined, the compound must be iso-

lated. For example, if the product of a reaction carried out in the laboratory is to be

identified, it must first be isolated from the solvent and from any unreacted starting

materials, as well as from any side products that might have formed. A compound

found in nature must be isolated from the organism that manufactures it.

Isolating products and determining their structures used to be daunting tasks. The

only tools chemists had to isolate products were distillation (for liquids) and sublima-

tion or fractional recrystallization (for solids). Now a variety of chromatographic tech-

niques allow compounds to be isolated relatively easily. You will learn about these

techniques if you take a laboratory course in organic chemistry.

At one time, identifying an organic compound relied upon determining its molecu-

lar formula by elemental analysis, determining the compound’s physical properties (its

melting point, boiling point, etc.), and simple chemical tests that indicated the presence

(or absence) of certain functional groups. For example, when an aldehyde is added to a

test tube containing a solution of silver oxide in ammonia, a silver mirror is formed on

the inside of the test tube. Only aldehydes do this. If a mirror forms, you can conclude