An efficient and convenient synthesis of N-substituted amides under heterogeneous condition using Al(HSO4)3 via Ritter reaction

Article abstract of DOI:10.1007/s12039-016-1036-x

An efficient and inexpensive synthesis of N-substituted amides from the reaction of aliphatic and aromatic nitriles with various benzylic alcohols (secondary and tertiary) and tert-butyl alcohol by refluxing nitromethane via the Ritter reaction catalyzed by aluminum hydrogen sulfate [Al(HSO₄)₃] is described. The catalyst which is an air-stable, cost-effective solid acid could be readily recycled by filtration and reused four times without any significant loss of its activity. [Figure not available: see fulltext.]

Full text of DOI:10.1007/s12039-016-1036-x

c
J. Chem. Sci. ꢀ Indian Academy of Sciences.
DOI 10.1007/s12039-016-1036-x
An efficient and convenient synthesis of N-substituted amides under
heterogeneous condition using Al(HSO4)3 via Ritter reaction
ELNAZ KARIMIAN, BATOOL AKHLAGHINIA∗ and SARA S E GHODSINIA
Department of Chemistry, Faculty of Sciences, Ferdowsi University of Mashhad, Mashhad 9177948974, Iran
e-mail: akhlaghinia@um.ac.ir
MS received 12 October 2015; revised 16 December 2015; accepted 26 December 2015
Abstract. An efficient and inexpensive synthesis of N-substituted amides from the reaction of aliphatic and
aromatic nitriles with various benzylic alcohols (secondary and tertiary) and tert-butyl alcohol by refluxing
nitromethane via the Ritter reaction catalyzed by aluminum hydrogen sulfate [Al(HSO4)3] is described. The
catalyst which is an air-stable, cost-effective solid acid could be readily recycled by filtration and reused four
times without any significant loss of its activity.
Keywords. Aluminum hydrogen sulfate [Al(HSO4)3]; Ritter reaction; N-substituted amides; heterogeneous
catalyst; nitriles; alcohols.
solid catalyst,3–5 formic acid under reflux,6 HBF4Et2O,7
2,4-dinitrobenzenesulfonic acid,8 HClO4-functionalized
silica-coated nanoparticles,9 sulfonic-acid functionalized
silica,10 P2O5/SiO2,11 NaHSO4/SiO2,12,13 liquid HF,14 TMS–
CN/H2SO4 as a reagent for the synthesis of formamides,15
t-BuOAc instead of t-BuOH as a carbocation precursor,16
Tf2O/ROH as an in situ source of ROTf,17 pentafluoropheny-
lammoniumtriflate (PFPAT),18 trifluoromethanesulfonic
acid (TfOH),19 Bi(OTf)3 and other metallic triflates,20–23
metal complexes,24 ionic liquids,25,26 nitrosonium salt to
effect the Ritter reaction of halides with nitriles,27 using of
gold (I) catalyst (this protocol is not suitable for solid nitrile
for which the nitrile is used as the solvent),28 silica-
bonded N-propyl sulphamicacid),29 nanocat-Fe-OSO3H,30
Amberlyst-(P-SO3H),31 Ca(HSO4)2,32 KAl(SO4)2.12H2O,33
H2SO4/ball mill,34 o-benzenedisulfonamide,35 sulfated
tungstate,36 and FeCl3/AgSbF637 Fe(ClO4)3/silica gel,38
4,5,6,7-tetrachlorobenzo[d][1,3,2]dioxaborol-2-ol,39 and
N-Alkyl-4-boronopyridinium halides.40 Although the
modified methods offer better chemo-selectivity and
milder, environmentally more acceptable conditions,
some of them suffer from at least one of the following
issues: limited availability of reagent, using hygro-
scopic reagents, low yield, tedious product isolation,
longer reaction time and competing side reactions.
In this paper we wish to report a comprehensive study
of the reactions between various organic nitriles and
alcohols in the presence of catalytic amount of Al(HSO4)3
to provide N-substituted amides via Ritter fashion.
1. Introduction
The classical Ritter reaction as one of the most impor-
tant carbon-nitrogen (C–N) bond-forming reaction
offers a particularly atom-economical approach to the
synthesis of amides. This reaction involves the reac-
tion of a carbocation precursor (tertiary alcohol or
substituted olefin), with nitriles in the presence of sto-
ichiometric amount of a strong acid, typically H2SO4
and often together with a highly ionizing solvent such
as AcOH.1,2 However, the Ritter reaction requires stoi-
chiometric amount of reagents, elevated temperatures,
harsh reaction conditions, long reaction times and based
on the structure of the reactants the reaction is accom-
panied by rearrangement and side products. Also, use
of an excess amount of corrosive sulfuric acid is the
main disadvantage of the classical Ritter reaction which
has limited its applicability to compounds containing
functional groups stable to acid. Despite this limitation,
the Ritter reaction has found widespread use in synthe-
sis. Over the years, due to the importance of amides as
building blocks in organic and medicinal chemistry and
to overcome the above mentioned limitations, several
variations have been reported for the Ritter reaction
to achieve higher yields with better chemo-selectivity
under milder and environmentally acceptable condi-
tions. These variations include the use of Nafion-H as a
∗For correspondence

Elnaz Karimian et al.
FT-IR (KBr): υmax/cm−1 3295 (N-H), 3060, 3029, 2925,
2. Experimental
2855, 1653 (C=O), 1525, 1451, 1278, 985, 742, 702,
1
514; H NMR (300 MHz, CDCl3, 25◦C) δ/ppm 7.86-
2.1 General
7.84 (m, 2H, PhCO), 7.57-7.29 (m, 13H, Ph), 6.75 (brd,
1H, J = 7.2 Hz, NH), 6.49 (d, 1H, J = 7.8 Hz,
CHNH); MS(EI) m/z (%) 287 (17) [M+], 104 (100)
[M+-C13H13N], 76 (80) [C6H4].
The purity determination of the products was accom-
plished by TLC on silica gel polygram STL G/UV 254
plates. The melting points of products were determined
with an Electrothermal Type 9100 melting point appara-
tus. The FT-IR spectra were recorded on an Avatar 370
FT-IR Therma Nicolet spectrometer. The NMR spectra
in CDCl3 were recorded on Bruker Avance 400 and 300
MHz instruments. Mass spectra were recorded with a
CH7A Varianmat Bremem instruments at 70 eV; in m/z
(rel%), Elemental analyses were performed using
Thermo Finnegan Flash EA 1112 Series instrument.
The catalyst was prepared and purified by the method
described in the literature.41 All yields refer to isolated
products after purification by recrystallization or thin
layer chromatography.
2.3b N-Benzhydryl-4-ethoxybenzamide (table 2, entry
2): White solid, yield 95%, M.p. 178-180◦C; FT-
IR (KBr): υmax/cm−13309 (N-H), 3057, 2973, 2930,
1637 (C=O), 1532, 1493, 1297, 1250, 1041, 849, 742,
1
696; H NMR (300 MHz, CDCl3, 25◦C) δ/ppm 7.80
(d, 2H, J = 8.7 Hz, Ph), 7.40-7.29 (m, 10H, Ph),
6.94 (d, 2H, J = 8.7 Hz, Ph), 6.61 (brd, 1H, J =
7.5 Hz, NH), 6.47 (d, 1H, J = 7.5 Hz, CHNH), 4.10
(q, 2H, J = 6.9 Hz, OCH2CH3), 1.46 (t, 3H, J =
7.2 Hz, OCH2CH3);13C NMR(75 MHz, CDCl3, 25◦C)
δ/ppm 166.03, 161.78, 141.65, 132.05, 128.88, 128.75,
127.53, 126.25, 114.28, 63.69, 57.39, 14.71. MS (EI)
m/z (%) 331 (19) [M+], 181 (72) [M+-C9H8O2], 120
(70) [M+-C14H11NO], 76 (41) [C6H4], 28 (64) [M+-
C2H4]; Elemental analysis data (%) for (C22H21NO): C
(79.62), H (7.23), N (5.43); found C (79.73), H (6.39),
N (4.23).
2.2 Preparation of aluminum hydrogen sulfate
Anhydrous aluminum chloride (33.4 g, 0.25 mol) was
placed into a 500 mL suction flask equipped with a
constant-pressure dropping funnel. The gas outlet was
connected to a vacuum system through an adsorbing
solution (water) and an alkali trap. Concentrated sul-
furic acid (98%, 73.5 g, 0.75 mol) was added drop-
wise over a period of 1 h at room temperature during
which HCl was evolved. After completion of the
addition, the mixture was stirred for 30 min, while
the residual HCl was eliminated by suction. Finally,
a pale-brown solid material was obtained (77.0 g)
and characterized according to the previously reported
procedure.41
2.3c N-Benzhydryl-3,4,5-trimethoxybenzamide (table 2,
entry 3): White solid, yield 90%, M.p. 200-201◦C
(Lit. 202-203◦C);43 FT-IR (KBr): υmax/cm−1 3291 (N-
H), 3068, 3031, 3002, 2937, 2839, 1628 (C=O), 1581,
1534, 1495, 1337, 1234, 1126, 1004, 757, 698,607;
1H NMR (300 MHz, CDCl3, 25◦C) δ/ppm 7.41-7.29
(m, 10H, Ph), 7.06 (s, 2H, Ph), 6.69 (brd, 1H, J = 7.5
Hz, NH), 6.46 (d, 1H, J = 7.5 Hz, CHNH), 3.91 (s, 9H,
OCH3);.MS (EI) m/z (%) 377 (5) [M+], 181 (46) [M+-
C10H12O4], 167 (54)[M+-C14H12NO], 77 (36) [C6H5],
28 (87) [M+- (C9H11O3+ C13H12N)].
2.3 Preparation of N-benzhydryl-2-phenylacetamide
in the presence of Al(HSO4)3
Al(HSO4)3 (0.5 mmol, 0.160 g) was added to a solution
of phenylacetonitrile (1 mmol, 0.117 g) in nitromethane
(2 mL) at 100◦C. Benzhydrol (1 mmol, 0.184 g) was
dissolved in nitromethane (2 mL) and added to the
reaction mixture dropwise during 3 h. Progress of the
reaction was monitored by TLC. After completion of
the reaction, the catalyst was removed by simple fil-
tration. The concentrated residue was recrystallized
from n-hexane to provide the corresponding amide (N-
benzhydryl-2-phenylacetamide) as a white solid (0.295
g) with 98% isolated yield.
2.3d N-Benzhydryl-4-methylbenzamide (table 2, entry 4):
White crystal, yield 85%, M.p. 176-177◦C (Lit. 176-
177◦C);6 FT-IR (KBr): υmax/cm−13316 (N-H), 3088,
3060, 3027, 2929, 2851, 1636 (C=O), 1610, 1524,
1
1494, 1311, 1033, 752, 737, 701, 576. H NMR (300
MHz, CDCl3, 25◦C) δ/ppm 7.75 (d, 2H, J = 8.1 Hz,
Ph), 7.41-7.25 (m, 12H, Ph), 6.71 (brd, 1H, J = 6.9
Hz, NH), 6.48 (d, 1H, J = 7.8 Hz, CHNH), 2.43
(s, 3H, CH3Ph). MS (EI) m/z (%) 301 (5) [M+],
182 (92) [M+-C8H7O], 166 (100) [M+-C8H9NO], 91
(42) [M+-C14H12NO], 77 (80) [C6H5], 29 (75) [M+-
2.3a N-Benzhydrylbenzamide (table 2, entry 1): White
crystal, yield 93%; M.p. 167-168◦C (Lit. 168-169◦C);42 (C13H12N+ C7H7)].

Synthesis of N-substituted amides using Al(HSO4)3
2.3e N-Benzhydrylthiophene-2-carboxamide (table 2, 3027, 2913, 2851, 1653 (C=O), 1524, 1493, 1449,
entry 5): Light brown solid, yield 95%, M.p. 168- 1052, 737, 702, 608; 1H NMR (300 MHz, CDCl3, 25◦C)
172◦C (Lit. 161◦C);44 FT-IR (KBr): υmax/cm−1 3327 δ/ppm 7.44-7.28 (m, 15H, Ph), 6.75 (brd, 1H, J = 7.2
(N-H), 3084, 3023, 3002, 2917, 1620 (C=O), 1541, Hz, NH), 5.45 (s, 1H, CHNH), 3.71 (s, 3H, CH2CO);
1510, 1494, 1317, 860, 715, 699. 1H NMR (300 MHz, MS (EI) m/z (%) 301 (3) [M+], 299 (59) [M+-2H], 166
CDCl3, 25◦C) δ/ppm 7.58 (d, 1H, J = 2.9 Hz, thio- (100) [M+-C8H9NO], 182 (93) [M+-C8H7O], 105 (73)
phene), 7.52 (d, 1H, J = 4.4 Hz, thiophene), 7.41-7.29 [M+- (C6H5+ C8H7O)], 91 (59) [C7H7], 77 (69) [C6H5],
(m, 10H, Ph), 7.10 (t, 1H, J = 3.8 Hz, thiophene), 28 (73) [M+- (C7H7+ C13H12N)].
6.59 (brd, 1H, J = 6.9 Hz, NH), 6.45 (d, 1H, J =
7.8 Hz, CHNH); 13C NMR (75 MHz, CDCl3, 25◦C)
δ/ppm 160.99, 141.28, 138.62, 130.27, 128.78, 128.35,
127.69, 127.65, 127.56, 57.38. MS (EI) m/z (%) 293
(5) [M+], 181 (82) [M+-C5H4OS], 166 (82) [M+-C5H5
NOS], 111 (100) [M+-C13H12N], 83 (23) [M+-C4H3S],
77 (83) [C6H5],28 (83) [M+- (C4H3S+ C13H12N)]; Ele-
mental analysis data (%) for (C18H15NOS): C (73.69),
H (5.15), N (4.77), S (10.93); found (%): C (73.17), H
(5.06), N (4.90), S (10.55).
2.3i N-Benzhydryl-2-(3-methoxyphenyl) acetamide
(table 2, entry 9): Brown solid, yield 92%, M.p.
124-126◦C (Lit. 125-127◦C);47 FT-IR (KBr): υmax/cm−1
3287 (N-H), 3129, 3060, 3027, 2921, 2851, 1649
(C=O), 1602, 1533, 1494, 1460, 1236, 1048, 752, 696;
1H NMR (300 MHz, CDCl3, 25◦C) δ/ppm 7.37-7.25
(m, 10H, Ph), 7.14-7.09 (m, 4H, Ph), 6.46 (brs, 1H,
NH), 6.23 (d, 1H, J = 8.4 Hz, CHNH), 3.77 (s, 3H,
OCH3Ph), 3.68 (s, 2H, CH2CO); MS (EI) m/z (%) 331
(3) [M+], 330 (32) [M+-1H], 181 (44) [M+-C9H10O2],
2.3f N-Benzhydrylacetamide (table 2, entry 6): White 166 (100) [M+-C9H11NO2], 91 (93) [M+- (C14H12NO+
solid, yield 95%, M.p. 109-110◦C (Lit. 110-111◦C);45 CH3O)], 77 (44) [C6H5], 28 (91) [M+- (C8H9O+
FT-IR (KBr): υmax/cm−1 3259 (N-H), 3186, 3054, 3023, C13H12N)].
2925, 1644 (C=O), 1544, 1493, 1450, 1370, 1288, 744,
1
699, 553, 469; H NMR (300 MHz, CDCl3, 25◦C)
δ/ppm 7.39-7.24 (m, 10H, Ph), 6.29 (brd, 1H, J =
2.3j N-(1-Phenylethyl)benzamide (table 2, entry 10):
White solid, yield 85%, M.p. 102-104◦C (Lit. 103-
8.1 Hz, NH), 6.09 (d, 1H, J = 7.2 Hz, CHNH), 2.09
(s, 3H, CH3CO); MS (EI) m/z (%) 225 (68) [M+],
181 (88) [M+-C2H4O], 147 (67) [M+-C6H6], 106 (82)
[M+- (C6H5+ C2H3O)], 77 (72) [C6H5], 43 (72) [M+-
C13H12N].
104◦C);36 FT-IR (KBr): υmax/cm−1 3346 (N-H), 3031,
2974, 2926, 1633 (C=O), 1521, 1489, 1275, 1013, 700,
1
647, 554; H NMR (300 MHz, CDCl3, 25◦C) δ/ppm
7.82-7.79 (m, 2H, PhCO), 7.56-7.32 (m, 8H, Ph), 6.36
(brd, 1H, J = 5.4 Hz, NH), 5.38 (qn, 2H, J = 7.2 Hz,
CHNH), 1.65 (d, 3H, J = 6.9 Hz, CH3CH); MS (EI)
2.3g N-Benzhydrylpentanamide (table 2, entry 7): m/z (%) 225 (16) [M+], 223 (93) [M+-2H], 120 (30)
White solid, yield 90%, M.p. 115-117◦C (Lit. 115- [M+-C7H5O], 105 (100) [M+-C8H10N], 77 (93) [C6H5],
117◦C);43 FT-IR (KBr): υmax/cm−1 3249 (N-H), 3186, 28 (93) [M+- (C6H5+ C8H10N)].
3050, 3028, 2957, 2926, 2859, 1638 (C=O), 1542,
1493, 1451, 1269, 1118, 743, 698, 556. 1H NMR (300
MHz, CDCl3, 25◦C) δ/ppm 7.39-7.24 (m, 10H, Ph),
2.3k 4-Ethoxy-N-(1-phenylethyl) benzamide (table 2,
6.29 (d, 1H, J = 8.1 Hz, CHNH), 6.07 (brd, 1H,
entry 11): Light brown crystal, yield 90%, M.p. 145-
146◦C; FT-IR (KBr): υmax/cm−1 3318 (N-H), 3064,
J = 7.5 Hz, NH), 2.28 (t, 2H, J = 7.8 Hz, CH2CO),
1.68 (qn, 2H, J = 7.6 Hz, CH2CH2Et), 1.38 (sx, 2H,
J = 7.5 Hz, CH2CH2CH3), 0.94 (t, 3H, J = 7.5 Hz,
CH2CH3); MS (EI) m/z (%) 267 (6) [M+], 265 (97)
[M+-2H], 224 (14) [M+-C3H7], 181 (97) [M+-C5H10O],
166 (100) [M+-C5H11NO], 106 (95) [M+- (C6H5+
C5H9O)], 77 (72) [C6H5], 57 (80) [M+-C14H12NO], 29
(90) [M+-C16H16NO].
3031, 2977, 2927, 2847, 1629 (C=O), 1533, 1505,
1306, 1259, 1180, 1042, 848, 698, 555. 1H NMR (300
MHz, CDCl3, 25◦C) δ/ppm 7.65 (d, 2H, J = 8.7 Hz,
Ph) 7.34-7.19 (m, 5H, Ph), 6.84 (d, 2H, J = 8.7 Hz,
Ph), 6.18 (brd, 1H, J = 7.2 Hz, NH), 5.26 (qn, 1H, J =
6.9 Hz, CHNH), 3.99 (q, 2H, J = 6.9 Hz, OCH2CH3),
1.52 (d, 3H, J = 6.9 Hz, CHCH3), 1.35 (t, 3H, J =
6.9 Hz, CH2CH3); 13C NMR(75 MHz, CDCl3, 25◦C)
δ/ppm 166.13, 161.59, 143.33, 128.74, 128.72, 127.42,
2.3h N-Benzhydryl-2-phenylacetamide (table 2, entry 126.61, 126.29, 114.20, 63.66, 49.11, 21.80, 14.72. MS
8): White solid, yield 98%, M.p. 161-162◦C (Lit. 161- (EI) m/z(%) 269 (8) [M+], 268 (68) [M+-1H], 223
162◦C);46 FT-IR (KBr): υmax/cm−1 3296 (N-H), 3056, (10) [M+-C2H6O], 164 (71) [M+-C8H9], 148 (100)

Elnaz Karimian et al.
[M+-C8H9O], 120 (80) [M+-C9H9O2], 77 (62) [C6H5],
28 (70) [M+- (C8H10N+ C8H9O)]; Elemental analy-
sis data (%) for (C17H19NO2): C (75.81), H (7.11), N
(5.20); found (%): C (76.92), H (7.23), N (5.43).
3028, 2974, 2929, 2868, 1648 (C=O), 1531, 1495,
1
1244, 1135, 699; H NMR (300 MHz, CDCl3, 25◦C)
δ/ppm 7.42-7.20 (m, 10H, Ph), 5.63 (brs, 1H, NH),
5.15 (qn, 1H, J = 7.2 Hz, CHNH), 3.62 (s, 2H,
CH2Ph), 1.42 (d, 3H, J = 6.9 Hz, CH3CH); MS
(EI) m/z(%) 239 (22) [M+], 238 (85) [M+-1H], 148
(10) [M+-C7H7], 91 (52) [M+-C9H10NO], 28(96) [M+-
(C8H10N+ C7H7)].
2.3l 3,4,5-Trimethoxy-N-(1-phenylethyl)benzamide
(table 2, entry 12): White solid, yield 80%; M.p. 177-
178◦C (Lit. 177.5-178◦C);48 FT-IR (KBr): υmax/cm−1
3310 (N-H), 3064, 3031, 2962, 2925, 2854, 1628
(C=O), 1580, 1542, 1499, 1338, 1240, 1125, 1002,
853, 697, 583, 547; 1H NMR (300 MHz, CDCl3, 25◦C)
δ/ppm 7.45-7.29 (m, 5H, Ph), 7.02 (s, 2H, Ph), 6.28
(brd, 1H, J = 7.5 Hz, NH), 5.36 (qn, 1H, J = 7.2
Hz, CHNH), 3.92 (s, 6H, OCH3), 3.90 (s, 3H, OCH3),
1.65 (d, 3H, J = 6.9 Hz, CH3CH); MS (EI) m/z
(%) 315 (4) [M+], 167 (81) [M+-C9H10NO], 119 (72)
[M+-C10H12O4], 104 (50) [M+-C10H13NO4], 76 (46)
[M+-C6H4], 28 (95) [M+- (C8H10N+ C9H11O3)].
2.3p N-(tert-Butyl)benzamide (table 2, entry 16):
White solid, yield 90%; M.p. 111-118◦C (Lit. 111-
118◦C);52 FT-IR (KBr): υmax/cm−1 3324 (N-H), 3068,
2965, 2937, 2770, 1636 (C=O), 1578, 1540, 1451,
1312, 1217, 1178, 1078, 1008, 719, 695, 579; 1H NMR
(400 MHz, CDCl3, 25◦C) δ/ppm 7.76-7.29 (m, 5H, Ph),
5.97 (brs, 1H, NH), 1.50 (s, 9H, C(CH3)3); MS (EI)
m/z(%)177 (40) [M+], 176 (77) [M+-1H], 161 (34)
[M+-CH4], 105 (100) [M+-C4H10N], 77 (92) [C6H5], 29
(96) [M+- (C4H10N+ C6H5)].
2.3m 4-Methyl-N-(1-phenylethyl) benzamide (table 2,
entry 13): Pale yellow solid, yield 85%, M.p. 125-
126◦C (Lit. 127-128◦C);49 FT-IR (KBr): υmax/cm−1
3358 (N-H), 3031, 2974, 2921, 2851, 1632 (C=O),
1527, 1502, 1109, 754, 698, 593; 1H NMR (300
MHz, CDCl3, 25◦C) δ/ppm 7.70 (d, 2H, J = 8.1 Hz,
Ph), 7.45-7.24 (m, 7H, Ph), 6.32 (brd, 1H, J = 6.9
Hz, NH), 5.37 (qn, 1H, J = 7.2 Hz, CHNH), 2.42
(s, 3H, CH3Ph), 1.63 (d, 3H, J = 6.9 Hz, CH3CH);
MS (EI) m/z(%) 239 (6) [M+], 238 (45) [M+-1H], 119
(97) [M+-C8H10N], 104 (31) [M+-C8H9NO], 91 (60)
[M+-C9H10NO], 77 (18) [M+-C6H5].
2.3q N-(tert-Butyl)-4-ethoxybenzamide (table 2,
entry 17): White solid, yield 93%, M.p. 100-105◦C;
FT-IR (KBr): υmax/cm−1 3354 (N-H), 3338, 3072,
2973, 2929, 2876, 2774, 1631 (C=O), 1607, 1545,
1506, 1451, 1318, 1255, 1217, 1178, 1119, 1046, 845,
768, 616;1H NMR (300 MHz, CDCl3, 25◦C) δ/ppm
7.69 (d, 2H, J = 8.7 Hz, Ph), 6.9 (d, 2H, J = 8.7
Hz, Ph), 5.91 (brs, 1H, NH), 4.08 (q, 2H, J = 6.9 Hz,
OCH2CH3), 1.48-1.42 (m, 12H, C(CH3)3, CH2CH3);
13C NMR (75 MHz, CDCl3, 25◦C) δ/ppm 166.46,
161.25, 128.44, 128.00, 114.08, 63.60, 51.43, 28.96,
14.73.MS (EI) m/z(%) 221 (25) [M+], 205 (46) [M+-
CH4], 176 (45) [M+-C2H5O], 164 (82) [M+-C4H9], 148
(100) [M+-C4H11N], 121(98) [M+-C5H10NO], 57 (70)
[C4H9], 30 (82) [C2H6]; Elemental analysis data (%)
for (C13H19NO2): C (70.56), H (8.65), N (6.33); found
(%): C (71.21), H (8.36), N (6.97).
2.3n N-(1-Phenylethyl)thiophene-2-carboxamide
(table 2, entry 14): White crystal, yield 85%, M.p.
126-127◦C (Lit. 127-128◦C);50 FT-IR (KBr): υmax/cm−1
3265 (N-H), 3080, 3031, 2975, 2929, 2864, 2827, 1622
(C=O), 1546, 1513, 1448, 1420, 1359, 1279, 1137,
826, 721, 696, 543;1H NMR (400 MHz, CDCl3, 25◦C)
δ/ppm 7.69-7.50 (m, 2H, thiophene), 7.42-7.23 (m, 5H,
Ph), 7.07 (t, 1H, J = 3.8 Hz, thiophene), 6.23 (brs,
1H, NH), 5.33 (qn, 1H, J = 7.2 Hz, CHNH), 1.57
(d, 3H, J = 7.1 Hz, CHCH3); MS (EI) m/z(%) 231
(17) [M+], 230 (93) [M+-1H], 215 (20) [M+-CH4],
154 (2) [M+-C7H8NOS], 120 (43) [M+-C5H4OS],111
(100) [M+-C8H11N], 104 (50) [M+-C5H6NOS], 83 (22)
[M+-C9H10NO], 77 (56) [C6H5].
2.3r N-(tert-Butyl)thiophene-2-carboxamide (table 2,
entry 18): White solid, yield 90%, M.p. 147-148◦C
(Lit. 148◦C);16 FT-IR (KBr): υmax/cm−1 (N-H), 3174,
3092, 3067, 2968, 2932, 2843, 1619 (C=O), 1539,
1511, 1453, 1363, 1308, 1217, 1110, 833, 745, 730,
1
448; H NMR (400 MHz, CDCl3, 25◦C) δ/ppm 7.47-
7.43 (m, 2H, thiophene), 7.07 (t, 1H, J = 3.6 Hz,
thiophene), 5.84 (brs, 1H, NH), 1.49 (s, 9H, C(CH3)3);
MS (EI) m/z(%)183 (16) [M+], 182 (75) [M+-1H],
2.3o 2-Phenyl-N-(1-phenylethyl)acetamide (table 2, 167 (75) [M+-CH4], 126 (76) [M+-C4H9O], 110 (100)
entry 15): White solid, yield 75%, M.p. 97-99◦C (Lit. [M+-C4H10N], 82 (60) [M+-C5H11NO], 57 (62) [M+-
97-99◦C);51 FT-IR (KBr): υmax/cm−1 3319 (N-H), 3059, C5H4NOS], 28 (76) [M+- (C4H10N+ C4H3S)].

Synthesis of N-substituted amides using Al(HSO4)3
2.3s 2-Phenyl-N-tritylacetamide (table 2, entry 19): the absence of any catalyst, performing the model reac-
White solid, yield 85%, M.p. 187-188◦C (Lit. 187- tion in solvent free condition and also in nitromethane
188◦C);53 1H NMR (300 MHz, CDCl3, 25◦C) δ/ppm (at room temperature and reflux) has not led to forma-
7.43-7.23 (m, 14H, Ph), 7.16-7.09 (m, 6H, Ph), 6.60 tion of the desired product (table 1, entry 1,4,5). Apply-
(brs, 1H, NH), 3.66 (s, 2H, CH2CO); MS (EI) m/z ing 0.5 mmol of catalyst in nitromethane and in solvent
(%)377 (31) [M+], 181(89) [M+-(C8H8O, C6H5)], free conditions has not produced N-benzhydryl-2-
164 (83), 90 (78) [M+-C20H17NO], 28 (79) [M+- phenylacetamide (table 1, entries 2 and 3). Based on
(C19H16N+ C7H7)].
our previous reports,55,56 in the present study, we also
found that in the presence of Al(HSO4)3, the rate of
addition of benzhydrol plays an important role in the
yield of N-benzhydryl-2-phenylacetamide. Quick addi-
tion of benzhydrol leads to the formation of excess
amount of symmetrical ether which was obtained from
self-condensation of benzhydrol. To minimize self-
condensation of benzhydrol in all of the following
reactions, addition of benzhydrol should be performed
dropwise, which consumes more time. In this man-
ner, N-benzhydryl-2-phenylacetamide was obtained as
the desired product. By applying 1/1 molar ratio of
phenyl acetonitrile/benzhydrol and using 0.5 mmol of
Al(HSO4)3, maximum yield of the desired product was
obtained by refluxing nitromethane without formation
of any by-product (table 1, entry 6). It is important to
note that the yield of the reaction was not increased by
applying other molar ratios of reactants, whilst sym-
metrical ether as by-product was produced considerably
(table 1, entries 7-8). To investigate the effect of catalyst
loading, formation ofN-benzhydryl-2-phenylacetamide
was carried out by refluxing nitromethane in the pres-
ence of different amounts of Al(HSO4)3. Additional
amounts of catalyst did not have any influence on the
reaction rate while lower amount of catalyst dimin-
ished the reaction rate concomitant with formation
of by-product (table 1, entries 9-10). In an effort to
develop better reaction conditions, different solvents
were screened for the preparation of N-benzhydryl-2-
phenylacetamide from the reaction of phenylacetoni-
trile with benzhydrol in the presence of 0.5 mmol of
Al(HSO4)3. No product was obtained when the reac-
tion was performed by refluxing in H2O and EtOH
(table 1, entries 11-12). Refluxing in solvents, such
as 1,2-dichloroethane, CHCl3, 1,4-dioxane, DMF and
toluene gave the desired product in low yield along
with formation of considerable amounts of by-product
(table 1, entries 13-17). As shown in table 1, under
the optimized reaction conditions (table 1, entry 6) and
in the presence of Al (HSO4)3/silica, N-benzhydryl-2-
phenylacetamide was produced in low yield after long
reaction time with the formation of symmetrical ether
(table 1, entry 18).
2.3t N-Tritylpentanamide (table 2, entry 20): White
solid, yield 90%, M.p. 183-184◦C(Lit.184-185◦C);54
1H NMR (300 MHz, CDCl3, 25◦C) δ/ppm 7.36-7.22
(m, 15H, Ph), 6.59 (brs, 1H, NH), 2.31 (t, 2H, J = 7.8
Hz, CH2CO), 1.66 (qn, 2H, J = 7.6 Hz, CH2CH2Et),
1.38 (sx, 2H, J = 7.5 Hz, CH2CH2CH3), 0.94 (t, 3H,
J = 7.5 Hz, CH2CH3); MS (EI) m/z (%) 343 (2) [M+],
76 (29) [M+-C18H21NO], 43 (49) [M+-C21H18NO],
57 (68) [M+-C20H16NO], 28 (100) [M+- (C19H16N
+ C4H9)].
3. Results and Discussion
Recently, we have reported thealkylation of 1,3-
dicarbonyl compounds and synthesis of unsymmetri-
cal ethers in the presence of Fe(HSO4)3.55,56 In the
course of our study, with the aim of finding efficient
and cost-effective catalysts,57 we found out thatN-
substituted amides were easily formed from the reaction
of aliphatic and aromatic nitriles with various benzylic
alcohols (secondary and tertiary) and tert-butyl alco-
hol in the presence of Al(HSO4)3 via the Ritter reaction
pathway (scheme 1).
At first, the reaction between phenylacetonitrile and
benzhydrol was chosen as model reaction. Then in
order to optimize the reaction conditions, in a set of
experiments the model reaction was carried out in dif-
ferent reaction conditions (table 1). To prove the cat-
alytic activity of Al(HSO4)3 in the Ritter reaction, in
To show the general application of the protocol and
with the optimal conditions at hand (table 1, entry 6),
the substrate scope of the Ritter reaction in the presence
Scheme 1. Synthesis of N-substituted amides in the pres-
ence of Al(HSO4)3.

Elnaz Karimian et al.
Table 1. Synthesis of N-benzhydryl-2-phenylacetamide in the presence of Al(HSO4)3 under different reaction conditions.
Molar ratio of
phenyl acetonitrile/
benzhydrol (mmol)
Catalyst
(mmol)
Temperature
(◦C)
Time
(h)
Conversiona
(%)
Selectivity
A/B
Entry
Solvent
1
2
3
4
5
6
7
8
1/1
1/1
1/1
1/1
1/1
1/1
1/1.5
1/2
1/1
1/1
1/1
1/1
1/1
1/1
1/1
1/1
1/1
1/1
0
0.5
0.5
0
None
None
r.t
r.t
r.t
r.t
24
24
24
24
24
3
3
3
3
3
3
3
3
3
4
4
4
24
0
0
0
0
0
>98
70
60
85
>98
0
0/0
0/0
0/15
0/0
0/15
CH3NO2
CH3NO2
CH3NO2
CH3NO2
CH3NO2
CH3NO2
CH3NO2
CH3NO2
H2O
C2H5OH
DCE
CHCl3
1,4-Dioxan
DMF
Toluene
0
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
0.5
0.5
0.5
0.4
0.6
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.05g
>98
80/ 20
40/60
90/ 10
>98
0/0
0/0
90/ 10
80/ 20
50/ 50
50/ 50
80/ 20
70/30
9
10
11
12
13
14
15
16
17
18b
0
80
80
50
50
60
24
CH3NO2
aThe corresponding data refer to the conversion of phenyl acetonitrile.
bThe reaction was performed in the presence of Al(HSO4)3/Silica.
of Al(HSO4)3 was investigated. Different N-substituted Al(HSO4)3 is attributed to the low stability of the cor-
amides were obtained from the reaction of aromatic, responding primary benzylic cation. Also, we tried to
heteroaromatic and aliphatic nitriles with secondary and expand the scope of the Ritter reaction for organic
tertiary benzylic alcohols and tertiary butyl alcohol with nitriles with secondary and primary aliphatic alcohols,
good to excellent yields (table 2). Also, all the reactions but the catalyst was ineffective in these reactions even
proceeded well in short reaction times with high selec- when excess amounts of catalyst (1 mmol of catalyst)
tivity without formation of any by-products. From this were used.
Table, it is clear that, in the presence of Al(HSO4)3 the
In our experiments, completion of the reaction was
reaction of benzhydrol was completed in short reac- confirmed by the disappearance of the organic nitriles
tion time than 1-phenylethanol due to more stability and then amide formation on TLC, followed by the
of the corresponding carbocation (compare entries 1-9 disappearance of nitrile CN stretching frequency at
with entries 10-15). This result led us to conclude that 2260-2220 cm−1 in the Fourier transform infrared
the more stable carbocation improved the reaction rate. (FT-IR) spectrum. The resulting N-substituted amides,
Comparatively owing to the steric hindrance, tertiary after purification via either re-crystallization or thin
butyl alcohol and tertiary benzylic alcohol reacted more layer chromatography were characterized using FT-IR,
slowly than benzhydrol and 1-phenyethanol (table 2, NMR, and mass spectrometry (MS). Absorption bands
entries 16-20). With effective conditions established at 3354-3249 and 1653-1619 cm−1 corresponding to
for the Ritter reaction, we next investigated the abil- NH, and CO groups of N-substituted amides, respec-
ity of Al(HSO4)3 to catalyze the formation of N- tively, confirmed the formation of the desired prod-
1
substituted amides from the reaction of organic nitriles ucts. In the H NMR spectra, the NH proton of N-
with primary benzylic alcohols. Even after a long period substituted amides was appeared at 6.75-5.63 ppm. In
of time, organic nitriles did not react with primary the 13C NMR spectra, signals around 166.46-160.99
benzylic alcohols under optimized reaction condi- ppm were assigned to the carbonyl carbon atom of N-
tions. Comparatively, low activity of primary benzylic substituted amides. Most of the products obtained were
alcohols towards organic nitriles in the presence of known compounds and characterized by comparison of

Synthesis of N-substituted amides using Al(HSO4)3
Table 2. Synthesis of structurally different N-substituted amides in the presence of Al(HSO4)3.
Entry
Nitrile
Alcohol
Product
Time (h)
Isolated yield (%)
1
2
2
2
93
95
3
4
5
6
2
2
1
2
90
85
95
95
CH3CN
7
8
9
CH3(CH2)2CN
2
3
90
98
92
1.5
10
11
2.5
2
85
90
12
13
2.5
3
80
85

Elnaz Karimian et al.
Table 2.
(continued)
Nitrile
Entry
Alcohol
Product
Time (h)
1.5
Isolated yield (%)
14
85
15
16
3
3
75
90
17
18
3
93
90
2.5
19
20
3
3
85
90
CH3(CH2)3CN
Scheme 2. Proposed mechanism for Ritter reaction in the presence of Al(HSO4)3.

Synthesis of N-substituted amides using Al(HSO4)3
Table 4. Ritter reaction of benzhydrol with phenyl ace-
tonitrile under optimized conditions in the presence of other
catalysts.
their melting points with those of the reported com-
pounds. Complete characterization of novel compounds
are also reported in this paper.
The suggested Ritter reaction mechanism was pub-
lished in 1948.1 Based on the Ritter reaction mech-
anism, we suggest that, initially the Brønsted acid
catalyzed the reaction with fast generation of the proto-
nated species I via the protonation of hydroxyl group of
alcohol which was followed by generation of the cor-
responding stable benzylic carbocation II. This idea is
supported by performing the reaction in the absence of
catalyst. Without any catalyst, the Ritter reaction did not
proceed even after a long period of time (table 1, entry
1). Nucleophilic attack of organic nitrile to benzylic car-
bocation II produced the nitrilium ion III. The positive
sites of benzylic carbocation II and nitrilium ion III
can interact with the negative oxygen of the solvent
which stabilized these two intermediates, while the
other reacting components cannot interact with sol-
vent. As the more stabilized intermediate was obtained
faster, CH3NO2 was chosen as a suitable solvent in
the present study. Hydrolysis of III affords the corre-
sponding N−substituted amide and releases the pro-
ton which re-enters to the catalytic cycle. Because
of dropwise addition of benzylic alcohol to the reac-
tion mixture, the formation of any symmetric ether
IV as by-product was prohibited. Also, dehydration of
II, which leads to alkene V, did not happen in the
Ritter reaction in the presence of Al(HSO4)3. Further
studies to elucidate the details of the mechanism are
ongoing (scheme 2).
Time Conversion Selectivity
Entry
Catalyst
(h)
(%)
Amide/Ether
1
2
3
4
5
6
7
8
9
FeCl3.6H2O
Fe(NO3)3.9H2O
CuCl2.2H2O
NiCl2.6H2O
ZnCl2
MnCl2.2H2O
AlCl3
3
3
24
24
4
24
24
24
3
70
50
0
0
20
0
0
0
>98
60/40
50/50
0
0
20/80
0
0
0
Al2O3
Al(HSO4)3
>98
significant impact on yields of productswith purity
similar to that obtained in the first run.
To show the efficiency of Al(HSO4)3 in Ritter
reaction, the model reaction in the presence of Al(HSO4)3
was compared with various metal salt catalysts such as
FeCl3.6H2O, Fe(NO3)3.9H2O, CuCl2.2H2O, NiCl2.6H2O,
ZnCl2, MnCl2.2H2O, AlCl3 and Al2O3(table 4). Per-
forming the reaction in the presence of CuCl2.2H2O,
NiCl2.6H2O, MnCl2.2H2O, AlCl3 and Al2O3 did not
produce the desired product in quantitative yield
(table 4, entries 3, 4, 6, 7, 8). Comparatively, Ritter reac-
tion in the presence of FeCl3.6H2O, Fe(NO3)3.9H2O
and ZnCl2 produced N-benzhydryl-2-phenylacetamide
in low yield concomitant with the formation of by-
products in considerable yield (table 4, entries 1, 2, 5).
Catalyst re-usability was assessed in the reaction of
phenylacetonitrile with benzhydrol. To this end, the
reaction was stopped after 3 h (i.e., at 100% conversion
of phenylacetonitrile) and the catalyst removed by fil-
tration and washed with acetone and 1,2-dichloroethane
several times. Table 3 shows the results obtained after
six re-use cycles. These results by themselves sug-
gest that the heterogeneous catalyst was used for 4
successive times in the new experiments without any
4. Conclusions
In conclusion, we have described that Al(HSO4)3 (as
an air-stable, cost-effective and reusable solid acid) cat-
alyzes the Ritter reaction of organic nitriles with ben-
zylic alcohols (secondary and tertiary) as well as the
Ritter reaction of organic nitriles with tertiary butyl
alcohol, respectively. In contrast to the existing meth-
ods using potentially hazardous catalysts/additives,
the present method (an inexpensive and eco-friendly
process which allows the preparation of various N-
substituted amides that can be useful synthons), offers
the following advantages: (i) Al(HSO4)3 is easy to pre-
pare from commercially available aluminum chloride
and sulfuric acid, (ii) Al(HSO4)3 as a heterogeneous
catalyst is recyclable without any significant impact on
the yields of the products, (iii) short reaction times,
(iv) ease of product isolation/purification, (v) no side
reactions and (vi) low cost and simplicity in process
handling.
Table 3. Synthesis of N-benzhydryl-2-phenylacetamide in
the presence of reused Al(HSO4)3.
Run
Time (h)
Conversion (%)
Isolated yield (%)
1
2
3
3
>98
>98
96
95
3
4
3
3
>98
>98
93
92
5a
6a
3/4.5
3/6
85/90
70/80
80/85
65/75
aThe second number in the third and fourth columns corre-
spond to conversion and yield after 4.5 and 6 h.

Elnaz Karimian et al.
23. Al-huniti M H and Lepore S D 2013 Adv. Synth. Catal.
Supplementary Information (SI)
355 3071
24. (a) Top S and Jaouen G 1981 J. Org. Chem. 46 78; (b)
Barton D H R, Magnus P D and Young R N 1973 J.
Chem. Soc., Chem. Commun. 331; (c) Mukhopadhyay
M, Reddy M M, Maikap G G and Iqbal J 1995 J. Org.
Chem. 60 2670
Supplementary Information is available at www.ias.ac.
in/chemsci.
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