Nuclear Magnetic Resonance Studies on Aromatic Compounds, IV
13C NMR Spectra of Some Disulpho-substituted 1- and 2-Naphthols
K a u k o R ä i s ä n e n * and L a u r i H. J. L a j u n e n
D epartm en t o f P h ysics*, D epartm ent o f Chemistry, U n iversity o f Oulu, SF-90100 Oulu 10, F inland
(Z. Naturforsch. 32 b, 818-820 [1977]; received March 22, 1977)
13C N M R, 1-Naphthols, 2-N aphthols
The 13C NM R spectra o f l-hydroxynaphthalene-3,6-disulphonic, 1-hydroxynaphthalene3,8-disulphonic, 2-hydroxynaphthalene-3,6-disulphonic, and 2-hydroxynaphthalene-6,8disulphonic acids were determ ined w ith and w ith ou t noise-decoupling. The fine sp littin g
caused b y th e long-range couplings pattern was used in the id en tification o f th e resonances
lines o f th e 13C nuclei. The com bined influence o f th e m esom eric effect o f th e h yd roxyl
group and th a t o f th e sulphonate groups have been discussed.
The 13C NMR spectra of the following dihydroxynaphthalenesulphonic acids were discussed in part
III of this series:1 2,3-dihydroxynaphthalene-6-sulphonic acid (abbreviated as 236DHNS), chromo­
tropic acid, and catechol-3,5-disulphonic acid (Tiron). In these three compounds we have analyzed
the contribution of the mesomeric effect of the
hydroxyl group. In the present work we have contin­
ued our investigations about aromatic sulpho-substitutet compounds with four different naphthalenedisulphonic acids, each containing one hydroxyl
group. The 13C NMR spectra of 1-hydroxynaphthalene-3,6-disulphonic (abbreviated here as
136 HNDS), 1-hydroxynaphthalene-3,8-disulphonic
(138HNDS), 2-hydroxynaphthalene-3,6-disulphonic
(236HNDS), and 2-hydroxynaphthalene-6,8-disulphonic (268 HNDS) acid swere recorded and discus­
sed.
Experimental
Reagents: All the naphtholsulphonates studied
were reagents from either E. Merck AG or Fluka AG,
and they were recrystallized several times from hot
water before use.
A pp a ra tu s and methods: The 13C NMR spectra of
the present compounds were determined from 20%
solutions with Jeol’s JNM -FX100 FT-spectrometer
(in DQD mode and at 25.06 MHz) both with and
without proton noise-decoupling. For all the spec­
tra, the respective settings were the same as those
reported in part I of this series2. All the measureR eq uests for reprints should be sen t to Dr. K.
D ep artm en t o f P h ysics, U n iversity of
Oulu, SF-90100 Oulu 10, Finland.
R ä is ä n e n ,
ments were carried out in D20 solution at 298 K
with TMS as an external standard for chemical
shifts. No corrections were made in the chemical
shifts for the differences in bulk susceptibility, be­
cause all the molecules studied had the same solvent.
Results and Discussion
In this work we investigated the contribution of
the hydroxyl and sulphonate groups to the distribu­
tion of ^-electrons around the carbon nuclei in
different disulphosubstituted 1- and 2-naphthols
with aid of the changes in chemical shifts related to
naphthalene measured from 13C NMR spectra. Our
interest was particularly focussed on the mesomeric
effect of these groups. The hydroxyl group has a
positive mesomeric effect and a negative inductive
effect. The inductive effect of the sulphonate group
is parallel, to that of the hydroxyl group, whereas
the mesomeric effect is reverse. All the other of these
effects are relatively small, except the mesomeric
effect of the hydroxyl group3. This effect was
dominantly observed in e.g. part III of this series1.
In all the sulphonate compounds studied in this
series, the relatively large positive chemical shifts
in the resonance lines of the carbons bonded directly
to the sulphonate group are observable (Table
I)1-2-4. These large shifts cannot be explained with
the mesomeric or inductive effect of the sulphonate
group. On the contrary, relatively small changes in
the chemical shifts induced by the sulphonate group
relative to naphthalene in ortho-, meta- and paracarbons have been noted. In explaining the changes
of these chemical shifts, especially the large shift
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K . Räisänen-L. H. J. Lajunen • Nuclear Magnetic Resonance Studies on Aromatic Compounds
819
Table I. 13C chem ical sh ifts of th e present disulpho-substituted naphthols relative to th e extern al standard
(TMS) a t 298 K . T he Table also show s th e 13C chem ical sh ifts o f naphthalene for com parison8.
Compound
C-l
C-2
C-3
C-4
N aphthalene
136H N D S
138H N D S
2 36 H N D S
268H N D S
128.10
153.60
152.76
112.49
108.40
125.95
107.45
111.73
153.04
157.86
125.95
142.12
141.87
126.75
120.75
128.10
119.36
120.04
128.26
133.18
in the carbon attached directly to the sulphonate
group, other effects must be taken in account, such
as the field or steric effect of this relative bulk
functional group. Since there are so many different
simultaneous effects, the entire influence of the
sulphonate group on the distribution of 71-electrons
is a sum of these effects, and hence the contribution
of the pure mesomeric effect cannot be resolved.
According to the semi-empirical theory based on
the ASMO-CI method5, the effect of the substitution
on the naphthalene MO’s depends entirely upon the
position where the substituent is attached to the
molecule. In 1- and 2-naphthols the electron density
is somewhat different in the corresponding carbon
nuclei. For example, the 7r-electron density around
C-2 in 2-naphthol is about 5% greater than that
around C-l in 1-naphthol5. However, these dif­
ferences in electron density cannot explain the
changes in the differences of the chemical shifts
between the corresponding carbon nuclei (being
about 23%) in the present compounds (Fig. 1).
-0160
136HNDS
-7.66
-8.06
138HNDS
-2.64
236HNDS
-1 6 8
-5.08
268HNDS
F ig. 1. T he substituent-induced differences o f th e
chem ical shifts in the 13C NM R spectra o f 136H N D S ,
138H N D S , 2 3 6 H N D S and 2 6 8 H N D S relative to
n a p h th alen e8.
Chemical sh ifts [ppm]
C-5
C -6
C-7
128.10
127.50
135.76
127.58
130.74
125.95
142.17
126.74
138.95
136.71
125.95
124.24
130.08
125.24
123.70
C -8
C-9
C-10
128.10
124.24
136.86
130.86
138.42
133.70
127.45
121.73
137.95
131.69
133.70
133.64
137.38
132.25
129.23
In 136HNDS and 138HNDS (Fig. 1) the changes
in the chemical shifts in the carbons C-l are about
7 ppm and 2 ppm smaller than those noted in the
carbons C-2 in 268HNDS and 236HNDS, respec­
tively. This is due to the sulphonate group which is
located in ortho-position to the hydroxyl group in
236HNDS. Because the mesomeric effect of the
sulphonate group is very small, as mentioned above,
the other possible effects (e.g. the field and steric
effects) diminish the electron density around the
carbon C-2.
It has been noted in earlier investigations1-2-4-6- 7
that a negative change is induced through the
carbon C-9 to the carbon C-8 by the mesomeric
effect of the hydroxyl group when this group is
substituted in carbon C-l in the naphthalene ring.
This is, however, an agent for the abnormal trans­
mission of the mesomeric effect along the carbon
chain. Thus, it seems that in 1-naphthols the distur­
bance of the mesomeric effect changes its “sign”
when it is transmitted through the electron cloud of
the carbon C-9. The changes in the chemical shifts
in the carbons C-8 in 136HNDS and 138HNDS are
— 3.86 and + 8 .7 6 ppm, respectively. In the case
of 2-naphthols the change of the chemical shift of
the carbon C-8 in 236HNDS and 268HNDS is
diminished in the former acid (— 2.76 ppm) and
increased in the latter (10.32 ppm) with respect to
their absolute values. However, the sings of these
shifts are same as in 136HNDS and 138HNDS.
Thus, it seems that in 2-naphthols the sign rule of
the mesomeric effect of the hydroxyl group is quite
correct along the carbon nuclei. The explanation for
this finding migth lie in the fact that the mesomeric
effect on the carbon in meta-position is significantly
smaller than that on the carbon in para-position6.
On the other hand, the bridge-end in the naphthalene
ring seems to disturb the normal progress of the
mesomeric effect of the hydroxyl group.
820
K. Räisänen-L. H. J. Lajunen • Nuclear Magnetic Resonance Studies on Aromatic Compounds
In 136HNDS the sulphonate groups are located
in the different halves of the naphthalene ring (in
the carbons C-3 and C-6). In comparing the changes
of the chemical shifts of 136HXDS induced by the
substituents in the carbon C-3 with those of
138HNDS, it can be observed that all the changes
in the chemical shifts of 13C nuclei are identical
within the limits of accuracy of the measurement. In
accordance with the earlier studies6, it is observed
that in 1-naphthols the change of the chemical
shifts is about zero in a carbon in meta-position. In
138HNDS the sulphonate group is attached to the
carbon C-3 (in meta-position with regard to the
hydroxyl group). Thus, it can be assumed that the
change in the chemical shift induced in the carbon
C-3 is almost entirely caused by the sulphonate
group. Judging from the present measurements,
one can further ascertain that the sulphonate groups
in position C-3 and C-6 do not disturb each other,
because the changes in the chemical shifts induced
by these two groups in the carbons C-3 and C-6 are
identical, within the limits of accuracy of the
measurements, to those in 138HNDS in the carbon
C-3. On the other hand, in 268HNDS both the
1 L a u r i H . J . L a j u n e n and K
auko
R
ä is ä n e n ,
to be
published.
2K
auko
R
ä is ä n e n
and L a u r i H . J . L a j u n e n , to be
published.
3 C. D .
J o h n s o n , The H am m et E quation, p. 13,
Cambridge U n iv ersity Press, 1973.
sulphonate groups are located in the same half of the
naphthalene ring (in meta-positions to each other),
and disturb greatly each other.
In 236HXDS the influences of the hydroxyl and
sulphonate groups on the 7r-electron density around
the carbon C-3 are almost equal but reverse, be­
cause the two groups are in ortho-position relative
to each other.
In conclusion, one can maintain that in disulphosubstituted 1- and 2-naphthols the hydroxyl group
induces great changes in the distribution of the
7r-electron cloud: a marked shift towards upfield
can be noted in the carbon nuclei in ortho-positions,
a weak shift towards downfield in the meta-carbons,
and a considerable shift towards upfield in the paracarbons.
The sulphonate group induces a great shift to­
wards upfield in the carbon to which it is attached.
In the ortho- and meta-carbons the shifts have the
same direction towards upfield, but are of relatively
small magnitude. The entire influence of these two
groups is very complicated, especially when the
hydroxyl and sulphonate groups are located in peripositions relative to each other.
4 K a u k o R ä i s ä n e n a n d L a u r i H . J . L a j u n e n , t o be
p u b lis h e d .
5 K . N i s h i m o t o , J. P h y s. Chem. 67, 1443 [1963].
6 L . E r n s t , C h em . B e r . 1 08, 2030 [1975].
7 L . E r n s t , J . M a g n . R e s o n . 22, 279 [1976].
8 T. D . A l g e r , D . M . G r a n t , a n d E . G . P a u l , J. A m .
C h e m . S o c. 8 8 , 5397 [1966].