Investigations into the nitric acid mediated dehalonitration of halophenols

Article abstract of DOI:10.1016/S0040-4039(03)01618-6

A reaction of nitric acid with bromophenols and iodophenols results in substitution of the halogen with a nitro group. The study indicates moderate reactivity for bromophenols and iodophenols, while chlorophenols were found to be sluggish in this reaction.

Full text of DOI:10.1016/S0040-4039(03)01618-6

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 6393–6395
Investigations into the nitric acid mediated dehalonitration of
halophenols
Subbarayappa Adimurthy, Sanjay S. Vaghela, Punita V. Vyas, Anjani K. Bhatt,
Gadde Ramachandraiah and Ashutosh V. Bedekar*
Central Salt and Marine Chemicals Research Institute, G.B. Road, Bhavnagar 364 002, India
Received 11 April 2003; revised 16 June 2003; accepted 27 June 2003
Abstract—A reaction of nitric acid with bromophenols and iodophenols results in substitution of the halogen with a nitro group.
The study indicates moderate reactivity for bromophenols and iodophenols, while chlorophenols were found to be sluggish in this
reaction.
© 2003 Elsevier Ltd. All rights reserved.
In recent years there has been considerable interest in
exploring the possibility of using aromatic bromides as
protecting groups in synthetic organic chemistry.1,2 This
notion is aided by the easy accessibility of several
aromatic bromo compounds as well as their high reac-
tivity for substitution reactions. One such reaction is
electrophilic debromination, brought about using
hydrobromic acid (HBr) in the presence of a suitable
bromine scavenger. Due to the considerable potential of
this subject, we were interested in studying the reaction
of halophenols, in general, with other electrophiles and
present our findings on the specific reaction with nitro-
nium ions in this communication. This reaction is simi-
lar to the investigations reported by Zincke3 and
others4,5 when bromophenols were exposed to the
source of nitronium ion or sodium nitrite,6 resulting in
the formation of the corresponding nitrophenols.
equivalent of concentrated nitric acid under ambient
conditions furnished a single product which was iden-
tified as 4-nitro-2,6-dibromophenol 2, while the same
reaction with two equivalents of nitric acid resulted in
the formation of 2-bromo-4,6-dinitrophenol 3, as
shown in Eq. (1). The substitution of bromine is selec-
tive in 1 where the para-bromine was substituted by a
nitro group. Similarly, when 1 was exposed to two
equivalents of nitric acid, the substitution of the sec-
ond bromine at the ortho position was achieved, albeit
more slowly, to give 3, in good yield, as a single
product.
It is known in the literature7,8 that in concentrated
nitric acid the equilibrium outlined in Eq. (2) is
present, providing a source of nitronium ions. The
subsequent reaction of a nitronium ion with 1 gives
intermediate 4, which loses a bromonium ion to give
the nitrophenol 2 as shown in Eq. (3).
The reaction of 2,4,6-tribromophenol 1 with one
(1)
Keywords: dehalonitration; halophenols; nitrophenols; nitric acid.
* Corresponding author. Fax: 91-0278-2567562; e-mail: avbedekar@yahoo.co.in
0040-4039/$ - see front matter © 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/S0040-4039(03)01618-6

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S. Adimurthy et al. / Tetrahedron Letters 44 (2003) 6393–6395
8, albeit in low yield. The other product isolated was
identified as 11 by the usual spectral and analytical
methods. The reaction of 1-iodo-2-naphthol 12 gave
mainly 8 in a good isolated yield. The higher conver-
sions of iodophenols to nitrophenols compared to other
halophenols could be because the iodonium ion is a
better leaving group. Reaction of 2-naphthol 13 under
identical reaction conditions furnished 8 in only 32%
yield as the main product (Chart 1).
(2)
Further exploration of the reaction of nitric acid with
other substrates such as 4-bromoanisole, 4-iodoanisole,
(3)
1-bromo-2-methoxynaphthalene,
1-iodo-2-methoxy-
naphthalene and 4-iodo acetanilide did not result in the
substitution of the halogen by a nitro group nor nitra-
tion at other positions. This result indicates the neces-
The reaction of tetrabromobisphenol 5 with two equiv-
alents of nitric acid gave the dinitro product 6 as the
major product, where one bromine from each aromatic
ring had been substituted, Eq. (4). In this reaction
along with 6 a small amount of a mononitro product,
resulting from a monosubstitution was also isolated.
sity of having
a free hydroxyl group for this
transformation which fits in with the mechanism sug-
gested in Eq. (3).
Thus we present our findings on the reactions of con-
centrated nitric acid with halophenols which forms
nitrophenols under simple reaction conditions.
We examined the reaction of nitric acid with 1-halo-2-
naphthols to compare the reactivity of three different
derivatives. The reaction of 1-chloro-2-naphthol 7 did
not furnish the substitution product 8, instead nitration
took place at other positions. We isolated the major
product and identified it as 6-nitro-1-chloro-2-naphthol
9. However, the reaction of 1-bromo-2-naphthol 10
with nitric acid gave the expected substitution product
Standard reaction conditions
Reaction of 1-iodo-2-naphthol: To a solution of 1-iodo-
2-naphthol (0.27 g, 1 mmol) in diethyl ether (15 mL) at
(4)
Chart 1.

S. Adimurthy et al. / Tetrahedron Letters 44 (2003) 6393–6395
6395
ambient temperature was added concentrated nitric
References
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hexane) as yellow crystals (0.125 g; 66%), and its struc-
ture was confirmed by melting point and NMR analysis
as well as by comparison by thin layer chromatography
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Acknowledgements
We wish to thank Dr. P. K. Ghosh for his help and
encouragement and to, the CSIR, New Delhi for finan-
cial assistance under the Quick Hire Scheme (A.V.B.).