Chemistry Reference
In-Depth Information
OH
O
O
O
O
-H
Phenol
Phenoxy radical
H
H
H
H
H
N
N
N
N
-H
etc.
Range of coupling reactions
Figure 8.7
Oxygen-hydrogen and nitrogen-hydrogen bond cleavage.
In the case of oxidation of phenols, the reaction can very quickly give
a complex mixture of products. This is because the phenoxy radical formed
on abstraction of the hydrogen radical, H
.
can give rise to carbon-carbon,
carbon-oxygen and oxygen-oxygen coupling reactions.
The pH at which the phenol is stored is also important since a
phenoxide ion, formed at high pH, can easily be oxidised to the phenoxy
radical (Figure 8.8).
Drugs containing phenolic groups include the analgesics morphine
(and related opiates) and paracetamol as well as the bronchodilator
salbutamol, widely used in the treatment of acute asthma. See Figure 8.9.
Drugs that contain two phenolic groups, such as adrenaline
(epinephrine) and other catecholamines such as noradrenaline (nor-
epinephrine) and isoprenaline are particularly susceptible to oxidation and
have to be formulated at acidic pH. All of these compounds are white crys-
talline solids that darken on exposure to air. Adrenaline forms the red
coloured compound adrenochrome on oxidation (Figure 8.10), which can
further polymerise to give black compounds similar in structure to melanin,
the natural skin pigment. Injections of adrenaline that develop a pink
colour, or that contain crystals of black compound, should not be used for
this reason. Adrenaline for injection is formulated as the acid tartrate
