Article
DOI: 10.1002/bkcs.10067
BULLETIN OF THE
E. J. Jeong and I.-S. H. Lee
KOREAN CHEMICAL SOCIETY
How Much Does the Hybridization of a Carbon Atom Affect the
Transmission of the Substituent Effect on the Chemical Shift?
*
Eun Jeong Jeong and In-Sook Han Lee
Department of Science Education, Kangwon National University, Chuncheon 200-701, South Korea.
*E-mail: ishl@kangwon.ac.kr
Received September 12, 2014, Accepted October 10, 2014, Published online January 5, 2015
1H and 13C NMR spectra of aryl esters of propionic acid, acrylic acid, and propiolic acid were systematically
examined to find out thesubstituent effect onthe chemical shift. The values of the chemical shift of the carbonyl
carbon showed aninverse correlationwith the Hammett σ values, andthemagnitude of theslope wasthe largest
with the propiolates. The α carbons of acrylates and propiolates also showed an inverse correlation with much
smaller values of the slopes than those of the carbonyl carbons; but those of the propionates showed absolutely
no correlation. However, the β carbons of acrylates and propiolates showed normal correlation with larger
values of the slopes. The signs and the magnitudes of the slopes may be understood by the transmission of
the substituent electronic effect through bonds as well as through space. The propiolyloxy group also showed
a significantly large effect on the 13C chemical shift values of the benzene ring.
Keywords: Hybridization of carbon atom, Substituent chemical shift, Transmission of electronic effect
Introduction
quite challenging because of the conjugate addition to the
C C triple bond.5 The self-coupling of the propiolates (3)
is also problematic in some cases (3a, e).6 The optimum con-
ditions for the preparations of 3a–k were the use of dicyclo-
hexylcarbodiimide (DCC) and dimethylaminopyridine
(DMAP) in CH2Cl2 solution at 0 ꢀC. The yields of 1, 2, and
3 are listed in Table 5.
It is widely known that hybridization of the carbon atom is one
of the key factors in organic chemistry. For example, Cδ Hδ+
bond polarization is most affected by the s character of the car-
bon atom. Any general organic chemistry textbook typically
lists the pKa values of CH3 CH2 H, CH2 CH H, and
HC C H as about 50, 44, and 25, respectively, and attributes
the increasing acidity to the magnitude of the s character.1
A similar analogy has been generally used to explain the acid-
ity of propionic acid, acrylic acid, and propiolic acid, whose
pKa values are 4.87, 4.25, and 1.93, respectively.2 Apparently,
the effect of the change in hybridization from sp2 to sp is far
more significant on the acidity than that from sp3 to sp2.
In the course of our investigation on the transmission of the
substituenteffectofm-andp-substitutedarylcompoundsonthe
protons and the carbon atoms in the side chain,3 we came to
examine the effect of the substituent on the NMR chemical shift
expecting that the Hammett substituent constant (σ) may be lin-
early related to the observed shift. At first, we were interested in
the dependence of the transmission of such effect on the nature
of the C C bond, i.e., single, double, and triple bonds.
Here we report the quite striking phenomena in the trans-
mission of the substituent effect in the aryl esters of propio-
nates (1), acrylates (2), and propiolates (3).
O
O
Z
Z
-
H
H
2
C
C
-
H
C
H
C
2
O
3
H
O
+
+
H
O
3
α
β
1
2
1'
O
O
Z
Z
O
H
O
H
H
C
α
C
β
H
C
H
C
O
H
2
2
2'
O
O
Z
Z
+
H
O
H
O
C
β
C
H
H
O
C
C
α
3
3'
-
,
; , m
H
e
-C
3
;
m
;
a, m N
O2
b, Br
,
d, m
H
-OC
Z:
-
c m-Cl;
3
,
; h,
g p-Br
,
-
Cl
;
; , -
j p
C
, k,
H H
3
f p-
p
i, p-
H
OC
;
N
O2
3
The 1H and 13C NMR spectra of the esters 1–3 were obtained
in0.1 MCDCl3 solution, andthechemicalshiftvaluesarelisted
in Tables 1 and 2, respectively. As shown in the tables, the dif-
ference in the chemical shift values between the substituted
compound (δZ) and the unsubstituted one (δH) is listed for each
compound instead of giving the actual value, so that the com-
parison among the individual proton and carbon atom can be
made readily. Also, the application of the single substituent
parameter equation below is possible7:
Results and Discussion
Although the propionates (1) could be easily prepared, the
synthesis of the acrylates (2) was not quite as straightforward
as reported in the literature.4 Impurities originating from the
dimerization had to be removed by column chromatography.
Furthermore, the preparations of the propiolates (3a–k) were
δZ −δH = ρσ
Bull. Korean Chem. Soc. 2015, Vol. 36, 295–299
© 2015 Korean Chemical Society, Seoul & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Wiley Online Library
295