Effect of medium on acid-catalyzed decomposition of N-(phenylazo)-substituted nitrogen heterocycles

Article abstract of DOI:10.1135/cccc19991654

Four N-(phenylazo)-substituted saturated nitrogen heterocyclics were synthesized and their structure was confirmed by ¹H and ¹³C NMR spectroscopy. The kinetics of their acid-catalyzed decomposition were studied at various concentrations of the catalyst (pivalic acid) in 40, 30, and 20% (v/v) aqueous ethanol at 25 °C. The values obtained for the observed rate constants were processed by the non-linear regression method according to the suggested kinetic models and by the method of principal component analysis (PCA). The interpretation of the results has shown that the acid-catalyzed decomposition of the heterocyclics under the conditions used proceeds by the mechanism of general acid catalysis, the proton being the dominant catalyst particle of the rate-limiting step. The decrease in the observed rate constant at higher concentrations of the catalyst was explained by the formation of a non-reactive complex composed of the undissociated acid and the respective N-(phenylazo)hetcrocycle. The effect of medium and steric effect of the heterocyclic moiety on the values of catalytic rate constant are discussed.

Full text of DOI:10.1135/cccc19991654

1654
Ludwig, Bednářová, Pařík:
EFFECT OF MEDIUM ON ACID-CATALYZED DECOMPOSITION
OF N-(PHENYLAZO)-SUBSTITUTED NITROGEN HETEROCYCLES
1,
2
Miroslav LUDWIG *, Iva BEDNÁŘOVÁ and Patrik PAŘÍK
Department of Organic Chemistry, Faculty of Chemical Technology, University of Pardubice,
1
Čs.legií 565, CZ-532 10 Pardubice, Czech Republic; e-mail: miroslav.ludwig@upce.cz,
2
patrik.parik@upce.cz
Received Jun e 9, 1999
Accepted Septem ber 2, 1999
Dedicated to Professor Otto Exner on the occasion of his 75th birthday in recognition of his contri-
bution to physical organic chemistry and chemometrics.We are proud to have had the possibility to
collaborate with him for a long time.
Four N-(ph en ylazo)-substituted saturated n itrogen h eterocyclics were syn th esized an d th eir
structure was con firm ed by 1H an d 13C NMR spectroscopy. Th e kin etics of th eir
acid-catalyzed decom position were studied at various con cen tration s of th e catalyst (pivalic
acid) in 40, 30, an d 20% (v/v) aqueous eth an ol at 25 °C. Th e values obtain ed for th e ob-
served rate con stan ts were processed by th e n on -lin ear regression m eth od accordin g to th e
suggested kin etic m odels an d by th e m eth od of prin cipal com pon en t an alysis (PCA). Th e in -
terpretation of th e results h as sh own th at th e acid-catalyzed decom position of th e
h eterocyclics un der th e con dition s used proceeds by th e m ech an ism of gen eral acid cataly-
sis, th e proton bein g th e dom in an t catalyst particle of th e rate-lim itin g step. Th e decrease in
th e observed rate con stan t at h igh er con cen tration s of th e catalyst was explain ed by th e for-
m ation of a n on -reactive com plex com posed of th e un dissociated acid an d th e respective
N-(ph en ylazo)h eterocycle. Th e effect of m edium an d steric effect of th e h eterocyclic m oiety
on th e values of catalytic rate con stan t are discussed.
Key w ord s: Reaction kin etics; Mech an ism ; Solven t effects; Triazen es; Ch em om etrics.
Th e in vestigation of relation sh ips between th e structure of organ ic com -
poun ds an d th eir reactivity in volvin g th e effect of m edium is an im portan t
task of ph ysical organ ic ch em istry. Suitable m odel system s for experim en tal
m easurem en ts with th e purpose of elucidation of th ese relation sh ips in -
volve com poun ds with triazen e skeleton . Th ese derivatives are readily solu-
ble in m ost organ ic solven ts (am fiprotic, protic) an d various water–organ ic
solven t m ixtures. Triazen es exh ibit h igh reactivity toward various reagen ts.
Th ere h ave been publish ed for exam ple studies of base-catalyzed cyclization
of 1-[2-(m eth oxycarbon yl)ph en yl]-3-(2-substituted ph en yl)triazen es in
Collect. Czech. Chem. Commun. (Vol. 64) (1999)

Effect of Medium
1655
aqueous m eth an olic buffers with pH 7.5–11.7 (ref.1) goin g by BAc2 m ech a-
n ism with specific base catalysis. Majority kin etic studies of structurally re-
lated triazen es exh ibit h igh reactivity toward electroph ilic reagen ts, th e
proton in particular. In th e presen t paper, we h ave ch osen th e N-(ph en yl-
azo)h eterocycles 1a–1d as th e m odel com poun ds.
8
14
8
13
9
13
9
12
1
2
3
1
2
3
4
4
7
12
7
N N N
N N N
O
6
8
5
9
10 11
6
8
5
9
10 11
1a
1b
14
15
13
12
1
2
3
1
2
3
13
12
4
4
7
7
N N N
N N N
6
5
10 11
6
5
10
11
1c
1d
Th is work follows our previous papers focusin g upon acid-catalyzed de-
com position of various triazen es in various type of m edia (for exam ple
refs2–5).
Th e triazen es of th is type h ave n ot yet received m uch atten tion in th e lit-
erature; in deed, N-(ph en ylazo)perh ydroazepin e h as n ot been cited at all.
Th e oth er derivatives are biologically active. An tiviral an d in secticidal prop-
erties were foun d6 with N-(ph en ylazo)piperidin e an d N-(ph en ylazo)-
pyrrolidin e. Th ey are effective again st m osaic viruses, dom estic flies, red
spider m ites, an d bean beetles. Ron destvedt an d Davis7 dealt in a com plex
way with studies of 3,3-dialkyl-1-ph en yltriazen es, i.e., com poun ds with th e
sam e basic skeleton as th at in th e N-(ph en ylazo)h eterocycles ch osen by us.
Som e of th ese derivatives sh owed an ti-tum our activity. Active tum our in h i-
bition was proved in th e case of 3,3-dim eth yl-1-ph en yltriazen e itself or in
its derivatives h avin g 4-n itro, 4-m eth oxy, 2-m eth yl, an d 3-trifluorom eth yl
groups in th e ph en yl rin g. However, th is activity was n ot foun d with
N-(ph en ylazo)h eterocycles. Som e triazen es appear as in term ediates in syn -
th eses of aryl iodides from an ilin es8, th ese syn th eses bein g distin guish ed by
h igh degrees of con version . Also described was a m eth od of preparation of
aryl iodides in volvin g N-(arylazo)pyrrolidin e as in term ediate. A reaction of
N-(ph en ylazo)piperidin e with tin radicals h as been reported9 to produce
ph en yl radicals.
A gen eral m eth od of preparation of triazen es con sists in th e azo couplin g
of th e correspon din g diazon ium salt to am in e in n eutral to m ildly basic
m edium 7. Th is procedure is also applicable to preparation s of th e m odel
N-(ph en ylazo)h eterocycles ch osen by us. Th e startin g m aterials for th ese
Collect. Czech. Chem. Commun. (Vol. 64) (1999)

1656
Ludwig, Bednářová, Pařík:
syn th eses are th e correspon din g saturated n itrogen h eterocycles an d
ben zen ediazon ium ch loride.
Com poun ds with triazen e skeleton are very easily decom posed in acid
m edia. Proton ation of th e m olecule can take place at an y of th e th ree n itro-
gen atom s presen t, as it follows from th e electron den sity distribution 10–12
,
but th e triazen e ch ain is on ly split after th e proton ation at N-3 atom , wh ere
th ere is n o double bon d. Th e proton ation s at th e two rem ain in g n itrogen
atom s are on ly side equilibria. On th e basis of fin din gs in paper3 dealin g
with sim ilar problem s of pivalic acid-catalyzed decom position of 3,3-dialkyl-
1-ph en yltriazen es in 40% (v/v) aqueous eth an ol, we can con sider two
m ech an ism s13,14 of acid-catalyzed decom position of th e m odel N-(ph en yl-
azo)h eterocycles in aqueous eth an olic m edia: th e specific acid catalysis (A1)
an d th e gen eral acid catalysis (A-SE2). Th e form er presum es th e proton ation
at N-3 atom of triazen e skeleton as a rapid pre-equilibrium an d subsequen t
decom position of th e proton ated in term ediate as th e rate-lim itin g step. Th e
latter in volves sim ultan eous proton tran sfer to N-3 an d th e N-2–N-3 bon d
splittin g in th e rate-lim itin g step. In both th e cases, th e products are th e
correspon din g diazon ium salts an d h eterocyclics, i.e., th e com poun ds used
in th e preparation of th e m odel substan ces.
An an alysis of m ech an ism of specific acid catalysis leads to th e con clu-
sion th at in creasin g steric requirem en ts on th e part of th e h eterocyclic m oi-
eties could possibly cause an acceleration of decom position of th e
proton ated substrate an d h en ce also an acceleration of th e wh ole reaction .
Th is con clusion directly follows from th e defin ition of A1 m ech an ism : it
in volves th e proton tran sfer from th e acid catalyst to sterically h in dered
N-3 atom in a rapid pre-equilibrium step; th erefore, th e steric effects of
h eterocyclic m oieties (h in derin g th e access of th e acid to th e reaction cen -
tre) do n ot affect th e overall reaction rate.
In th e gen eral acid catalysis, wh ich was con firm ed for com poun ds with
triazen e skeleton by m easurin g th e kin etics of decom position of substituted
3-m eth yl-1,3-diph en yltriazen es an d subsequen t m ath em atical m odellin g of
th is reaction 13,14, th ere are two possible path ways: th e catalysis by th e pro-
ton an d th at by gen eral acid. Th e catalysis by th e dim er of carboxylic acid
is n ot con sidered likely in aqueous eth an olic m edia. In th e case of gen eral
acid catalysis, an in creasin g slowin g down of th e reaction with in creasin g
steric requirem en ts of th e h eterocyclic m oieties sh ould be observable, be-
cause th is m ech an ism in volves th e proton tran sfer from a sterically h in -
dered acid to th e reaction cen tre in th e rate-lim itin g step, th e steric
h in dran ce at th e reaction cen tre th us bein g fully m an ifested.
Collect. Czech. Chem. Commun. (Vol. 64) (1999)

Effect of Medium
1657
EXPERIMENTAL
Spectroscopy
Th e 1H an d 13C NMR spectra of th e N-(ph en ylazo)h eterocyclics prepared were m easured on
an AMX 360 spectrom eter (Bruker) at 360.14 an d 90.56 MHz, respectively, as 5% CDCl3
pyrrolidin e an d m orph olin e derivatives or (CD3)SO piperidin e an d perh ydroazepin e deriva-
tives solution s. Th e UV-VIS spectra of th e N-(ph en ylazo)h eterocyclics dissolved in 40% (v/v)
aqueous eth an ol were m easured on a diode-array spectroph otom eter HP 8452A (Hewlett–
Packard). Th e NMR ch em ical sh ifts, referen ced to th e solven t used, th e wavelen gth s of th e
lon gest-wave UV-VIS m axim a are presen ted below.
Syntheses of Intermediates and Model Substances
Preparation of Perhydroazepine by Reduction of Hexano-6-lactam (Modified According to ref.15
)
Syn h ydride (272.5 m l, 0.99 m ol) was placed in a 1-liter Erlen m eyer flask equipped with an
Y-part with a reflux con den ser an d a droppin g fun n el. A solution of h exan o-6-lactam
(19.1 g, 0.41 m ol) in toluen e (200 m l) was added dropwise in to th e reducin g agen t stirred
with a m agn etic stirrer (th e reaction is stron gly exoth erm ic). After addin g all th e lactam , th e
reaction m ixture was stirred for an oth er 2 h an d th en refluxed. After coolin g an d pourin g
in to a m ixture of h ydroch loric acid an d ice (500 m l; 1 : 1), th e organ ic ph ase was separated
an d extracted with dilute h ydroch loric acid (2 × 200 m l; 1 : 1). Th e aqueous ph ase was
m ixed with th e two extracts an d th e resultin g m ixture was extracted with eth er. Th e extract
was evaporated on a vacuum evaporator to a sm all volum e, alkalin ized with con cen trated
sodium h ydroxide solution , an d steam -distilled. Th e distillate was saturated with solid potas-
sium h ydroxide, an d th e separated perh ydroazepin e was extracted with eth er (200 m l). Th e
eth ereal extract was evaporated an d th e residue was distilled to give 35 m l (0.31 m ol)
perh ydroazepin e (75% yield).
Preparation of N-(Ph en ylazo)h eterocycles – (Gen eral Procedure Modified Accordin g to ref.16
)
A solution of sodium acetate (16.4 g, 0.2 m ol) in water (20 m l) was m ixed with th e corre-
spon din g h eterocyclic (0.2 m ol) in a 500 m l beaker. Th e solution th us form ed was stirred at
a tem perature below 5 °C, an d a cold solution of ben zen ediazon ium ch loride (0.2 m ol) pre-
pared in a usual way was added, m ain tain in g th e pH of reaction m ixture at about 7. Th e
separated solid was collected by suction , wash ed with water, dried, an d purified by
recrystallization .
N-(Phenylazo)piperidine (1a). Yellow crystals. Yield: 16%, m .p. 38–40 °C (ben zen e), (ref.17
gives 43 °C). For C11H15N3 (189.3) calculated: 69.81% C, 7.99% H, 22.20% N; foun d: 69.98% C,
8.06% H, 22.88% N. 1H NMR: 7.20 m , 1 H (H-7); 7.38–7.41 m , 4 H (H-5, H-6, H-8, H-9);
1.66 s, 6 H (H-11, H-12, H-13); 3.77 s, 4 H (H-10, H-14). 13C NMR: 120.23, 2 C (C-5, C-9);
125.51, 1 C (C-7); 128.89, 2 C (C-6, C-8); 150.36, 1 C (C-4); 23.77, 5 C (C-10, C-11, C-12,
C-13, C-14). UV: 314 n m .
N-(phenylazo)morpholine (1b). Oran ge crystals. Yield: 19%, m .p. 25–27 °C (ben zen e), (ref.18
gives 29–30 °C). For C10H13N3O (191.2) calculated: 62.81% C, 6.85% H, 21.97% N; foun d:
62.94% C, 6.93% H, 21.95% N. 1H NMR: 7.17 m , 1 H (H-7); 7.31–7.35 m , 2 H (H-6, H-8);
Collect. Czech. Chem. Commun. (Vol. 64) (1999)

1658
Ludwig, Bednářová, Pařík:
7.42–7.45 m , 2 H (H-5, H-9); 3.73–3.76 m , 4 H (H-11, H-12); 3.80–3.83 m , 4 H (H-10, H-13).
13C NMR: 121.15, 2 C (C-5, C-9); 126.42, 1 C (C-7); 128.84, 2 C (C-6, C-8); 150.04, 1 C
(C-4); 47.94, 2 C (C-10, C-13); 66.31, 2 C (C-11, C-12). UV: 290 n m .
N-(phenylazo)pyrrolidine (1c). Yellow crystals. Yield: 12%, m .p. 47–49 °C (dieth yl eth er),
(ref.7 gives 47–49 °C). For C10H13N3 (175.2) calculated: 68.54% C, 7.48% H, 23.98% N;
foun d: 68.85% C, 7.44% H, 23.17% N. 1H NMR: 7.10 m , 1 H (H-7); 7.28–7.32 m , 2 H (H-6,
H-8); 7.38–7.40 m , 2 H (H-5, H-9); 1.98–2.02 m , 4 H (H-11, H-12); 3.77 s, 4 H (H-10, H-13).
13C NMR: 120.32, 2 C (C-5, C-9); 125.11, 1 C (C-7); 128.79, 2 C (C-6, C-8); 150.39, 1 C
(C-4); 23.79, 4 C (C-10, C-11, C-12, C-13). UV: 314 n m .
N-(phenylazo)perhydroazepine (1d ). Yellow crystals. Yield: 16%, m .p. 19–21 °C (dieth yl
eth er). For C12H17N3 (203.3) calculated: 70.90% C, 8.43% H, 20.67% N; foun d: 71.23% C,
e
8.51% H, 20.70% N. 1H NMR (a axial, equatorial): 7.14 m , 1 H (H-7); 7.35–7.36 d, 4 H
(H-5, H-6, H-8, H-9); 1.58–1.61m , 4 H (H-12, H-13); 1.77 s an d 1.87 s, 2 H an d 2 H (H-11a,
H-14a an d H-11e, H-14e); 3.76 t an d 4.00 t, 2 H an d 2 H (H-10a, H-15a an d H-10e, H-15e).
13C NMR: 120.09, 2 C (C-5, C-9); 124.79, 1 C (C-7); 128.80, 2 C (C-6, C-8); 150.90, 1 C
(C-4); 24.41, 27.68, 28.38, 29.69, 4 C (C-11, C-12, C-13, C-14); 48.04, 53.61, 2 C (C-10,
C-15). UV: 310 n m .
Kinetic Measurements, Determination of pKHA of Acid Catalyst
Pivalic acid (an alytical grade, Fluka) was dissolved in 40, 30, or 20% (v/v) aqueous eth an ol
to obtain th e h igh est con cen tration an d (by dilutin g with th e sam e solven ts) th e lower con -
cen tration s used for kin etic m easurem en t. Th e con cen tration of acid catalyst was th en deter-
m in ed titrim etrically with 0.1 M tetrabutylam m on ium h ydroxide usin g Titralab3 autom atic
titrator with poten tiom etric in dication (Radiom eter) an d a glass an d SCE electrodes with sat-
urated m eth an olic KCl solution .
Th e sam e apparatus was also used to determ in e th e pKHA value of pivalic acid in th e re-
spective water–eth an ol m ixtures (40%, pKHA = 6.33; 30%, pKHA = 5.95; 20%, pKHA = 5.65),
usin g ben zoic acid as th e stan dard (its pKHA values were taken from ref.19: 40%, pKHA
5.35; 30%, pKHA = 5.07; 20%, pKHA = 4.80).
=
Th e pivalic acid-catalyzed decom position of N-(ph en ylazo)h eterocyclics was m easured in
40, 30, or 20% (v/v) aqueous eth an olic solution usin g th e sam e spectroph otom eter as above.
Th e m easurem en ts were carried out at th e wavelen gth s of th e respective lon gest-wave
UV-VIS m axim a. A 1-cm quartz cell was ch arged with ca 2 m l of a pivalic acid an d, after at-
tain in g th e tem perature of 25.0 ± 0.1 °C in th e cell com partm en t, 1–10 µl of N-(ph en ylazo)-
h eterocyclic com poun d solution in dioxan e was in jected with a Ham ilton syrin ge, an d th e
cell con ten t was th orough ly m ixed. Th e absorban ce decrease was followed at th e
above-m en tion ed wavelen gth for a period of four h alf-lives of th e reaction . Th e experim en -
tal data obtain ed were evaluated an d th e rate con stan ts were obtain ed usin g th e optim iza-
tion program in cluded in th e program set OPGM. We also carried out m easurem en ts to
con firm th e fact th at th e acid-catalyzed decom position of th e N-(ph en ylazo)h eterocyclics
proceeds as a pseudo-first-order reaction . In con n ection with our previous paper3 dealin g
with th e acid-catalyzed decom position of 3,3-dialkyl-1-ph en yltriazen es with pivalic acid in
40% (v/v) aqueous eth an ol at 25 °C, we h ave n ow also m easured acid-catalyzed decom posi-
tion of 3,3-dicycloh exyl-1-ph en yltriazen e in 30 an d 20% (v/v) aqueous eth an ol un der th e
sam e con dition s. Th e observed rate con stan ts of all decom position s are given in Table I.
Collect. Czech. Chem. Commun. (Vol. 64) (1999)

Effect of Medium
1659
TABLE I
Observed rate con stan ts for acid-catalyzed decom position of N-(ph en ylazo)h eterocyclics
1a–1d in various solven ts
40% (v/v) aqueous eth an ol, kobs 103, s–1
Substrate/cHA
0.0101
0.0308
0.0503
0.1007
0.1510
0.1985
1a
1b
1c
1d
0.4878
0.4545
1.1205a
0.3989
1.0750
0.7740
1.9643a
0.6718a
1.8460
1.3470
2.6170
0.9306a
2.3220
1.7310
3.0930
1.0380
2.1800
1.9740
3.3730
1.1450
2.8690
1.9920
4.1200a
1.4550
Substrate/cHA
0.2489
0.2992
0.3300
0.3607
0.3999
0.4502
1a
1b
1c
1d
2.8930
2.1410
4.6250a
1.5940
2.9460
2.1540
4.8610
1.5670
2.8850
2.0930
4.7980
1.5320
2.8620
2.1160
4.5970
1.4115a
2.7130
2.0560
4.1930a
1.3240a
2.5860
1.9870
4.2980a
1.3500
Substrate/cHA
0.5006
0.5593
0.5872
0.6432
0.6966
1a
1b
1c
1d
2.4990
2.0770
3.9760
1.2730
2.3625a
1.9830
3.6490a
1.0610a
2.3150
1.8980
3.7880a
1.1160
2.2110
1.9280
3.6460
1.0830
2.1420
1.8990a
3.5500a
0.9766
30% (v/v) aqueous eth an ol, kobs 102, s–1
Substrate/cHA
0.0101
0.0309
0.505
0.1010
0.1515
0.1992
1a
0.4059
0.2848
0.3758
0.1768
1.8770
0.5819
0.4311
0.6466
0.2536
3.5190
0.7698
0.5262
0.8015
0.3187
4.2870
0.9188
0.6246a
0.9748
0.4088a
5.5700
1.0230
0.7396
1.0140
0.4208a
6.9800
1.0380
0.7698
1.0370
0.3938
6.5980
1b
1c
1d
DCTAZb
Substrate/cHA
0.2497
0.3002
0.3507
0.4012
0.4517
0.5022
1a
1.0080
0.7641
1.0160
0.3927
6.4110
0.9181
0.6970
1.0120
0.3800
5.3650
0.7224
0.6397
0.9455
0.3585
4.5470
0.6488
0.5820
0.8850
0.3030
2.8920
0.5799
0.5392
0.8326
0.2605
2.6390
0.5847
0.5195a
0.8302
0.2562
2.3380
1b
1c
1d
DCTAZb
Collect. Czech. Chem. Commun. (Vol. 64) (1999)

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Ludwig, Bednářová, Pařík:
TABLE I
(Continued)
30% (v/v) aqueous eth an ol, kobs 102, s–1
Substrate/cHA
0.5611
0.5892
0.6453
0.7014
1a
0.5239
0.4652a
0.7909
0.2295
1.6850
0.5020
0.4785
0.7654
0.2211
1.6410
0.4676
0.4273a
0.7305
0.2076
1.4120
0.4398
0.3924a
0.6586a
0.1817
1.2300
1b
1c
1d
DCTAZb
20% (v/v) aqueous eth an ol, kobs 102, s–1
Substrate/cHA
0.0101
0.0302
0.0504
0.0907
0.1492
0.1996
1a
0.9313
0.6200
0.8040
0.4251
0.0993
1.6660
1.0500
1.2120
0.6910
0.1668
2.0540
1.2380
1.5250
0.8197
0.3264a
2.4990
1.4390
2.0820
0.9588
0.4851a
2.9280
1.7730
2.1840
1.0420
0.6138
3.1720
1.9230
2.5090
1.0440
0.7294
1b
1c
1d
DCTAZb
Substrate/cHA
0.2500
0.3004
0.3508
0.3991
1a
3.3370
2.0140
2.5220
1.0380
0.8024
3.3220
2.0580
2.5550
0.9085
0.9491
3.0630
1.9370
2.4900
0.7718
1.1183a
2.9495a
1.7410
2.4630
0.6020a
0.9485a
1b
1c
1d
DCTAZb
a
b
Average value from m ore m easurem en ts; 3,3-dicycloh exyl-1-ph en yltriazen e.
RESULTS AND DISCUSSION
Syntheses, Spectral and Kinetic Results
Th e syn th esis of N-(ph en ylazo)h eterocyclics was accom plish ed by th e clas-
sic azo couplin g of ben zen ediazon ium salt an d th e correspon din g saturated
n itrogen h eterocycle. Aqueous sodium acetate proved to be th e best cou-
plin g m edium for all th e four derivatives prepared. pH ≈ 7 h ad to be kept
th rough out th e couplin g reaction with particular atten tion to avoid its low-
Collect. Czech. Chem. Commun. (Vol. 64) (1999)

Effect of Medium
1661
erin g to th e acid region . It was also im portan t to keep th e m ixture cold to
preven t th e diazon ium salt from decom position , especially at th e begin -
n in g of th e couplin g reaction . Oth erwise, th e reaction proper presen ted n o
problem s.
Th e structure of N-(ph en ylazo)h eterocyclics was con firm ed by th eir 1H
an d 13C NMR spectra. Th e results of NMR m easurem en ts alon g with assign -
m en t of in dividual ch em ical sh ifts to th e correspon din g h ydrogen or car-
bon atom s are given in Experim en tal. In th e NMR spectra of N-(ph en ylazo)-
h eterocycles, som e ch em ical sh ifts of h ydrogen or carbon atom s coalesce
in to a sin gle sign al due to dyn am ic ch an ges in th e m olecule. On th e oth er
h an d, th e 1H NMR spectrum of N-(ph en ylazo)perh ydroazepin e clearly dif-
feren tiates between axial an d equatorial h ydrogen atom s. In th e 1H NMR
spectrum of N-(ph en ylazo)m orph olin e, ch em ical sh ifts of th e pairs of
ch em ically equivalen t h ydrogen atom s in th e m orph olin e m oiety are very
close, wh ich preven ts th eir un am biguous assign m en t.
For th e acid-catalyzed decom position of N-(ph en ylazo)h eterocyclics m ea-
sured in 40, 30, or 20% (v/v) aqueous eth an ol, we ch ose pivalic acid as a
catalyst because of its con siderable steric requirem en ts. Th e observed rate
con stan ts kobs of th e acid-catalyzed decom position of N-(ph en ylazo)-
h eterocycles with pivalic acid in 40, 30, an d 20% (v/v) aqueous eth an ol are
presen ted in Table I alon g with th e values foun d for 3,3-dicycloh exyl-
1-ph en yltriazen e in 30 an d 20% (v/v) aqueous eth an ol. A graph ical repre-
sen tation for N-(ph en ylazo)m orph olin e decom position in 30% (v/v) aque-
ous eth an ol is given as an exam ple in Fig. 1 as a plot of kobs vs th e pivalic
acid con cen tration . Th e experim en tal kobs values are given as poin ts.
Th e graph ical treatm en t of experim en tally m easured data sh ows th at in -
creasin g con cen tration of pivalic acid causes a virtually lin ear in crease in
0.008
k
obs
0.006
0.004
FIG. 1
0.002
Th e relation between cHA an d kobs for
N-(ph en ylazo)m orph olin e in 30% aqueous
eth an ol; ● experim en tal data,
fun ction al depen den ce (20)
calculated
0.000
0.0
0.2
0.4
0.6
c
HA
Collect. Czech. Chem. Commun. (Vol. 64) (1999)

1662
Ludwig, Bednářová, Pařík:
kobs up to a certain con cen tration wh ich depen ds on th e solven t used. Th is
lim itin g con cen tration lies in th e in terval from 0.15 to 0.30 m ol l–1 for all
th e m edia. Above th is lim itin g con cen tration , th e kobs value in creases n o
lon ger with in creasin g con cen tration of pivalic acid but it begin s to decrease.
Curves of sim ilar sh ape were also foun d3 in th e case of acid-catalyzed decom -
position of 3,3-dialkyl-1-ph en yltriazen es with pivalic acid as a catalyst in
40% (v/v) aqueous eth an ol.
For all th e derivatives studied an d all th e con cen tration s used, th e kobs
values in 40% (v/v) aqueous eth an ol are gen erally th e lowest, in creasin g
with th e decreasin g eth an ol con ten t in th e reaction m ixture. Th e acid-
catalyzed decom position is th e fastest in 20% (v/v) aqueous eth an ol. Th e
largest ratio of th e m axim um kobs values in 20 an d 40% (v/v) aqueous eth a-
n ol, 11.4, was foun d with N-(ph en ylazo)piperidin e. Th e ratios of th e oth er
com poun ds decrease in th e order: N-(ph en ylazo)m orph olin e 9.6,
N-(ph en ylazo)perh ydroazepin e 6.6, an d N-(ph en ylazo)pyrrolidin e 5.3.
Wh en com parin g th e kobs values depen din g on th e con cen tration of th e
acid catalyst cHA for a series of N-(ph en ylazo)h eterocycles in 40% (v/v)
aqueous eth an ol, we can see th at an exch an ge of th e h eterocycle affects th e
decomposition rate. The order of kobs values for the whole concentration range
of pivalic acid is as follows: kobs (N-(phenylazo)pyrrolidine) > kobs (N-(phenyl-
azo)piperidin e) > kobs (N-(ph en ylazo)m orph olin e) > kobs (N-(ph en ylazo)-
perhydroazepine). This order corresponds to the increasing ring size of the
heterocycles. Although the morpholine and piperidine rings are of the same
size, the kobs values obtain ed for N-(ph en ylazo)m orph olin e were lower, obvi-
ously due to a reduced electron den sity on N-3 atom . Th e electron egative
oxygen atom in th e m orph olin e rin g m akes th e N-3 n itrogen atom in
N-(ph en ylazo)m orph olin e less n ucleoph ilic an d less basic th an th at in
N-(ph en ylazo)piperidin e (pKB = 11.12 an d 8.49 for piperidin e an d
m orph olin e, respectively20). Th is order is in accordan ce with th e fin din g of
our previous paper3 con cern in g th e dialkyl derivatives of th e Ph –N=N–NR2
type wh ere th e followin g order of rate con stan t values was obtain ed for
alkyl groups R: kobs (m eth yl) ≈ kobs (eth yl) > kobs (isopropyl) > kobs (butyl) >
kobs (cycloh exyl).
If th e decom position of N-(ph en ylazo)h eterocyclics is m easured in 30%
(v/v) aqueous eth an ol, th en th e order rem ain s un ch an ged: kobs
(N-(ph en ylazo)pyrrolidin e)
>
kobs (N-(ph en ylazo)piperidin e)
>
kobs
(N-(ph en ylazo)m orph olin e) > kobs (N-(ph en ylazo)perh ydroazepin e). How-
ever, th e differen ces between th e in dividual depen den ces are sm aller.
N-(Ph en ylazo)piperidin e beh aves alm ost iden tically with N-(ph en ylazo)-
pyrrolidin e up to th e con cen tration of 0.22 m ol l–1 of pivalic acid, but
Collect. Czech. Chem. Commun. (Vol. 64) (1999)

Effect of Medium
1663
above th at (0.5–0.7 m ol l–1), its kobs values are alm ost iden tical with th ose
of N-(ph en ylazo)m orph olin e. Th is is probably due to a com bin ation of sol-
vation effects of th e m ixed solven t an d com plex form ation with th e
un dissociated acid.
In 20% (v/v) aqueous eth an ol, it is n o lon ger true th at th e decom position
of th e com poun ds with th e sm allest h eterocycle is th e fastest, an d th at th e
in crease in th e rin g size is associated with slowin g down of th e
acid-catalyzed decom position . Th e order of rate con stan ts is: kobs
(N-(ph en ylazo)piperidin e)
>
kobs (N-(ph en ylazo)pyrrolidin e)
>
kobs
(N-(ph en ylazo)m orph olin e) > kobs (N-(ph en ylazo)perh ydroazepin e). Th e
ch an ges in th e order are obviously due to specific solvation effects of water,
wh ich gen erally m akes less operative oth er effects m an ifested in solven ts
with a h igh er con ten t of organ ic com pon en t.
Wh en com parin g th e depen den ces of kobs vs cHA in in dividual m ixtures,
on e arrives at in terestin g results. In 40% (v/v) aqueous eth an ol, th e
depen den ces for in dividual com poun ds reach th eir m axim a at th e con cen -
tration of acid catalyst about 0.3 m ol l–1, wh ereas th ose in 30% (v/v) aque-
ous eth an ol at about 0.2 m ol l–1 an d th ose in 20% (v/v) aqueous eth an ol at
about 0.25 m ol l–1. At th ese con cen tration s, we determ in ed th e differen ces
between th e experim en tal kobs values of th e in dividual depen den ces. In
40% (v/v) aqueous eth an ol, th e largest differen ce is between th e
depen den ces for N-(ph en ylazo)pyrrolidin e an d N-(ph en ylazo)piperidin e
(0.0019 s–1), wh ereas th e differen ce between N-(ph en ylazo)piperidin e an d
N-(ph en ylazo)m orph olin e is on ly 0.0008 s–1, an d th at between th e
depen den ces for N-(ph en ylazo)m orph olin e an d N-(ph en ylazo)perh ydro-
azepin e is still sm aller, on ly 0.0006 s–1. On th e oth er h an d, in 30% (v/v)
aqueous eth an ol th is tren d is opposite. Th e differen ce between th e
depen den ces for N-(ph en ylazo)pyrrolidin e an d N-(ph en ylazo)piperidin e is
zero, th at between N-(ph en ylazo)piperidin e an d N-(ph en ylazo)m orph olin e
is 0.0027 s–1, th at between N-(ph en ylazo)m orph olin e an d N-(ph en ylazo)-
perh ydroazepin e bein g th e h igh est, 0.0038 s–1. In 20% (v/v) aqueous eth a-
n ol, th e h igh est kobs values are sh own by N-(ph en ylazo)piperidin e, th e dif-
feren ce between N-(ph en ylazo)piperidin e an d N-(ph en ylazo)pyrrolidin e
bein g 0.0082 s–1.Th e differen ce between th e depen den ces for N-(ph en yl-
azo)pyrrolidin e an d N-(ph en ylazo)m orph olin e am oun ts 0.0051 s–1, an d
th at between N-(ph en ylazo)m orph olin e an d N-(ph en ylazo)perh ydroazepin e
0.0098 s–1.
Of in terest are th e depen den ces for 3,3-dicycloh exyl-1-ph en yltriazen e in
40 (ref.3), 30, an d 20% (v/v) aqueous eth an ol. Th is com poun d differs from
th e N-(ph en ylazo)h eterocyclics, because th e above-m en tion ed rule, viz. th at
Collect. Czech. Chem. Commun. (Vol. 64) (1999)

1664
Ludwig, Bednářová, Pařík:
in 40% (v/v) aqueous eth an ol, th e acid-catalyzed decom position of all th e
five substrates con sidered is th e slowest an d it in creases with decreasin g
eth an ol con ten t, does n ot h old un am biguously h ere. Th e tren d is rough ly
retain ed up to th e con cen tration of acid catalyst of 0.23 m ol l–1, but at
h igh er con cen tration s, an abrupt decrease in kobs for 20 an d 30% (v/v)
aqueous eth an ol occurs an d th e order of com poun ds is ch an ged. A com par-
ison of kobs for 3,3-dicycloh exyl-1-ph en yltriazen e with th at for th e
N-(ph en ylazo)h eterocyclics in th e in dividual m ixtures reveals th at th e de-
com position of dicycloh exyl derivative is th e fastest in 40% (v/v) aqueous
eth an ol, wh ereas in 30% (v/v) aqueous eth an ol it is between N-(ph en yl-
azo)m orph olin e an d N-(ph en ylazo)perh ydroazepin e. In 20% (v/v) aqueous
eth an ol, th e slowest reaction takes place at th e con cen tration of cHA
=
0.30 m ol l–1.
Mathematical Modelling of kobs vs cHA Dependence
As already stated above, th e decom position of N-(ph en ylazo)h eterocyclics
can proceed by eith er of two basic m ech an ism s13,14. In th e first on e, th e
substrate S is split in to products P by th e A-SE2 m ech an ism eith er by th e re-
action of th e substrate with th e proton
kH
S + H+
or with th e un dissociated acid HA
S + HA
P ,
(1)
kHA
P .
(2)
Th e existen ce of a dim eric form of th e acid catalyst is im probable in th e
m edia studied.
Th e secon d m ech an ism is specific acid catalysis (A1)
KH
k
S + H+
SH+
P .
(3)
Th e m ech an ism s given , h owever, can n ot explain th e decrease in ob-
served rate con stan t at h igh er con cen tration s of th e pivalic acid catalyst.
Th is decrease is probably due to th e lowerin g of con cen tration of th e reac-
Collect. Czech. Chem. Commun. (Vol. 64) (1999)

Effect of Medium
1665
tive form of N-(ph en ylazo)h eterocyclic com poun d in th e reaction m ixture
caused by form ation of a little reactive com plex. Th is com plex can be com -
posed of N-(ph en ylazo)h eterocycle an d th e un dissociated acid in th e ratios
of 1 : 1, 1 : 2, 1 : 3 etc.21. Th e form ation of such com plexes, den oted as SK1,
SK2, SK3, can be expected in fast pre-equilibria followed by very slow product-
form in g processes.
k
KSK1
KSK2
KSK3
′
HA
S + HA
S + SK1
S + SK2
SK1
SK2
SK3
P
P
P
(4)
(5)
(6)
k
′′
HA
k
′′′
HA
Proposed m ech an ism for N-(ph en ylazo)piperidin e with form ation of com -
plexes SK1 an d SK2 is sch em atically suggested in Sch em e 1. Th en th e con -
cen tration s of pivalic acid an d N-(ph en ylazo)h eterocycle in Eqs (1)–(6) can
be expressed as follows.
H
N
H
N
H
N N
N
N
+
CH3
H3C
C
COOH
N
N
N
N N N
CH3
2
complex SK2
(CH3)3CCOOH
(CH3)3CCOOH
CH3
H3C
C
COOH
N
N N
CH3
complex SK1
SCHEME 1
Collect. Czech. Chem. Commun. (Vol. 64) (1999)

1666
Ludwig, Bednářová, Pařík:
cHA = [H+] + [HA] + [SK1] + [SK2] + [SK3] + [SH+]
(7)
(8)
cS = [S] + [SK1] + [SK2] + [SK3] + [SH+]
KSK1 = [SK1]/[HA][S]
(9)
KSK2 = [SK2]/[HA][SK1] = [SK2]/KSK1[HA]2[S]
KSK2 = [SK3]/[HA][SK2] = [SK3]/KSK1KSK2[HA]3[S]
KH = [SH+]/[H+][S]
(10)
(11)
(12)
(13)
KHA = [H+][A–]/[HA] = [H+]2/[HA]
Usin g Eqs (7)–(12) alon g with th e reaction rate equation th e overall rela-
tion sh ip for th e depen den ce of th e observed rate con stan t on con cen tra-
tion s of pivalic acid an d proton can be derived (Eq. (14)). Of th at, th e
con cen tration s of all species on th e righ t-h an d side of Eq. (7), except for
[HA], were n eglected relative to experim en tally determ in ed dissociation
con stan ts of pivalic acid in 40, 30, an d 20% (v/v) aqueous eth an ol (KHA
=
5.012 · 10–7, 1.122 · 10–6, an d 2.139 · 10–6, respectively) an d of th e con cen -
tration ratio of N-(ph en ylazo)h eterocycle an d acid catalyst in th e reaction
m ixture.
3
2
+ kK + kH [H + ]
H
′′′
′′
′
kHA KSK3 KSK2 KSK1 cHA + kHA KSK2 KSK1 cHA + kHA KSK1 + kHA
c
)
(
(
)
HA
kobs
=
(14)
1 + KSK3 KSK2 KSK1 cH3 A + KSK2 KSK1 cH2 A + KSK1 cHA + K H +
H[ ]
Th ough bein g com plex, th e equation m akes it possible to set up a n um -
ber of m odels with th e acid catalyst con cen tration s raised to differen t pow-
ers in both th e n um erator an d den om in ator. Usin g Eq. (13) an d an
Collect. Czech. Chem. Commun. (Vol. 64) (1999)

Effect of Medium
1667
experim en tally determ in ed dissociation con stan t of pivalic acid in given
m edia (see Experim en tal), it is th en possible to write th e proton con cen tra-
tion s in th e m edia as follows:
in 40% (v/v) aqueous eth an ol … [H+] = 0.000684 cHA
in 30% (v/v) aqueous eth an ol … [H+] = 0.00106 cHA
in 20% (v/v) aqueous eth an ol … [H+] = 0.00150 cHA
(15)
(16)
(17)
.
Th e lowerin g of th e kobs values at h igh con cen tration s of th e acid catalyst
can be due to th e existen ce of SK1, SK2 or SK3 com plexes on ly if th ese com -
plexes do n ot react furth er or react m uch m ore slowly th an th e oth er spe-
cies (i.e. kH′ A = kH′′ A = kH′′′A = 0). Th e reaction accordin g to Eq. (3) will n ot
proceed eith er h en ce kHA = 0. On th e basis of th ese facts, Eq. (14) can be
sim plified to Eq. (18).
kK + kH H +
(
)
[ ]
H
kobs
=
(18)
1 + KSK3 KSK2 K
SK1 cH3 A + KSK2 KSK1 cH2 A + KSK1 cHA + KH H +
[ ]
Equation (18) was solved by n on -lin ear regression obtain in g a n egative
value of param eter KH (ph ysically m ean in gless). Th erefore, th e term s in -
volvin g th e KH param eter were excluded, i.e., th e m ech an ism of specific
acid catalysis described by Eq. (3) does n ot sh ow up (k = 0). By gradual ex-
cludin g th e statistically in sign ifican t param eters, we obtain ed th e resultin g
m ath em atical m odel with two param eters con tain in g th e term cHA in th e
n um erator (after in troducin g for [H+] from Eqs (15), (16), or (17)) an d th e
term cHA , cHA , cH2 A , or cH3 A in th e den om in ator. Th ese m odels were applied
to th e depen den ces of kobs vs cHA for th e respective com poun ds. For th e
N-(ph en ylazo)h eterocyclics, th e suitable m odel in volves th e cH2 A term in th e
den om in ator (Eqs (19), (20), an d (21) for 40, 30, an d 20% (v/v) aqueous
eth an ol, respectively); h en ce th e m ech an ism on ly in volves n on -reactive
com plexes SK1 an d SK2. For th e acid-catalyzed decom position of
N-(ph en ylazo)h eterocyclics in 20% (v/v) aqueous eth an ol, a suitable m odel
seem s to in volve th e SK3 com plex (Eq. (22)). In th is m edium , it is im possi-
Collect. Czech. Chem. Commun. (Vol. 64) (1999)

1668
Ludwig, Bednářová, Pařík:
ble to un am biguously decide wh ich of th e m odels is m ost probably valid
because th e existen ce of n on -reactive com plex is m an ifested particularly in
th e region of decreasin g depen den ce of kobs vs cHA. Th is part of curve, h ow-
ever, is n ot sufficien tly depicted, sin ce th e depen den ce can on ly be m ea-
sured up to th e catalyst acid con cen tration cHA = 0.4 m ol l–1 (it is im possible
to prepare th e solution s of h igh er con cen tration s). Obviously, m ore proba-
ble is a m odel in volvin g th e n on -reactive com plexes form ed by a lower
n um ber of m olecules of th e un dissociated acid.
0.000684kH cHA
kobs
kobs
kobs
kobs
=
=
=
=
, wh ere KK = KSK1KSK2
, wh ere KK = KSK1KSK2
, wh ere KK = KSK1KSK2
, wh ere KK = KSK1KSK2KSK3
(19)
(20)
(21)
(22)
1 + KK cH2 A
0.00106kH cHA
1 + KK cH2 A
0.00150kH cHA
1 + KK cH2 A
0.00150kH cHA
1 + KK cH3 A
Th e results obtain ed by n on -lin ear regression , th e catalytic rate con stan ts
kH, th e com plex stability con stan ts KK, th eir stan dard deviation s, an d resid-
ual sum s of squares (RSS) for th e N-(ph en ylazo)h eterocyclics in th e given
m edia are given in Table II. An exam ple of in terpretation of th e results is
th e depen den ce of kobs vs cHA in Fig. 1 (th e curve is th e fun ction al depen d-
en ce of kobs on cHA calculated accordin g to th e respective m odel, th e poin ts
sh owin g th e experim en tal values).
In 40% (v/v) aqueous eth an ol, th e catalytic rate con stan t kH decreases
with in creasin g size of h eterocyclic m oiety, wh ich un am biguously con firm s
th e effect of substituen ts at th e N-3 atom on th e rate of acid-catalyzed de-
com position of th e N-(ph en ylazo)h eterocyclic. Th e effect observed can be
in terpreted by steric h in dran ce to th e approach of solvated proton to th e
N-3 atom , wh ich supports th e claim of bim olecular reaction m ech an ism
with gen eral acid catalysis. Alth ough th e piperidin e an d m orph olin e rin gs
are of th e sam e size, th e kH value is h igh er for N-(ph en ylazo)piperidin e th an
for N-(ph en ylazo)m orph olin e. Obviously, th is is due to a lower n ucleoph ili-
city an d basicity of th e N-3 atom in N-(ph en ylazo)m orph olin e com pared
Collect. Czech. Chem. Commun. (Vol. 64) (1999)

Effect of Medium
1669
with N-(ph en ylazo)piperidin e as a con sequen ce of th e presen ce of th e
electron egative oxygen atom in th e m orph olin e rin g an d th e ch an ge in
electron den sity distribution . In 30 an d 20% (v/v) aqueous eth an ol, th e cat-
alytic rate con stan t kH does n ot decrease an y lon ger with in creasin g rin g
size of th e h eterocyclic m oiety. Obviously, h ere th e steric requirem en ts of
bulky substituen ts are outweigh ed by specific solvation effects of water
wh ich gen erally lower th e in fluen ce of oth er effects operatin g in 40% (v/v)
aqueous eth an ol (i.e., a solven t with h igh er proportion of eth an ol). In 40,
30, an d 20% (v/v) aqueous eth an ol, th e m ixed com plex stability con stan t
TABLE II
Optim ized param eters kH (l m ol–1 s–1), KK (l2 m ol–2), th eir stan dard deviation s (s), residual
sum of squares (RSS) an d n um ber of experim en tal poin ts (n) obtain ed by n on -lin ear regres-
sion
Substrate
kH
s(kH)
KK
s(KK)
RSS 107
n
40% (v/v) aqueous eth an ol, m odel (19)
1a
10.93
7.38
0.328
0.221
0.315
0.168
0.370
4.34
2.87
3.42
4.77
2.30
0.255
0.244
0.201
0.354
0.157
2.29
2.25
17
17
25
22
22
1b
1c
15.75
5.60
10.69
13.64
3.52
1d
DCTAZb
17.46
30% (v/v) aqueous eth an ol, m odel (20)
1a
32.60
21.22
29.28
13.30
18.51
0.978
0.424
1.171
0.266
0.370
12.91
8.19
0.796
0.359
0.524
0.570
2.570
33.52
15.95
69.56
4.42
16
21
18
18
16
1b
1c
6.69
1d
DCTAZb
11.82
40.04
25.82
20% (v/v) aqueous eth an ol, m odel (21)
1a
1b
1c
1d
59.23
35.59
44.98
25.47
1.185
1.068
1.349
1.019
5.65
5.50
0.397
0.567
0.494
1.422
9.26d
14.87d
7.15d
11
10
10
11
4.91
16.27
3.03d
a
b
c
d
3,3-Dicycloh exyl-1-ph en yltriazen e; experim en tal data from ref.8; m odel (23); RSS · 106.
Collect. Czech. Chem. Commun. (Vol. 64) (1999)

1670
Ludwig, Bednářová, Pařík:
KK of SK1 an d SK2 com plexes is n ot on ly related to th e steric requirem en ts
of h eterocyclic m oieties but also sign ifican tly affected by th e participation
of solvatin g m olecules of water an d eth an ol.
Th e m eth od of n on -lin ear regression was also used for th e experim en tal
data of acid-catalyzed decom position of 3,3-dicycloh exyl-1-ph en yltriazen e.
In 40% (v/v) aqueous eth an ol, th e depen den ce of kobs on cHA for th is com -
poun ds was described in a previous paper3 by application of a m odel in -
volvin g SK1 an d SK2 (Eq. (19)). However, for th e depen den ce in 30% (v/v)
aqueous eth an ol, th e m odel (20) is n o lon ger sufficien t. We h ave foun d a
suitable m odel in volvin g, besides SK1 an d SK2, also th e SK3 com plex an d
described by Eq. (23).
0.00106kH cHA
kobs
=
, wh ere KK = KSK1KSK2KSK3
(23)
1 + KK cH3 A
Th e data for 20% (v/v) aqueous eth an ol resisted all optim ization attem pts
for an y of th e m odels, th e com plex stability param eter KK always bein g sta-
tistically in sign ifican t. Th is is a con sequen ce of an in sufficien t ran ge of
m easurem en t of th e kobs vs cHA depen den ce. However, th e experim en tal
data allow a con clusion th at a furth er in crease in th e acid catalyst con cen -
tration would cause a very steep decrease in kobs, an d th e curve of th is
sh ape could th en be fit by a m odel in volvin g apparen tly a n on -reactive
com plex con tain in g th ree or even m ore m olecules of th e un dissociated
acid. Th e com plex stability con stan t KK would be h igh .
Th e kin etic data obtain ed for th e N-(ph en ylazo)h eterocyclics were also
treated by th e m eth od of prin cipal com pon en t an alysis22 (PCA). Th e source
m atrix for th e given m edium was arran ged in such a way th at th e rows an d
colum n s correspon ded to th e in dividual substrates an d th e acid catalyst
TABLE III
Values of score vectors t from PCA calculation in various solven ts
Substrate
40% Eth an ol
30% Eth an ol
20% Eth an ol
1a
1b
1c
1d
0.461
0.248
1.000
0.000
0.740
0.478
1.000
0.000
1.000
0.410
0.673
0.000
Collect. Czech. Chem. Commun. (Vol. 64) (1999)

Effect of Medium
1671
con cen tration s, respectively. Th e m atrix was filled to 100%. Accordin g to
th e statistical criteria, on ly on e laten t variable is sign ifican t in all th e m e-
dia, describin g 97.8, 94.8, an d 98.9% of variability in 40, 30, an d 20% (v/v)
aqueous eth an ol, respectively. From th e order of values in th e score vectors
(Table III), it is obvious th at th e sin gle laten t variable describin g th e vari-
ability of source m atrix in volves both th e steric effect of th e h eterocyclic
m oiety an d th e solven t effect. Obviously, in 40% (v/v) aqueous eth an ol th e
order of values in th e score vector is given predom in an tly by th e rin g size
of th e h eterocycle wh ich is associated with steric requirem en ts. In 30%
(v/v) aqueous eth an ol, th e order rem ain s th e sam e, but in 20% (v/v) aque-
ous eth an ol, th e solven t effect of th e m ixture probably begin s to be appar-
en t in addition to th e steric effect, wh ich alters th e order of th e given
substrates. Th e sam e con clusion an d th e sam e order of substrates followed
from th e qualitative description of th e kobs vs cHA depen den ce, too.
CONCLUSION
Results obtain ed on proposed m ath em atical m odels with un am biguous
ch em ical m ean in g sh ow th at acid-catalyzed decom position un der used con -
dition proceeds by m ech an ism A-SE2 (gen eral acid catalysis). Th e proton is
dom in an t catalyst affectin g in rate lim itin g step. Very in terestin g situation
of decreasin g of observed rate con stan t at h igh er con cen tration of acid cata-
lyst was explain ed by th e form ation of n on -reactive com plex of two (in
sam e cases th ree) m olecules of un dissociated acid catalyst an d th e respec-
tive m odel h eterocyclic triazen e. Values of catalytic rate con stan ts an d re-
sults of PCA sh ows th at th e rin g size of h eterocyclic m oiety affectin g
substan tially on reaction rate.
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