Organic Chemistry

Summary 329

Absorbance at 287 nm

pH

pH = pKa

Figure 8.10
The absorbance of an aqueous solution of
phenol as a function of pH.

The of a compound can be determined by UV/Vis spectroscopy if either the
acidic form or the basic form of the compound absorbs UV or visible light. For exam-
ple, the phenoxide ion has a at 287 nm. If the absorbance at 287 nm is determined
as a function of pH, the of phenol can be ascertained by determining the pH at
which exactly one-half the increase in absorbance has occurred (Figure 8.10). At this
pH, half of the phenol has been converted into phenoxide. Recall from Section 1.20
that the Henderson–Hasselbalch equation states that the of the compound is the
pH at which half the compound exists in its acidic form and half exists in its basic
form
UV spectroscopy can also be used to estimate the nucleotide composition of DNA.
The two strands of DNA are held together by both A–T base pairs and G–C base pairs
(Section 27.4). When DNA is heated, the strands break apart. Single-stranded DNA
has a greater molar absorptivity at 260 nm than does double-stranded DNA. The melt-
ing temperature of DNA is the midpoint of an absorbance-versus-temperature
curve (Figure 8.11). For double-stranded DNA, increases with increasing numbers
of G–C base pairs because they are held together by three hydrogen bonds, whereas
A–T base pairs are held together by only two hydrogen bonds. Therefore, can be
used to estimate the number of G–C base pairs. These are just a few examples of the
many uses of UV/Vis spectroscopy.

Tm

Tm

1 Tm 2

([HA]=[A-]).

pKa

pKa

lmax

pKa

Absorbance at 260 nm

Temperature (°C)

T = Tm

Figure 8.11

The absorbance of a solution of DNA as a

function of temperature.

Summary

Dienes are hydrocarbons with two double bonds.
Conjugated double bondsare separated by one single
bond. Isolated double bondsare separated by more than
one single bond. Cumulated double bondsare adjacent to
each other. A conjugated diene is more stable than an
isolated diene, which is more stable than a cumulated
diene. The least stable alkene has the greatest -¢H°value.

A diene can have as many as four configurational isomers:

E-E,Z-Z,E-Z, and Z-E.

An isolated diene, like an alkene, undergoes only 1,2-

addition. If there is only enough electrophilic reagent to add

to one of the double bonds, it will add preferentially to the

more reactive bond. A conjugated diene reacts with a limit-

ed amount of electrophilic reagent to form a 1,2-addition

PROBLEM 24

Describe how one could determine the rate of the alcohol dehydrogenase catalyzed oxida-

tion of ethanol by NAD+(Section 20.11).