Organic Chemistry

The enzyme lactate dehydrogenase catalyzes the reduction of pyruvate by NADH

to form lactate. NADH is the only species in the reaction mixture that absorbs light at

340 nm, so the rate of the reaction can be determined by monitoring the decrease in

absorbance at 340 nm (Figure 8.9).

328 CHAPTER 8 Reactions of Dienes • Ultraviolet and Visible Spectroscopy

Absorbance at 240 nm

Time

CH 3 CH 2 NO 2

nitroethane

++HO− CH 3 CH

nitroethane anion

NO 2 H 2 O

−

max^ = 240 nm

Figure 8.8N
The rate at which a proton
is removed from nitroethane is
determined by monitoring
the increase in absorbance at
240 nm.

Absorbance at 340 nm

Time

+++

pyruvate max = 340 nm

lactate

dehydrogenase

CH 3 CCOO−

O

NADH H+

lactate

CH 3 CHCOO−

OH

NAD+

Figure 8.9N
The rate of reduction of pyruvate
by NADH is measured by
monitoring the decrease in
absorbance at 340 nm.

8.13 Uses of UV/VIS Spectroscopy

Reaction rates are commonly measured using UV/Vis spectroscopy. The rate of

any reaction can be measured, as long as one of the reactants or one of the prod-

ucts absorbs UV or visible light at a wavelength at which the other reactants and

products have little or no absorbance. For example, the anion of nitroethane has a

at 240 nm, but neither (the other product) nor the reactants show any

significant absorbance at that wavelength. In order to measure the rate at which

hydroxide ion removes a proton from nitroethane (i.e., the rate at which the ni-

troethane anion is formed), the UV spectrophotometer is adjusted to measure

absorbance at 240 nm as a function of time instead of absorbance as a function of

wavelength. Nitroethane is added to a quartz cell containing a basic solution, and

the rate of the reaction is determined by monitoring the increase in absorbance at

240 nm (Figure 8.8).

lmax H 2 O