Thermolysis and photolysis of N-benzoylhydrazone derivatives

Article abstract of DOI:10.1007/s00706-011-0532-4

Two selected N-benzoylhydrazones were subjected to thermolysis by refluxing at 200 °C. Benzil, benzoic acid, biphenyl, benzanilide together with the corresponding ketones, nitriles, substituted methanes, and imines were isolated. Similar treatment of a third hydrazone at 250 °C afforded, in addition to the previous products, toluene, bibenzyl, stilbene, and 2-phenylindole. Photolysis of the same hydrazones in acetonitrile gave the previously reported products but in different ratios along with azine derivatives and substituted methanes. A free radical mechanism involving homolysis of the N-N and C-N bonds is suggested, substantiated by trapping of the phenyl radical with isoquinoline, to account for the formation of the identified products.

Full text of DOI:10.1007/s00706-011-0532-4

Monatsh Chem (2011) 142:1021–1027
DOI 10.1007/s00706-011-0532-4
ORIGINAL PAPER
Thermolysis and photolysis of N-benzoylhydrazone derivatives
•
Abd El-Aal M. Gaber Khalid S. Khairou
Received: 28 December 2009 / Accepted: 30 May 2011 / Published online: 28 June 2011
Ó Springer-Verlag 2011
Abstract Two selected N-benzoylhydrazones were sub-
jected to thermolysis by refluxing at 200 °C. Benzil,
benzoic acid, biphenyl, benzanilide together with the cor-
responding ketones, nitriles, substituted methanes, and
imines were isolated. Similar treatment of a third hydra-
zone at 250 °C afforded, in addition to the previous
products, toluene, bibenzyl, stilbene, and 2-phenylindole.
Photolysis of the same hydrazones in acetonitrile gave the
previously reported products but in different ratios along
with azine derivatives and substituted methanes. A free
radical mechanism involving homolysis of the N–N and
C–N bonds is suggested, substantiated by trapping of the
phenyl radical with isoquinoline, to account for the for-
mation of the identified products.
Moreover, benzophenone hydrazone was photolyzed in the
presence of air to yield diphenylmethane as the major
photoproduct, together with benzophenone, benzophenone
imine, and benzophenone azine as minor products [4].
Aldazines undergo N–N cleavage resulting in the forma-
tion of a nitrile and an imine [5]. Interestingly, Bhatnagar
and George [6] have reported that hydrazones of aldehydes
and ketones are oxidized by manganese dioxide to give the
corresponding ketones and biphenyl through a free radical
mechanism.
Furthermore, the hydrazone derivatives are widely
applied in the treatment of diseases such as tuberculosis,
leprosy, mental disorders, and are also used as herbicides,
insecticides, and rodenticides; they can be used as plasti-
cizers and antioxidants [7].
Keywords Thermolysis ꢀ Photolysis ꢀ
The biological activity of benzoylhydrazones [8, 9]
prompted us to clarify the behavior of these compounds when
subjected to high temperature or light radiation, and reinves-
tigate such reactions in an effort to gain further information
about more generalized photolytic and thermolytic mecha-
nisms. Benzoylhydrazones selected in this study contain the
biologically active moiety CO–NH–N=C [10].
N-benzoylhydrazones ꢀ Free radicals ꢀ 2-Phenylindole
Introduction
Thermolysis of the phenylhydrazones of aldehydes and
ketones has been a matter of controversy since Chattaway
et al. [1] reported the thermal breakdown of benzaldehyde
phenylhydrazone into N-benzylideneaniline, stilbene,
ammonia, benzene, and nitrogen. Also, thermolysis of
phenylhydrazone derivatives by refluxing gave ammonia,
benzonitrile, aniline, ketones, 7-methyl-3-phenylindole,
and 9-phenylacridine [2]. Photochemistry of hydrazones
has been extensively studied by Binkley et al. [3].
Results and discussion
A number of preliminary experiments were carried out to
determine the proper temperature for thermolysis. The
decomposition of 1a and 1b starts only above 180 °C and
above 230 °C for 1c. It was found that 200 °C is the lowest
temperature at which the conversion of N-benzoylhydraz-
ones 1a and 1b (250 °C for 1c) was complete at the end of
10 h thermolysis.
Abd El-Aal M. Gaber (&) ꢀ K. S. Khairou
Chemistry Department, Faculty of Applied Science,
Umm Al-Qura University, Makkah, Saudi Arabia
e-mail: gaber1371@hotmail.com
Thermolysis of benzophenone N-benzoylhydrazone (1a)
at 200 °C for 10 h gives benzonitrile (2a), benzanilide (3),
123

1022
Abd El-Aal M. Gaber, K. S. Khairou
benzoic acid (6), benzophenone (10a), benzil (7), diph-
enylmethane (4a), and benzophenone N-phenylimine (9a)
in addition to water, ammonia, and carbon monoxide as
shown in Scheme 1.
tautomerizes into the enolic form and finally produces
benzonitrile and hydroxyl radical [12, 13]. In addition, it
may couple with a phenyl radical, which is readily avail-
able in the reaction medium, to give benzanilide (3).
Moreover, homolysis of the C–N bond (Scheme 1, route b)
is less favorable than the N–N bond as predicted from bond
energy values 213.5 and 300.6 kJ mol-1, respectively [14].
Homolysis of the C–N bond produces benzophenone hyd-
razonyl and benzoyl free radicals. The benzoyl radical is
the precursor of benzoic acid (6) and benzil (7) through
the processes of oxidation and dimerization [15, 16],
respectively. Also, the mechanism of formation of
The nature of the identified products in the hydrazone
pyrolyses (Table 1) points to a free radical mechanism. For
example, the products formed during the thermolysis of
hydrazone 1a involving the homolysis of the N–N bond
(Scheme 1, route a) form benzophenone iminyl and
phenylamidyl radical pairs. The benzophenone iminyl
radical undergoes further fragmentation into phenyl radi-
cals and benzonitrile (2a) [11]. The phenylamidyl radical
O
Ph
N
NH-C-Ph
a, R = Ph
b,R= CH3
R
(b)
(a)
1a, 1b
Δ
(b)
(a)
O
OH
O
Ph
R
O
O
Ph
dimer.
Ph-C-NH
NH
+
Ph-C-C-Ph
Ph-C=N
N
NH + Ph-C
R
7
-CO
Ph
-H
+OH
R-CN + Ph
Ph-COOH
Ph-CN + OH
H
6
2a
2a, 2b
Ph
R
O
N
N
Ph
Ph-C-NH-Ph
N
NH2
N
3
Δ
R
8a, 8b
Ph
R
+ N2
O2
Δ
N
Ph
4a, 4b
Ph
R
Ph
N
+ NH2
O + N2 + H2O
10a, 10b
5
R
H
Ph
N
NH3
Ph
R
Ph
9a, 9b
Scheme 1
123

Thermolysis and photolysis of N-benzoylhydrazone derivatives
1023
reaction and is consistent with the proposed free radical
mechanism [19] (Scheme 1).
Table 1 Thermolysis products of N-benzoylhydrazones 1a–1c (%
yield)
Productsa
Reactant
Similarly, thermolysis of benzaldehyde N-benzoylhyd-
razone (1c) at 250 °C for 10 h leads to the formation of
toluene (4c), benzanilide, benzoic acid, benzil, bibenzyl
(12), stilbene (13), and 2-phenylindole (11) together with
benzonitrile (2a) as major products in addition to water,
ammonia, and carbon monoxide as shown in Scheme 2 and
Table 1.
1a (R = Ph) 1b (R = CH3) 1c (R = H) 1ac
Benzoic acid (6)
Benzil (7)
12
7
11
8
14
10
12
8
10
6
Bibenzyl (12)
Stilbene (13)
Benzanilide (3)
Nitriles 2
–
–
–
–
–
–
Another competing pathway for the thermolysis of
benzaldehyde N-benzoylhydrazone (1c) is the homolysis of
the C–N bond (Scheme 2, route b) furnishing benzalhyd-
razonyl and benzoyl free radicals. The benzalhydrazonyl
radical may undergo isomerization to the corresponding
benzylazoalkane, as observed by other workers, initiated
by free radicals present in the reaction medium [20]. The
isomerized benzylazoalkane undergoes a-scission to form
N2 and the benzyl radical, which can form toluene (4c) and
bibenzyl (12) through H-abstraction and dimerization as
shown in Scheme 2. Stilbene (13) can be formed by free
radical dehydrogenation [21, 22] of bibenzyl, whereas the
benzoyl radicals produce benzoic acid and benzil through
the same mechanism suggested in Scheme 1.
13
15
13
11
–
12
12
14
13
–
8
9
18
–
13
12
10
11
Imines 9
Ketones 10
–
1-Phenylisoquinoline
(5)
–
2-Phenylindole (11)
–
–
12
8 (23)b
–
Substituted methanes 4 10
12
5
8
4d
Recovered hydrazones
6
2
Unresolved residue/g
0.08
0.1
0.05
0.1
a
NH3 and CO were detected by chemical means. H2O formed in less
than 1%
b
Heated in decalin as a solvent (23% yield of toluene)
c
Heated in isoquinoline as a radical trap
d
The formation of benzonitrile (2a) (Scheme 2, route b)
can account for the decomposition of benzaldehyde
hydrazone [23] to give benzaliminyl and ammonia. The
benzaliminyl radical is considered to be the precursor of
benzonitrile and water via reaction with the hydroxyl rad-
ical, which is readily available in the reaction medium.
However, substituted indoles are not formed in the case of
compounds 1a and 1b (R = Ph, CH3). This result reflects
the difference in behavior as a factor of substituents in the
hydrazone series, i.e., absence of a benzyl radical, which is
responsible for the indole formation.
Recovered isoquinoline was collected at b.p. 75–82 °C (7 mbar);
n2D0 = 1.6051; picrate derivative, m.p. and mixed melting point
(m.m.p.) 219–221 °C
diphenylmethane (4a) may be described as a Wolff–Kish-
ner reduction involving an initial hydrogen migration to
form the azo compound followed by loss of nitrogen and
radical recombination [17] (Scheme 1). On the other hand,
the benzophenone hydrazonyl radicals may abstract
hydrogen to form benzophenone hydrazone (8a) which
decomposes to diphenyliminyl radicals and ammonia [1].
The diphenyliminyl radical may couple with a phenyl
radical to form benzophenone N-phenylimine (9a) [18].
Furthermore, oxidation of benzophenone hydrazone by
traces of oxygen in the medium gives rise to benzophenone
(10a), nitrogen, and water as reported earlier [3], as shown
in Scheme 1.
A possible pathway for the formation of 2-phenylindole
(11) is the isomerization of benzaliminyl radical followed
by coupling with benzyl radical (C–C rather than C–N) to
form benzyl phenylketimine. Hydrogen abstraction at the
benzylic position followed by intramolecular cyclization
and hydrogen abstraction produces 2-phenylindole [24] as
shown in Scheme 2.
Analogous results were also obtained in the thermolysis
of acetophenone N-benzoylhydrazone (1b) under similar
conditions: acetonitrile (2b), benzanilide, benzoic acid,
acetophenone (10b), benzil, ethylbenzene (4b), and ace-
tophenone N-phenylimine (9b) in addition to water,
ammonia, and carbon monoxide were formed. The for-
mation of these products is also consistent with the
mechanism suggested in Scheme 1.
Thermolysis of benzaldehyde N-benzoylhydrazone (1c)
in the presence of decalin as a hydrogen donor solvent
under the conditions used afforded a great increase in the
yield of toluene (4c, formed by H-abstraction, 23% yield)
in addition to other minor products. However, the forma-
tion of benzonitrile from both routes a and b correlates with
its high yield among the identified products.
Thermolysis of benzophenone N-benzoylhydrazone (1a)
by refluxing at 200 °C in the presence of isoquinoline as a
radical scavenger afforded the same products mentioned
above, in addition to 1-phenylisoquinoline (5, 11% yield).
This confirms the formation of phenyl radicals during the
Photolysis of N-benzoylhydrazones 1a–1c in acetonitrile
at ambient temperature for 24 h gave benzonitrile, benzoic
acid, and benzil, in addition to the corresponding azines
14a–14c, ketones 10a–10c, and substituted methanes
4a–4c as shown in Scheme 3 and Table 2. The formation
123

1024
Abd El-Aal M. Gaber, K. S. Khairou
O
Ph
H
N
NH-C-Ph
(a) (b)
1c
Δ
(a)
(b)
O
O O
OH
O
dimer.
OH
Ph-C-C-Ph
Ph-CH2-N=N
Ph-CH=N-NH Ph-C
+
Ph-C-NH
Ph-CH=N + Ph-C=N
7
Δ
-CO
-H
H
Δ
Ph
O
Ph-COOH
Ph-CN
Ph-C=NH
Ph-CH=N-NH2
6
Ph-CH2 + N2
Ph-C-NH-Ph
Ph-CN + OH
Ph-CH=N
Ph
3
Δ
+Ph-CH2
dimer.
H
Ph
Ph-CH=N + NH2
NH
-H
Ph-CN + H2O
(as shown in route a)
Ph-CH2-CH2-Ph
Ph-H2C
12
+ OH
disprop.
Ph-CH3
4c
-H2
CH
H2O + Ph-CN + NH3
C
Ph
Ph-CH=CH-Ph
N
H
2a
13
cycl.
-H
Ph
Ph
N
H
N
H
H
11
Scheme 2
of azines 14a–14c only upon photolysis of the investigated
hydrazones 1a–1c indicates the homolysis of the N–N bond
in the excited state forming an iminyl free radical
(Scheme 3, route a) followed by dimerization within the
solvent cage. Also, bibenzyl, stilbene, benzanilide, and
imines were formed only in the thermolysis reactions as a
result of multistage bond fission induced by further heating
of the primary radicals. Benzil, benzoic acid, benzonitrile,
and substituted methanes are formed in all cases.
Conclusions
The thermolysis and photolysis of N-benzoylhydrazones
1a–1c are a complex process in the condensed phase. At least
three independent primary thermolysis processes have been
observed, viz. (a) homolysis of the C–N bond to provide
radicals which gave benzonitrile, benzil, benzanilide, imi-
nes, and ketones; (b) homolysis of the N–N bond to give
radicals which cyclize to 2-phenylindole; (c) homolysis of
the N–CO bond to form benzoic acid, bibenzyl, and stilbene.
The complexities are compounded by intermolecular sec-
ondary processes which lead to the observed products. In
addition, a free radical mechanism is suggested, substanti-
ated by trapping of the phenyl radical with isoquinoline, to
account for the formation of the identified products.
The formation of benzoic acid (6) and benzophenone
(10a) can be explained on the basis of hydrolysis by water
present in the reaction medium (Scheme 3). The yield of
benzoic acid in photolysis is almost twice that obtained in
thermolysis. This may be due to the long period of pho-
tolysis (24 h) compared with thermolysis (10 h).
123

Thermolysis and photolysis of N-benzoylhydrazone derivatives
1025
O
Ph
R
N
NH-C-Ph
(a) (b)
R= Ph, CH3, H
1a-1c
h
υ
(a)
(b)
H2O
O
Ph
R
O
Ph
R
Ph
R
N-NH2
+ Ph-COOH
-NH
C-Ph
+
N
N
+
Ph-C-NH
8a-8c
6
dimer.
O2
dimer.
OH
Ph
Ph
_
O O
Ph-C-C-Ph
7
N=N
Ph-C=N
O
+ N2 + H2O
R
Ph
R
Ph
R
R
10a-10c
N
N
Δ
14a-14c
Ph
+ N2
Ph-CN + OH
R
2a
4a-4c
(as shown in Scheme 1)
Photolysate
Scheme 3
Preparative column chromatographic separations were
performed using a 120 9 2.5-cm glass column packed with
alumina and using the following solvents successively:
petroleum ether (40–60 °C), petroleum ether (60–80 °C),
and mixtures thereof (1:1 and 1:2 v/v); petroleum ether
(60–80 °C)/benzene (1:2 and 2:3 v/v), benzene/ether (2:1
v/v), and finally methanol. Gas–liquid chromatography
(GLC) was carried out using a Perkin-Elmer model Sigma
3B apparatus, 8 9 1/20-inch column, packed with 30% SE
30 on a chromosorb W (35–80 mesh), thermal conductivity
detector, using nitrogen as a carrier gas. GC/MS analyses
were carried out using a Finnigan MAT SSQ 7000 spec-
trophotometer with 5% (phenylmethyl)polysiloxane using
a 30-m DB-1 capillary column. Products were identified by
co-injection with authentic materials and/or by comparison
with known GC/MS library fragmentation patterns.
Table 2 Photolysis products of hydrazone derivatives 1a–1c (%
yield)
Products
Reactant
1a
1b
1c
Benzoic acid (6)
Benzil (7)
21
23
27
12
10
11
Ketones 10
10
12
10
Benzonitrile (2)
Methanes 4
20
18
15
13
14
16
Azine derivatives 14
Residue/g
18
16
17
0.02
0.03
0.01
Experimental
General procedures
Irradiations
All melting points were determined on a Gallenkamp
apparatus. The IR spectra were recorded on a Pye-Unicam
spectrometer model SP 3-100 G. 1H NMR spectra for some
reaction products were obtained using an EM-390
200 MHz NMR spectrometer. Thin-layer chromatography
was carried out using glass plates (10 9 3 cm) coated with
silica gel (25–40 mesh) eluted with ether/pentane (1:4 v/v).
Solvents were Merck Uvasol grade and used as received.
The irradiation apparatus comprised an immersion well
reactor (50 cm3, solidex glass) allowing evacuation, with a
central 125-W high pressure mercury lamp (Philips HPK
125), magnetic stirring, nitrogen atmosphere after degas-
sing the reactant solution, tap water as coolant, and ambient
123

1026
Abd El-Aal M. Gaber, K. S. Khairou
temperature. This apparatus provides a line spectrum with a
range of light greater than 300 nm in the irradiated
solution.
chromatography using petroleum ether (60–80 °C)/ben-
zene (5:1 v/v); m.p. and m.m.p. 162 °C. Benzophenone N-
phenylimine (9a) [32], eluted with benzene; m.p.
110–114 °C. Benzophenone (10a), eluted from column
chromatography using petroleum ether (60–80 °C)/ben-
zene (1:2 v/v); m.p. 49–50 °C; 2,4-DNP derivative, m.p.
and m.m.p. 238 °C. 1-Phenylisoquinoline (5) [33], eluted
from column chromatography using petroleum ether
(60–80 °C)/benzene (2:3 v/v); m.p. and m.m.p. 95 °C;
picrate derivative, m.p. 164 °C. 2-Phenylindole (11) [34],
eluted with successive portions of petroleum ether
(60–80 °C)/benzene (1:2 v/v); m.p. 188–190 °C; picrate
derivative, m.p. and m.m.p. 127 °C. Benzophenone azine
Starting materials
Benzophenone N-benzoylhydrazone (1a) [25], crystallized
from ethanol, m.p. 116–117 °C (Ref. [25] 118 °C). Ace-
tophenone N-benzoylhydrazone (1b) [26], crystallized
from ethanol, m.p. 153–155 °C (Ref. [26] 155 °C). Benz-
aldehyde N-benzoylhydrazone (1c) [27], crystallized from
ethanol, m.p. 204–206 °C (Ref. [25] 205 °C).
1
General method for the thermal decomposition
of hydrazones 1a–1c
(14a) [35], m.p. 163–164 °C; identified by H NMR and
m.m.p. as compared with an authentic sample. Acetophe-
none azine (14b) [36], crystallized on standing and
recrystallized from ethanol; m.p. 122–124 °C. Benzalazine
(14c) [37], m.p. 92–93 °C; a spot with the same Rf value
(0.8) as compared with an authentic sample was visualized
by TLC.
The appropriate hydrazone 1a or 1b (0.046 mol) was
heated under reflux either alone or in 5 cm3 isoquinoline at
200 °C, while hydrazone 1c was heated either alone or in
decalin as a solvent at 250 °C for 10 h. The products NH3
and CO were detected by Nessler’s reagent and palladium
chloride [26], respectively. The pyrolysate was separated
via fractional distillation up to 180 °C whereby water,
toluene, ethylbenzene (4b), and acetonitrile (2b) were
isolated, followed by distillation under reduced pressure to
afford the following compounds.
General method for the photolysis
of N-benzoylhydrazone derivatives 1a–1c
A solution of N-benzoylhydrazone derivatives 1a–1c
(0.004 mol) in 100 cm3 acetonitrile was irradiated at
ambient temperature in a Pyrex apparatus under nitrogen
until 1a–1c completely disappeared (24 h according to
TLC monitoring). The photolysate was separated into
amine and neutral products by extracting with hydrochloric
acid (0.1 M) as previously described [38]. Samples were
analyzed by GLC, and products were identified by com-
parison with authentic samples and quantified using
nitrobenzene as an internal standard. The results are sum-
marized in Table 2.
Toluene (4c), b.p. 60–65 °C (4 mbar); its GLC revealed a
single peak at 1.0 min comparable with an authentic sample
using an 8 9 1/20-inch SE 30 column at 90 °C. Acetophe-
none (10b), b.p. 82–85 °C (4 mbar); n2D0 = 1.5325; 2,4-
dinitrophenylhydrazone (2,4-DNP derivative), m.p. and
m.m.p. 250 °C. Diphenylmethane (4a) [28], b.p. 95–100 °C
(4 mbar); n2D0 = 1.5788; on oxidation with K2Cr2O7/H2SO4
gave benzophenone; 2,4-DNP derivative, m.p. 238 °C.
Acetophenone N-phenylimine (9b) [29], b.p. 180–185 °C
(4 mbar); m.p. 41–42 °C. Benzonitrile (2a), b.p. 41–45 °C
(4 mbar); n2D0 = 1.527; on hydrolysis gave benzoic acid,
m.p. and m.m.p. 121 °C.
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