Organic Chemistry

If a compound has enough conjugated double bonds, it will absorb visible light
and the compound will be colored. -Carotene, a precursor of vita-
min A, is an orange substance found in carrots, apricots, and sweet potatoes. Lycopene
is red and is found in tomatoes, watermelon, and pink grapefruit.

An auxochromeis a substituent that when attached to a chromophore, alters the
and the intensity of the absorption, usually increasing both; OH and groups
are auxochromes. The lone-pair electrons on oxygen and nitrogen are available for
interaction with the pelectron cloud of the benzene ring, and such an interaction

lmax NH 2

1 lmax 7 400 nm 2 b

-carotene

λmax = 455 nm

lycopene

λmax = 474 nm

Section 8.11 Effect of Conjugation on lmax 325

3-D Molecule:

1,3,5,7-Octatetraene

The increases as the number of

conjugated double bonds increases.

Lmax

Table 8.3 Values of max and for Ethylene and Conjugated Dienes

Compound max (nm) (M−^1 cm−^1 )

CH 2 165 15,000

217 21,000

H 2 C

256 50,000

290 85,000

334 125,000

364 138,000

∆Enonconjugated ∆Econjugated

nonconjugated electrons

CO

HOMO

LUMO *

conjugated electrons

CC

CO

HOMO

LUMO^ *

>Figure 8.7

Conjugation raises the energy of

the HOMO and lowers the energy

of the LUMO.

Conjugation raises the energy of the HOMO and lowers the energy of the LUMO, so
less energy is required for an electronic transition in a conjugated system than in a
nonconjugated system (Figure 8.7). The more conjugated double bonds there are in a
compound, the less energy is required for the electronic transition, and therefore the
longer is the wavelength at which the electronic transition occurs.
The values of the transition for several conjugated dienes are shown
in Table 8.3. Notice that both the and the molar absorptivity increase as the num-
ber of conjugated double bonds increases. Thus, the of a compound can be used
to predict the number of conjugated double bonds in the compound.

lmax

lmax

lmax p:p*