Chemistry Reference
In-Depth Information
Back to the “classical” quantum structure-reactivity interpretations, among phenols,
pyrocatechol (II) and pyrogallol (IV) are the most reducing ones. Consequently, they
are used as photographic developer. The alkaline solution of the 1,2,3-trihydroxy
phenol (IV) quickly and quantitatively absorbs oxygen, serving for gas analysis
(Nenitzescu
1966
, Part II, Chap. 2). Phloroglucinol (V), resorcinol (III) and naphtols
(VI-VIII) have the practical and theoretical support in their tendency to ketolyse
(Avram
1994
, Chap. 19). Along with the extension of the conjugated bonds system,
the maximum absorption bands moves toward visible (batochromic effect) (Hen-
drickson et al.
1976
, Chap. 7). In this respect, naphtols are more suitable as coupling
agents (Avram
1995
, Chap. 22; Nenitzescu
1966
, Part II, Chap. 5). The structural
indices calculated for different atoms in hydroxy arenes (Table
11.14
) allow us to
establish some structure-property correlations in organic chemical reactions.
The values
ρ
O
>ρ
C
;F
O
>
F
C
correlate with the higher oxygen electronegativity
than that of the carbon atom (Putz
2006
, 2007) and also with the potential contours
presented in Fig.
11.10
.
The reactions occurring at nucleus with electrophilic substitution mechanism:
halogenation, nitrosation, sulfonation, nitration, alkylation, and coupling take place
preferentially at carbon atoms possessing the highest electron density. Therefore,
the electrophilic reagents turn to C(2) and C(4) of phenol, C(1) of 2-naphthol- 3,6-
sodium disulfonate, C(4) of 1-naphthol. The 1,3,5-trihydroxylbenzene carbon atoms
are favored by high electron densities, but certainly in substitution reactions they
experience steric hindrance (Avram
1994
, Chap. 19). The high free valences of some
atoms in these molecules indicate their increased reactivity in reactions with both
ionic and radicalic mechanisms. The low free valences of the carbon atoms to which
hydroxyl groups are bounded confirm that, opposed to alcohols, hydroxyarenes do
not participate to reaction during which carbon-oxygen bond is broken (Nenitzescu
1966
, Part II, Chap. 2).
Increased reactivity of the hydroxyl group of these compounds show the het-
erolytic cleavage of the bond during neutralization, etherification, esterification,
and also colligation of some intermediary formed radicals (Avram
1994
, Chap. 19;
Nenitzescu
1966
, Part II, Chap. 2).
11.6
Structure and Reactivity of Aromatic Compounds Involved
in Azo Dyes Synthesis
Our interest for such compounds resides in their participation to diazotisation and
coupling reactions, both as substrates and electrophilic reactants. The aromatic
amines and mononuclear/polynuclear hydroxyarenes undergo electrophilic substi-
tution reactions with a higher rate than their reference compounds, benzene and
naphthalene, respectively: k
Ar
−
Y
/k
ArH
>
1(Y
=
OH, NH
2
) (Nenitzescu
1968
, Part
IV). This behavior is due to the ring activation by conjugation of the non-participating
electrons of nitrogen and oxygen atoms with the
electrons of the aromatic ring
(Avram
1994
, Chap. 19). In this respect, the aromaticity of these molecules can be
π
