vogel_practical_ochem_3.pdf

1144 PRACTICAL ORGANIC CHEMISTRY [A,
form and carbon tetrachloride ; water, diethyl ether, ethanol and methanol
are used only if considerations of solubility make it necessary. In general,
polar solvents should be avoided. The selected solvent must be free
from absorbing impurities ; " spectroscopically pure " solvents can be
purchased. The possibility of interaction between the solvent and the
compound under examination must always be considered. When the
absorption spectrum of a pure substance is determined in each of a series
of solvents, a slight difference in the location and the intensity of the
absorption bands is usually observed with variation in solvent. The
small solvent effects depend upon the nature of the solvent, the type of
absorption band (K~ or jR-band *), and the nature of the solute (polar or
non-polar). Marked changes in the nature of the absorption may be due
to chemical interaction with the solvent, complex formation, dissociation
or to equilibration of two tautomers in solution. Table VI gives the
lowest wave length (m(x) at which a number of purified solvents transmit
ultraviolet radiation in 1 cm. cells.
TABLE VII. LOWEST WAVE LENGTH (mjj.) AT WHICH SOLVENTS TRANSMIT
ULTRAVIOLET RADIATION
Solvent
ct/cZoHexane
n-Hexane
Carbon tetrachloride
Chloroform .
Benzene
Wave
Length
195
200
257
237
280
Solvent
Water
Ethanol
isoPropanol .
Methanol
Diethyl ether
Wave
Length
191
204
205
225
225
SURVEY OF DATA
Chromophores and auxochromes. Absorption of light in the visible
and ultraviolet regions is due to the excitation of relatively loosely-bound
electrons, such as in multiple bonds or of lone pairs. The classical term
for an arrangement of multiple bonds in adjacent positions (i.e., separated
by one single bond) and for the electronic interaction resulting therefrom
is " conjugation ". The unsaturation electrons of multiple bonds are the
re-electrons and we may adopt the term n-n conjugation for this case. In
saturated organic compounds containing elements other than carbon and
hydrogen, e.g., nitrogen, oxygen or halogen, unshared ^-electrons are
present as well as the a-valency electrons. The non-bonding ^-electrons
are held rather less firmly than c-electrons. We may also have a n-p
conjugation with certain groups, such as —NR2; this is now realised to be
* Two or more chromophores (see below) in conjugation produce intense bands, which
are termed X-bands (from the German Konjugation) in the neighbourhood of 230 m/i.
Low intensity bands at longer wave lengths, probably due to single chromophores, are
termed i?-bands (from German Radikal).