An efficient conversion of alcohols to alkyl bromides using pyridinium based ionic liquids: A green alternative to appel reaction

Article abstract of DOI:10.14233/ajchem.2018.21086

Pyridinium based ionic liquids namely 4-alkylpyridinium bromides were prepared and used for the conversion of alcohols to alkyl bromides in the presence of p-toluenesulphonic acid in the absence of volatile organic compounds. This solvent free procedure promises to be a much improved and environmentally benign alternative to the Appel reaction.

Full text of DOI:10.14233/ajchem.2018.21086

Asian Journal of Chemistry; Vol. 30, No. 3 (2018), 651-654
A
SIAN
J
OURNAL OF HEMISTRY
C
https://doi.org/10.14233/ajchem.2018.21086
An Efficient Conversion of Alcohols to Alkyl Bromides Using Pyridinium
Based Ionic Liquids: A Green Alternative to Appel Reaction
*
PRANAB J. DAS , JUPITARA DAS and DIMPEE DAS
Department of Chemistry, Gauhati University, Guwahati-781 014, India
*Corresponding author: E-mail: pranabjdas52@gmail.com
Received: 10 October 2017; Accepted: 8 January 2018;
Published online: 31 January 2018;
AJC-18760
Pyridinium based ionic liquids namely 4-alkylpyridinium bromides were prepared and used for the conversion of alcohols to alkyl
bromides in the presence of p-toluenesulphonic acid in the absence of volatile organic compounds. This solvent free procedure promises
to be a much improved and environmentally benign alternative to the Appel reaction.
Keywords: Appel reaction, Ionic liquid, Solvent free synthesis, Alkylbromides, Aliphatic alcohol.
The need for environmentally acceptable processes has
fuelled great interest towards the use of benign reagents espe-
cially the use of ionic liquids to promote reactions. Generally
reactions promoted by ionic liquids results in reduced reaction
time, simplifies product recovery and greatly improves turn
over. Leadbeater et al. [10] used imidazolium based ionic
liquids, which played the dual role as a reagent and as a solvent
in microwave mediated technique for the conversion of
alcohols to the alkyl halides in the presence of mineral acids.
Ren and Wu [11] used 1,3-dimethylimidazolium halide as
reagent for the conversion of n-butanol and n-octanol to the
corresponding halide in the presence of bronsted acids such as
HCl, H2SO4 and CH3SOOH. Petten et al. [12] used halodehydro-
genation technique using ionic liquid for the conversion of
long chain alcohols to their respective alkyl halides, Ranu and
Jana [13]reported a stereoselective one pot conversion combi-
ning the use of ionic liquid and ultrasound and finally a stereo-
selective one pot conversion of N,N’-diisopropylcarbodiimide
was reported [14] besides others [15,16].
INTRODUCTION
Alkyl halides are widely used as industrial solvents and
also as essential precursors for the preparation of a variety of
fine chemicals. The classical procedure for the conversion of
alcohols to alkyl haides is termed as the Appel reaction and
in the classical example requires the use of NaBr and H2SO4
under harsh reaction conditions [1]. Being and environmentally
unacceptable process, this synthesis has limited synthetic
applicability today. A direct conversion of alcohols to alkyl
halides is difficult due to poor leaving group characteristics of
the hydroxyl group, however several improved procedures have
been reported to overcome this drawback. An elegant proce-
dure for the conversion was reported wherein triphenylphos-
phine and tetrahaloalkanes were used [2]. Other notable syn-
thetic procedures are the Mitsunobo reaction [3], the reaction
of 2,4,6-trichloro-[1,3,5]-triazine (TCT) in DMF followed by
addition of CH2Cl2 solution of the reactant alcohol [4], the
reaction of alcohol with 2,3-trichloro-5,6-dicyanobenzo-quinone
in the presence of triphenyl phosphine, CH2Cl2 and a phase
transfer catalyst [5]. ROMP-gel supported triphenyl-phosphine
[6], phosphorus(V) mediated nucleophilic substitution reaction
[7] and finally the use of fluorous phosphine [8] besides others.
The previously reported procedures suffer from the use of toxic
and expensive reagents and further the use of triphenylphosphine
makes product recovery difficult and time consuming specially
due to the in situ formation of the phosphane oxide. In order
to avoid the use of triphenylphosphine, an elegant phosphine
free halogenation protocol using a visible light photoredox
catalyst Ru(bpy)3Cl2 was reported [9].
EXPERIMENTAL
GC/MS was carried out on a Perkin Elmer Clarus 600 gas
chromatograph and Clarus 600C mass spectrometer (column
30.0 m × 250 µm). 1H was done on a Bruker 300 MHz instru-
ment using CDCl3 as the solvent, 4-methylpyridine, ethyl-
bromide, n-butylbromide, n-octylbromide, substrate alcohols
and p-toluenesulphonic acid were purchased from Sigma
Aldrich and were used as received. The ionic liquids, 1-n-alkyl-
4-methylpyridinium bromide, were prepared from 4-methyl-
pyridine and characterized.

652 Das et al.
Asian J. Chem.
Thermogravimetric analysis (TGA): TGA experiments
which could be identified by running GC with authentic sample.
Products were examined for their purity by TLC using silica
gel plates and n-hexane:petroleum ether (40-60) (9:1) as the
eluent.
were performed using a TGA-DSC1, Mettler Toledo instru-
ment. The samples were weighed and placed in a platinum
crucible. They were then heated in a stream of nitrogen atmos-
phere, from room temperature to 700 °C with a heating rate
usually of 10 °C/min.
General method for preparation of ionic liquids: Three
different pyridinium based ionic liquids namely 1-ethyl-4-
methylpyridiniumbromide, 1-n-butyl-4-methylpyridiniumbromide
and 1-n-octyl-4-methyl pyridinium bromide were prepared by
simply mixing 4-methylpyridine (γ-picoline) and the appro-
priate alkyl halide and stirring the mixture in the dark for 20-
24 h (Scheme-I). The yield of the ionic liquids was found to
be almost quantitative.
Recovery of 1-n-butyl-4-methylpyridinium-p-toluene-
sulphonate: After separation of the products, the precipitated
ionic liquid was found to be transformed to the p-toluene-
sulphonate. For further purification, 25 mL petroleum ether
(40-60 °C) was added to this and stirred for 5 min to remove
any trace of alkyl bromides. The petroleum ether layer on
evaporation afforded the 1-n-butyl-4-methyl-pyridinium-p-
toluenesulphonate, which was recovered and stored in dessi-
ccator. The recovered product was identified as the sulphonate
1
1
form by H NMR spectra. H NMR (300 MHz, CDCl3): δH
ppm 8.915 (d, 2H, J = 6.3 Hz, ArH), 7.740-7.685 (m, 4H,
ArH), 7.112 (d, 2H, J = 7.8 Hz,ArH), 4.612 (t, 2H, J = 7.2 Hz,
2H, NCH2), 2.520 (s, 3H,ArCH3), 2.303 (s, 3H,ArCH3), 1.867-
1.767 (m, 2H, CH2), 1.292-1.192 (m, 2H, CH2), 0.840 (t, 3H,
J = 7.2 Hz, CH3); 13C NMR (75 MHz, CDCl3): δ ppm 158.38,
143.51, 139.58, 128.50, 125.50, 60.44, 32.94, 21.61, 20.93,
18.79, 13.10. The sulphonate form of the ionic liquid was
converted to the bromide form by treatment with 30 % HBr.
Stirring
R
Br
+
24-30 hr
N
R
N
R = C2H5, C4H9, C8H17
Br
Scheme-I
RESULTS AND DISCUSSION
In all reported synthetic methods, the imidazolium based
ionic liquids were use as the promoter. These ionic liquids are
costly, less effective and the methods reported requires high
pressure and long reaction time and the procedures are limited
to a few selected alcohols and could not be generalized. Search
for inexpensive, efficient and environmentally benign methods
for carrying out the Appel reaction is still being explored. In
order to overcome the disadvantages associated with the use
of immidazolium based ionic liquids, we examined the possi-
bility of the use of pyridinium based ionic liquids for promoting
the Appel reaction for the conversion of alcohols to the alkyl
bromides. The results reported herein indicate the superiority
of pyridinium based ionic liquids vis-a-vis imidazolium coun-
terparts. The pyridinium based ionic liquids used in this study
were prepared by a simple reported procedure [17] using low-
cost and easily available starting compounds. Three different
ionic liquids were prepared starting from 4-methylpyridine
and corresponding alkyl halides namely 1-ethyl-4-methylpyri-
diniumbromide [1-E-4-MPyr]Br, 1-n-butyl-4-methyl-pyridinium
bromide [1-n-B-4Mpyr]Br and 1-n-octyl-methylpyridinium-
bromide [1-n-O-4-Mpyr]Br and characterized. Results indicate
that all the ionic liquids have thermal stability upto 250 °C.
Thermogravimetric curves of the three ionic liquids indicate
that 1-n-butyl-4-methyl pyridinium-bromide had a melting
range of 110-280 °C and this being close to the melting range
of usual ionic liquids was considered to be the most appropriate
of the three ionic liquids for the present synthesis. The TGA
curves are shown in Fig. 1.
1-Ethyl-4-methylpyridinium bromide: m.f.: C8H12NBr,
white solid, melting range 250-300 °C; 1H NMR (300 MHz,
CDCl3, Me4Si) (δH ppm): 9.322 (2H, d, J = 6.6 Hz, pyr-CH),
7.756 (2H, d, J = 6.3Hz, pyr-CH), 4.803 (2H, q, J =7.2Hz,
CH2), 2.503 (3H, s, CH3),1.532 (3H, t, J = 7.2 Hz, CH3); 13C
NMR (75 MHz, CDCl3) (δ ppm): 158.53, 143.85, 128.72,
56.12, 22.03, 17.03.
1-n-Butyl-4-methyl pyridinium bromide: m.f.: C10H16NBr,
nature: white waxy solid, melting range 110-280 °C; 1H NMR
(300 MHz, CDCl3, Me4Si) (δH ppm): 9.363 (2H, d, J = 6.6 Hz,
pyr-CH), 7.840 (2H, d, J = 6.0 Hz, pyr-CH), 4.812 (2H, t, J =
7.2 Hz, CH2), 2.577 (3H, s, CH3), 1.972-1.872 (2H, m, CH2),
1.381-1.282 (2H, m, CH2), 0.853 (3H, t, J = 7.2 Hz, CH3); 13C
NMR (75 MHz, CDCl3) (δ ppm): 158.51, 144.17, 128.70,
60.59, 33.51, 22.08, 19.09, 13.36.
1-n-Octyl-4-methyl pyridinium bromide: m.f.: C14H24NBr,
nature: light pink waxy solid, melting range 300-400 °C; 1H
NMR (300 MHz, CDCl3, Me4Si) (δH ppm): 9.338 (2H, d, J =
6.3 Hz, pyr-CH), 7.893 (2H, d, J = 6.3 Hz, pyr-CH), 4.872
(2H, t, J = 7.5 Hz, CH2), 2.645 (3H, s, CH3), 2.022-
1.941 (2H, m, CH2), 1.300-1.204 (10H, m, CH2), 0.824 (3H,
t, J = 6.9 Hz, CH3); 13C NMR (75 MHz, CDCl3) δ: 158.54,
144.11, 128.75, 60.93, 31.69, 31.45, 28.82, 25.84, 22.35,
22.09, 13.87.
General procedure for the conversion: A mixture of 5
mmol alcohol, 5 mmol p-toluenesulphonic acid, 5 mmol 1-n-
butyl-4-methylpyridiniumbromide was heated in an oil bath
with constant stirring to different temperatures (Table-2), the
progress of the reactions monitored by GC after 0.5, 1.0, 1.5,
2.5 and 5 h. In the GC experiments, aliquots of reaction mixture
drawn, diluted with 1 mL of diethylether, solution filtered and
GC recorded. After complete conversion as indicated by GC,
25 mL of n-hexane was added to precipitate out the ionic liquid.
The filtrate on evaporation of the solvent afforded the bromides,
The ionic liquids were used as a promoter for a solvent
free conversion of a variety of alcohols to the corresponding
bromides. Initially the efficiency of the individual ionic liquids
were screened for their utility. The conversion of octan-1-ol
to 1-bromooctane using the three ionic liquids individually
was examined as a model reaction and the results summarized

Vol. 30, No. 3 (2018)
An Efficient Conversion of Alcohols to Alkyl Bromides Using Pyridinium Based Ionic Liquids 653
100
90
80
70
60
50
40
30
110
100
90
80
70
60
50
40
30
20
10
0
110
100
90
80
70
60
50
40
30
20
10
0
(a)
(b)
(c)
Mass change: 48 %
Mass change: 95 %
Mass change: 85 %
-10
-10
0 100 200 300400 500 600 700 800 9001000
Temperature (°C)
0 100 200 300400 500 600 700 800 900 1000
Temperature (°C)
0 100 200 300400 500 600 700 800 9001000
Temperature (°C)
Fig. 1. TGA curve of (a) 1-n-butyl-4-methylpyridiniumbromide (b) 1-ethylpyridiniumbromide (c) 1-n-octylpyridiniumbromide
in Table-1. Results indicates the better efficiency, in terms of
better yield of products, of 1-n-butyl-4-methyl-pyridiniumbromide
[1-n-B-4Mpyr]Br over other ionic liquids used. Consequently
all subsequent reactions were carried out with using [1-n-B-
4Mpyr]Br.
of the reaction was monitored by GC of aliquots drawn from
the reaction mixture at time intervals of 0.5, 1.5, 2.5 and 5 h.
The reaction is shown in Scheme-II and results obtained for a
variety of alcohols summarized in Table-2. The reaction time
and temperature necessary for complete conversion, varied
with the nature of the substrate alcohol.
TABLE-1
Conclusion
EFFICIENCY OF DIFFERENT PYRIDINIUM BASED
IONIC LIQUID FOR CONVERSION OF OCTAN-1-OL TO
n-OCTYLBROMIDE AFTER 5 h OF REACTION TIME
The use of pyridinium based ionic liquid for the Appel
reaction offers an easy access to the alkylbromides. The pro-
ducts could be isolated by simple filtration of the precipitated
ionic liquids. The precuts were identified performing GC with
authentic sample. The ionic liquids could be recycled. Simple
reaction condition, absence of costly reagents, high turnover
of products makes this procedure an improved practical alter-
native to the conventional Appel reaction.
Reaction
temp. (°C)
% Conversion
after 5 h*
Entry
1
Ionic liquid
1-Butyl-4-methylpyridinium
bromide
1-Ethyl-4-methylpyridinium
-bromide
1-Octyl-4-methylpyridinium
-bromide
130
130
130
100
2
3
65
70
*% conversion obtained from GC experiments.
ACKNOWLEDGEMENTS
In a typical procedure, an equimolar mixture of alcohol,
p-toluenesulphonic acid and 1-n-butyl-4-methylpyridinium-
bromide was taken in an round bottom flask and heated to the
optimized temperature for range of reaction time. The progress
One of the author, JD is grateful to CSIR, New Delhi for
grant of Junior Research Fellowship and another author, DD
is grateful to University Grand Commission, New Dehi for
providing the fellowship under BSR programme.
TABLE-2
SYNTHESIS OF ALKYL BROMIDES FROM ALCOHOLS USING AS THE
IONIC LIQUID [1-B-4-M-Pyr]Br AS THE BROMINATING AGENT
Entry
Reactants
Butan-1-ol
Pentan-1-ol
Pent-2-ol
Hexan-1-ol
Cyclohexanol
Heptan-1-ol
Octan-1-ol
Benzyl alcohol
4-chlorobenzyl alcohol
Products
n-Bromobutane
n-Bromopentane
2-Bromopentane
n-Bromohexane
Bromocyclohexane
n-Bromoheptane
n-Bromooctane
Benzyl Bromide
4-Chlorobenzyl Bromide
Reaction temp. (°C)
Time (h)
5.0
5.0
7.0
4.0
5.0
5.0
2.5
0.5
Conversion (%)
1
2
3
4
5
6
7
8
9
100
130
120
130
140
130
140
140
140
100
100
100
100
100
100
100
100
100
1.0
N
PTSA
SO3
N
R
Br
R
OH
+
+
+
H2O
Br
alcohols
alkyl bromides
Where R= CH3(CH2)n- where n= 3,4,5,6,7
= C6H11, C6H5CH2, 4-ClC6H4CH2-
Scheme-II: Conversion of alcohols to alkylbromides using pyridinium based ionic liquids

654 Das et al.
Asian J. Chem.
9. J.M.R. Narayanam, J.W. Tucker and C.R.J. Stephenson, J. Am. Chem.
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