Some Applications of Redox Reactions in Qualitative Analysis
Numerous applications of redox reactions exist in qualitative analysis. Some fall in the realm of organic chemistry, and others in that of inorganic chemistry.
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Some Applications of Redox Reactions in Qualitative Analysis
Numerous applications of redox reactions exist in qualitative analysis. Some fall in the realm of organic chemistry, and others in that of inorganic chemistry.
21.1
Organic Analysis
21.1.1
Colorimetric Analysis
The formation of colored derivatives often results from one or several redox reactions. • Phenols and amines endowed with a free ortho or para position can be characterized with 3-methylbenzothiazoline-2-one hydrazone (mbth);
S N NH2 N mbth
CH3
mbth gives a coupling reaction with phenol in an oxidizing medium. A colored azo derivative forms. The coupling reaction may be achieved in acidic or alkaline medium in the presence, for example, of ceriumIV and ammonium double sulfate:
OH S O
N NH2+ O
CeIV
S N N
N
N
CH3
CH3
O
...etc.
An acute examination of this reaction shows that it proceeds through two oxidization steps. The first one involves the oxidization of mbth, which gives a strongly electrophilic diazonium salt:
J.-L. Burgot, Ionic Equilibria in Analytical Chemistry, DOI 10.1007/978-1-4419-8382-4_21, © Springer Science+Business Media, LLC 2012
405
406
21 Some Applications of Redox Reactions in Qualitative Analysis
S
S N NH2 N
N NH
–2e–,–H+
N CH3 diazonium salt
CH3
The actual coupling reaction (that occurring between the diazonium salt and phenol) is the second step. It is characterized by a loss of two electrons and by a loss of one or two protons according to the acido-basic form phenol was under before the reaction. At least formally, one may imagine a pathway through the intermediary product S N N
N H
OH
–2e–,–2H+
azoic
CH3
hypothetic scheme which loses two electrons. We notice, however, that the two successive reactions have not been seen so far. Aromatic amines give colored derivatives of the same kinds as those obtained with phenols through an analogous reaction. Their formation provides evidence of their presence. They may be formed under the influence of the oxidizing power of FeIII . The analogy of behavior exhibited by amines and phenols is not surprising since the formation of colored derivatives in both cases involves an electrophilic substitution reaction, which is easy because of the very comparable electron-donating effects of hydroxy and amino groups.
S N N
N
N CH3 • The detection of aldehydes with mbth also involves several redox reactions. The formed colored derivatives are cations. Taken as a whole, they result from the condensation of two mbth molecules with one of aldehyde. One of their possible limit structures is the following one:
CH3 N
CH3 R
N
N N C N N S
... etc.
S
It is rational to think that in a first step, a first mbth molecule is condensing with the aldehyde molecule to give the corresponding hydrazone.
21.1 Organic Analysis
407
CH3
CH3
N RCHO + 2HN
N
N
RCH
N
N
S
S
Furthermore, in the oxidizing medium (presence of FeIII ), the hydrazone is oxidized, giving one molecule of the diazonium salt already described:
CH3
CH3
N
N –
N – NH2 – 2e – H
N – NH
+
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