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1396
J.
Chem. SOC.
A),
1969
Chemistry
of Nitryl
Chloride. Part 1
B y Ram Chand Paul, Darshan Singh , and
K .
C. Malhotra, Department of Chemistry, Panjab University,
Chandigarh-14, India
Nitryl chloride dissolves readily i n sulphuric and disulphuric acids. Cryoscopic, conductometric,and Raman
and
i r spectral studies of its solutions have confirmed t h a t
it
gives
nitronium
ions.
It
forms complexes with strong
electron acceptors like boron trichloride
and
anti.mony pentachloride. From
their
conductance
n
nitromethane and
i r
spectra the complexes have been f ound
to
be ionic.
Nitryl
chloride also forms complexes with organic tertiary
bases.
The ionic nature of these complexes has also been confirmed by conductances
a n d i r
spectra. We con-
clude t h a t nitryl chloride i s a source of nitronium ions rather t h a n positive chlorine, as was postulated earlier.
AN improved method of preparation of nitryl chloride
and its convenient working range (m.p.
-145 ;
b.p.
-15 ) have increased its importance in organic syn-
t l ~ e s i s . ~ ~ ~t is easily hydrolysed and from the products
it has been concluded5 that chlorine is bonded to
oxygen. However, microwave 6 and i.r. spectra
7
indi-
cated
it
to be
a
Y-shaped planar molecule in which
chlorine is attached to nitrogen. The N-Cl bond length
of 1.98 is longer than the sum of two covalent radii.
The N-Cl bond energy8 is 30.0 kcal./mole which is
considerably lower than the normal N-C1 covalent bond
energy of 38.4 kcal./mole.
Thus the N-Cl bond in
nitryl chloride is weaker than normal.
Nitryl chloride
has a low specific conductivity (
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Inorg. Phys. Theor. 1397
room temperature. Excess of nitryl chloride and the
solvent
was
allowed to escape, leaving pale crystals 0.8 .).
These were repeatedly washed with dry carbon tetrachloride
to remove traces of nitryl chloride and were dried
in ucccuu.
They decomposed at 112 (Found: Sb,
3 1 .1 ;
C1, 55.6.
KO,Cl,SbCl, requires Sb
31 -92 ;
C1, 55.94%).
All
attempts
to purify the c ompound failed
as
it was insoluble in most
solvents or reacted with them.
in a trap
at -40'.
Excess of n itryl chloride was added.
chloride can behave as in reactions (3), (a), or 5 )depend-
No&1 -k 2H~S~o,-
N02C1
+
5H2S20,
-- .
N0,Cl + 3H2Sz07-
NO(OH)+Cl
+
HS3010- + H,SO,
(3)
C1++
NO+
+ 2HS O -
+ 4H,SO,
(4)
NO2+
+
HS03C1+ HS,Olo-
+
2H,SO,
5 )
Boron trichloride
1 .0 g.)
was dissolved in sulphur dioxide
The mixture was left a t
-40
for 2 hr. and then brought to
room temperature. The excess
of
nitryl chloride and
sulphur dioxide
was
allowed to escape leaving white crystals
(decomp.
86').
These were repeatedly washed with carbon
tetrachloride and dried
in
uucuo. The
cowplex
could not be
recrystallised as i t is insoluble in the conventional solvents
(Found: B, 5.1; C1,
71.7.
NO,Cl,BCl, requires
B,
5 - 5 2 ;
C1, 7
1.35 ).
Complexes
of
nitryl chloride with organic tertiary bases
were prepared as usual. A known weight of ni tryl chloride
was dissolved in acetonitrile and t o this excess of the base
in acetonitrile was added dropwise with constant stirring.
Pale yellow compounds separated in most cases but some-
times carbon tetrachloride or light petroleum had to be
added to isolate them. The mixtures were left for
1 hr.
and filtered in a dry atmosphere. The solids were re-
peatedly washed with dry carbon tetrachloride and dried
in.
VU CU O .
Analyses are given in Table
3.
Molar conduct-
ances were measured a t 25'.
RESULTS A N D DISCUSSION
By analogy with nitrosyl chloride
l
(NOC1+ NO+
Cl-) one would expect nitryl chloride to yield positive
nitronium ion and negative chloride ion, yet at
low
temperatures in the solid, liquid, and gaseous state
it
reacts with ammonia to yield chloramine and ammonium
nitrite,13J4 suggesting t ha t it gives positive chlorine and
negative nitri te ions. As nitronium
is
a strongly
electrophilic cation,
it
cannot be stabilised in
a
strongly
nucleophilic medium. Therefore the behaviour of
nitrylcliloride was studied in the weakly nucleophilic
sulphuric and disulphuric acids, which can stabilise such
cations.15J6
Nitryl chloride dissolves exothermically in these acids
to give highly conducting solutions.
Its
solubility is
much higher in disulphuric than in sulphuric acid.
Disulphuric acid is itself strongly dissociated and
ionised, and in
it
the bases are completely protonated
as l 7 in
l ) ,
nd the ionisable chlorides are quantitat ively
B
+ 2HzS20,w
BH+
+ HS3OI0- + H,SO,
1)
converted into chlorosulphuric acid as in 2). Nitryl
KC1 +
3HzS207-+
K+
+ HS3010-
+ HS0,Cl + 2H,SO,
2)
A . 3. Burg and G. W. Campbell, J .
A m e r . C h e m .
Soc. 1948,
13 NI. Schmeisser, 2 anorg Chem.,
1948,
255,
33.
14 H. H. Batey and H. H. Sisler, J . A m e r . C h e w .
Suc.
1952,
15 R.
J.
Gillespie and
J.
B. Milne, I n o v g .
Chem.
1966,
5,
1577.
16
J.
Ba n , R.
J.
Gillespie,
R.
Kapoor, and K. C. Malhotra,
70, 1964.
74,
3408.
Canad .
J .
Chern.
1968,
46,
149.
ing upon i ts nature. If it has sufficiently basic oxygen i t
may be protonated by strong acids (equation 3), or if it
is a source
of
positive chlorine ions it may react by
equation (4), or if a source of positive nitronium ion
by equation
5) .
Conductometric and cryoscopic studies
in disulphuric acid show that nitryl chloride behaves
as in reaction 5 ) , so it is a source of nitronium ions
rather than positive chlorine ions. Cryoscopic titrations
of these solutions against sulphur trioxide show that two
moles
of
sulphuric acid are formed per mole
of
nitryl
chloride, in agreement with reaction (5). The Raman
spectrum of
a
concentrated solution of nitryl chloride in
disulphuric acid shows a sharp absorption band at
1400
cm.2 which is characteristic of the nitronium ion in
strongly acidic media.l* The i.r. spectrum of this solu-
tion shows
a
sharp band at
2350
cm.-l indicating nitron-
ium ions.
No
other i.r.-active bands could be observed
for nitronium ions owing to the complicated spectrum
of
disulphuric acid. These observations confirm that
nitryl chloride behaves like potassium chloride in
disulphuric acid.
TABLE
Conductivities a t 25 and freezing points
of
solutions
of nitryl chloride
In R,S,O, In H2S0,
102(Molality)
(ohm-f crm-1) F.p. (ohm-1cm.-l) F.p.
1.0 3.9826 34.98 1.1426 10.17
2.0 4.2792
34-88 1.3184 9-92
3-0 4.5580 34.73 1.6220 9.64
4.0 4.8826 34.60 1.7646 9.35
5 0
5.2383 34-46
1.9888
9-06
6-0 5.5626
34.29
2.2260 8-78
7.0 5,9324 34.12 2.4388 8.46
8.0
6.3045 33-96
2.6400 8.15
9.0 6.6098 33-61 2.7433
7 . 8 0
10.0 6.9523 33.40 2-9406
7.58
12.0
7.6504 33.22 3.1488
14.0 8-2366 32-85
16.0 8.8580 32.43
18.0 9.43 16 32.03
20.0 9-8958 31.60
1 0 3 ~
1 0 K
Unlike nitryl fluoride,lg the solubility
of
nitryl chloride
in
lOOyo
sulphuric acid is limited, so conductance and
cryoscopic studies have been confined (Table 1) to dilute
solutions
(0.12
molal). At low concentration, nitryl
chloride produces
ca 5
particles
(V 5 )
and two ions
of
R. J.
Gillespie and K.
C.
Malhotra,
J . CJzem. SOC
A ) ,1968,
18 R. J. Gillespie, J. B. Milne, and J .
B.
Senior, Inorg. Chem.
l9
G.
Hetherington, D.
R.
Hub, and
P.
L. Robinson,
J .
C h e m
1933.
1966, 5, 1233.
SOC.
955, 4041.
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1398
J .
Chem. SOC.
A),
1969
high mobility
y
2) in sulphuric acid. (The value of
v is never 5 and
y
is two only at lower concentrations.)
These values steadily decrease at higher concentration
which can be explained if the reaction takes place in
two stages
(6)
and
7).
Reaction
(6)
is irreversible and
N0,Cl
-H2S04 O,+ +
HC1 +
HSOd-
(6)
HC1+ 2H,SO, ----L H3T0 HS0,Cl +
HSO,-
7)
reaction
7)
reversible. Gillespie and his co-workers
2o
have shown tha t hydrogen chloride can be quantitatively
removed from sulphuric acid by a stream of dry nitrogen.
Thus reaction
7)
is never complete
so
values of v and
y
are low. These low values cannot be attr ibu ted to the
formation of ion pairs
(e.g.,
reaction 8) because the
NO2+
HSO,- _ NO,HSO,
8)
dielectric constant of sulphuric acid is high and the
solutions are dilute. There is no evidence that nitryl
chloride reacts
(9)
with hydrogen sulphate to form sul-
phate ions, as postulated by Robinson and his co-
N0,Cl
-
HSO - O,
-
HC1
+
SO
(9)
workers
l9
for nitry l fluoride in sulphuric acid. Since
our solutions are dilute,
y
is not significantly lower than
2.
Dissociation
of
nitryl chloride
as
2N0,Cl
_t N204
k
C1,
in these strong acids is ruled out as
it
would lead to
higher values of
v
and
y
in sulphuric acid owing to
reaction
(10).
We conclude that nitryl chloride ionises
N,O, -
3H2S04--t
NO2+ NOi H 3 + 0 3HS0,-
(10)
in strongly acidic media to produce nitronium ions.
Further information on the nitryl chloride molecule
comes from
a
study of t he coniplexes of nitryl chloride
with Lewis acids and bases. An attempt to isolate its
complexes with Lewis acids at
-75
failed
l4
but the
strong electron acceptor sulphur trioxide forms a complex
NOc,C1,S206, hose structure has not been studied.
We
have isolated complexes with boron trichloride
and
antimony pent achloride in liquid sulphur dioxide.
These complexes are moisture-sensitive and decompose
before melting. Their conductance in nitrobenzene
changes with time, either because of dissociation or
fur ther nitra tion of the solvent. Their solutions in
nitromethane are fairly stable and their molar con-
ductance, comparable with those of uni-univalent
electrolytes, suggests that they are ionic.
Seel and his
co-workers21 obtained similar results for
a
complex of
nitryl chloride and antimony pentachloride in liquid
chlorine. 1.r. spectra also support the ionic nature of
these complexes. The strong fundamental bands
361,
795, and 1685 cm.-l due t o the NO, group are absent
while two new bands a t 2355 and 3760 cm.-l are present.
The band2, at 2355 crn.-l is due to the asymmetric
20 R.
J.
Gillespie
and E. ,4.
Robinson,
J . Amev .
Cheilz.
SOC.,
i965 ,s7 , 2428.
2 1
IT
F. Seel, J . Norgradi, and R. Posse, 2
anovg. Chern.
1952,
269, 198.
stretching vibration of nitronium ion (Table
2).
The
bending mode
(v2)
a t
540
cm.-l has not been observed on
a spectrometer having sodium chloride optics. Accord-
ing to Raman selection rule, nitronium ion must have
TABLE
Principal bands
in
the i.r. spectra of complexes
of
nitryl
chloride and
Lewis
acids
NO,+BCl,-
NO,+SbCI,-
v1
+ v 3 N 0 , + ) .........
3760 w 3750
v,(NO,+) ............... 2360 \-.s. 2355
2v3 BC14)- ............
1460
vl
+ v p +
vQ .........
1380
2vl 2v ............... 1275
vi3 ........................ 690 670
one i.r.-inactive and Raman-active symmetrical stretch-
ing mode at
1400
cm.-l which is absent from the present
spectrum, but the band a t 3760 cm.-l may be the com-
bination tone of this band and
v3 (1400 + 2360).
Simi-
lar observations have been made by Marcus and Fresco
23
in the spectrum of nitric acid and it s solutions in various
solvents. The absence of an absorption band a t 650
shows the absence of N-Cl bonds. All these
absorption bands confirm the presence of nitronium
ions in these complexes. The other bands present are
due to the anions. The broad and strong bands at
1455,
1380,
1275, and
690 cm.-l
are due to the tetra-
chloroborate ions (BC1J-.24 However, for hexachloro-
antiinonate ion (SbClJ-, except for one sharp band at
670
cm.-l
(v3)
no other band was observed because of the
limited transparency of th e optical window. We con-
clude tha t boron trichloride and antimony pentachloride
form
ionic complexes by accepting chloride ions from
nitryl chloride
e.g.,
reactions
11
and
(12),
confirming
that nitryl chloride
is
a source of nitronium ion.
N0,Cl
+
SbC1, NO,Cl,SbClJ
-+
K0&1
+
BC1, __t (N0,C1,BC13) -+
NOZ+-
SbC1,- 11)
KO,-
+ BC1,- (12)
Though acid halides are well known to form addition
complexes with organic tertiary bases, yet no complex
of
nitryl chloride with organic tertiary bases has been
reported. 1.r. spectra of complexes of acid halides and
ter tia ry bases further support the mode of ionisation
of
acid halides. We therefore prepared
a
number of
coin-
plexes
of
nitryl chloride with organic tertiary bases and
oxygenated bases (Table 3). Conductometric titrations
between pyridine, quinoline, rx-picoline, morpholine, and
piperidine and nitryl chloride have been carried out in
acetonitrile to ascertain the stoicheiometry
of
these com-
plexes. There are two breaks in the conductance-
composition curve at molar ratio
1 : 1
and 1
:2,
cor-
responding to NO,Cl,B and
NO,C1,2B.
The initial
22
D. Cook,
S.
J. Kuhn,
and
G. A.
Olah, J .
Chenz. Phys. 1960,
33,
1669.
23 R. A. Marcus and
J. M.
Fresco,
J .
C h e m . Phys. 1957,
27,
564.
24
T. C. Waddington
and
F. Klanberg, J . C h e m .
SOL 1960,
2339.
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Inorg.
Phys.
Theor. 1399
increase in the conductance of the solution is due to the
solubility of the 1 : 1 complex, which may be ionic,
and the decrease beyond 1 :
1
composition is due to the
insolubility of 1 :2 coniplexes. Molar conductances of
these complexes in various solvents (where solubility
permitted) indicate that the complexes are ionic.
How-
ever urea and dimethyl sulphoxide complexes have
conductances below those for
1
:
1
electrolytes,
so
these
complexes are only weakly ionic.
formation of pyridiniuin chloride (PyH)+Cl-,= and in the
complexes of pyridine with selenium tetrachloride 26
and acetyl chloride.27 The bands a t
1620,
1520, 1480,
1360, 1260,
1060, and 760 cm.-l are due to the free
pyridinium ion. A broad band at 2360 cm.-l due
to NO, has shifted to
2150
cm.-l owing to combination
with base. The band
at
1670 cm.-l, which is one
of
the
fundamental bands of the NO, group, has completely
disappeared. We conclude that pyridine combines (13)
TABLE 3
Complexes of nitryl chloride with bases
Colour aiid Molar conduc tance
physical
ALP.
at
25
C1 ( ) C1
( /)
Base ( ) Base
(yo)
Base Stoicheionietry stat e (decomp.) (ohm-1cm. mole-l) Found Required Fou nd Required
Pyridine
Pyridine
Quinoline
a-Picoline
Piperidine
Diethylamine
Triethylamine
Morpholine
Ethylene-
diamine
Dimethyl
sulphoxide
Urea
NO,Cl, (C5H,N)2 Yellow solid
2N0,Cl,C,H5N Pa le yellow
NO,CI,
(C,H,N), Yellowish orange
NO,CI,C,H,N Yellow solid
NO,Cl, (C,H,,N)
,
Yellow solid
NO,Cl[(C,H,),NH], Wh ite solid
NO,Cl[(C,H,),N], Pale yellow
NO,Cl, (C,H,NO) Pa le yellow
NO,CI(C,H,N,), Dark red liquid
NO,CI,
(C,H,SO) I'ellow liquid
SO CI (N,H,CO), Sel low liquid
solid
solid
solid
solid
148'
58
156
164
116
102
90
124
-
-
-
*
In nitroinethane.
Further support of the ionic nature of nitryl chloride is
provided by the spectra of its complexes with organic
tertiary bases. For brevity, only the spectrum of the
pyridine compound (C,H,N),,N02C1 is reported (Table
4).
The spectra of all these complexes indicate that
TABLE
4
1.r.
spectra (cm.-l) of
the
complex of
nitryl chloride
with pyridine
Pyridine
3005s
1583m
1483
1439s
1370vw
1220s
1152s
1072s
1034s
995s
784s
705s
-2ssignnient
C-H
C=C and
C=N
Stretch
C=C
and C=X
C-H in p lane deforniatioii
Pyridine ring breathing
Out-of-ring deformation
NO,Cl, (C,H,K)
2940s
2150111
1620s
1520s
1480s
1360s
1260s
1200s
1
1 7 0 w
1060s
1030w
1010s
930
855vs
7 6 0 ~ s
spectral bands of pure components change on complex
formation. In these complexes absorption bands of
pyridine change similarly to those which occur on the
z5
N.
N. Greenwood and I
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1400
J .
Chem.
SOC. A) , 1969
acid the proton (H+) s a far stronger acid than NO,+ and
hence NO,+ is replaced by the proton in these complexes.
Nitryl chloride can be represented by structures
(111)-(V).
Structure
(VI)
is not reasonable as it
CI CI
I
CI i-
ll
violates Paulings adjacent charge rule and the N-Cl
bond is even longer than ordinary single bonds. Struc-
ture
(111)
(IV)
would be expected t o have a dipole
moment of 2.5
D.
This could be reduced to the reported
0.53
D
if there is a significant contribution of structure
(V). sp
Hybridisation of the chlorine-nitrogen bond
would produce a dipole moment opposite to that of the
NO,
group, which is contrary to the observed value.
Though we cannot draw definite conclusions about the
structure of nitryl chloride from the chemical evidence,
yet we believe that there must be
a
significant contribu-
tion
of
structure (V) to the bond polarities. This is sup-
ported by calculations by Millen and Sinnott
28
of
quadrupole coupling constants, who found that the N-C1
bond in nitryl chloride is
12%
ionic. Therefore, nitryl
chloride can be represented to a fair approximation as
(111)
f p
IV)
- (V). The chlorine atom in nitrosyl
chloride is clearly negative.
In
nitryl chloride the
addition of a second oxygen atom to the nitrogen a tom
considerably reduces its polarity but we conclude that
it has not made it positive.
[8/1751
Received November
18th, 19681
D.
J. Millen and K. M. Sinnott, J . C h e w . SOL 1958, 350.
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