Kinetics and mechanism of the reaction of benzyl halides with zinc in dimethylformamide

Article abstract of DOI:10.1002/poc.1114

The reaction of metal zinc with various benzyl halides in dimethylformamide has been studied. The low-temperature ESR-spectroscopy is used to show that reaction of zinc with benzyl bromide and benzyl iodide occurs to form benzyl radicals. Benzyl chloride reacts with zinc at 77 K to form benzyl radicals and ion-radical pairs RX^- - Zn⁺. Nevertheless these pairs disappear at temperatures higher than 110 K. The investigation of the reaction stereochemistry and the use of a radical trap provide evidence in favor of a radical mechanism. The optical activity has been proved in the course of the formation of organozinc halide from (+)-R-1-halogen-1-phenylethane at low temperature. The kinetic features of the reaction have been studied. Benzyl iodide reacts with zinc at transport-controlled rate. The reaction of zinc with benzyl chloride and benzyl bromide in DMF occurs according to Langmuir-Hinshelwood scheme, the adsorption of reagents taking place on similar active centers of metal. The kinetic and thermodynamic parameters of the reaction have been clarified. The comparison of kinetic parameters with those reported in literature evidences that the limiting step of reaction is halogen atom transfer (inner sphere electron transfer). Copyright

Full text of DOI:10.1002/poc.1114

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY
J. Phys. Org. Chem. 2006; 19: 664–675
Published online 26 November 2006 in Wiley InterScience
(
www.interscience.wiley.com) DOI: 10.1002/poc.1114
Kinetics and mechanism of the reaction of benzyl
halides with zinc in dimethylformamide
Anatoly M. Egorov*
Tula State University, 92 Prospekt Lenina, 300600 Tula, Russia
Received 18 March 2006; revised 22 June 2006; accepted 10 July 2006
ABSTRACT: The reaction of metal zinc with various benzyl halides in dimethylformamide has been studied. The low-
temperature ESR-spectroscopy is used to show that reaction of zinc with benzyl bromide and benzyl iodide occurs
to form benzyl radicals. Benzyl chloride reacts with zinc at 77 K to form benzyl radicals and ion-radical pairs
ꢁ
ꢀ
þ
ꢀ
RX ꢁ Zn . Nevertheless these pairs disappear at temperatures higher than 110 K. The investigation of the reaction
stereochemistry and the use of a radical trap provide evidence in favor of a radical mechanism. The optical activity has
been proved in the course of the formation of organozinc halide from (þ)-R-1-halogen-1-phenylethane at low
temperature. The kinetic features of the reaction have been studied. Benzyl iodide reacts with zinc at transport-
controlled rate. The reaction of zinc with benzyl chloride and benzyl bromide in DMF occurs according to Langmuir–
Hinshelwood scheme, the adsorption of reagents taking place on similar active centers of metal. The kinetic and
thermodynamic parameters of the reaction have been clarified. The comparison of kinetic parameters with those
reported in literature evidences that the limiting step of reaction is halogen atom transfer (inner sphere electron
transfer). Copyright # 2006 John Wiley & Sons, Ltd.
KEYWORDS: zinc; benzyl halides; dimethylformamide; benzylzinc halides; mechanism; ESR spectra; kinetics;
stereochemistry; halogen atom transfer; inner-sphere electron transfer; benzyl radicals; ion-radical pairs; Langmuir–
Hinshelwood scheme; adsorption
INTRODUCTION
Organozinc halides have been proved to be preparatively
shifts are given in ppm relative to tetramethylsilane as
internal standard. The accuracy of chemical shifts was
ꢂ0.01 ppm. Elemental analyses were carried out on ‘Carlo
1,2
4
useful for a number of reactions.
mechanism of their formation is uncertain and the kinetic
However, the
Erba 1100’ instrument according to standard procedure.
The ESR spectra were recorded at 77K on a Radiopan
radiospectrometer in films of zinc co-condensates with
3
and thermodynamic parameters are poorly investigated.
The experimental conditions normally used in carrying out
the reaction reflect the accumulated experience of synthetic
5,6
benzyl halides (1:50) according to literature at frequency
of 9 GHz without saturation and amplitude broadening.
Zinc vapor was prepared by evaporating the purified metal
1,2
chemists.
The lack of information prevents wide
ꢁ
5
application of benzyl zinc halides for industrial purposes.
In this paper the mechanism of benzyl halide reaction with
zinc in dimethylformamide (DMF) has been investigated.
The kinetics of this reaction have been studied in detail.
from corundum crucible at 570–600 K 10 mm Hg. The
ꢁ
1
rate of evaporation of Zn was 0.2mmol min . Benzyl
halides were evaporated at 273–308 K. The IR spectra were
measured on an IMPACT 400d (NICOLET) and Perkin–
Elmer 325 spectrophotometers; the samples were prepared
in KBr and in suspension in mineral (vaseline) oil. Specific
polarized light plane rotation was measured on an A-1 EPO
automatic polarimeter (d ¼ 0.018).
EXPERIMENTAL
Equipment and analytical measurements
The purity of initial substances was monitored and the
quantitative analysis of organic reaction products was
carried out using gas chromatography (GC). The
1
H-NMR spectra were recorded on a Jeol LTD FX-90 Q
spectrometer using 25–30% solutions in CDCl . Chemical
3
7
conditions of GC analysis were described previously.
The reaction products were isolated by preparative liquid
chromatography (LC) on a Tsvet-304 chromatograph
equipped with a UV detector (l ¼ 254 nm) using
steel column (l ¼ 250 mm, d ¼ 4 mm). Silasorb 600
(Chemapol, Czech Republic, particle sizes 15–25 mm)
*Correspondence to: A. M. Egorov, Tula State University, 92 Prospekt
Lenina, 300600 Tula, Russia.
E-mail: yegorov_am@mail.ru
Contract/grant sponsor: Russian Federation’s Ministry of Education;
Contract/grant numbers: UR 05.01.012 and UR 05.01.419.
Copyright # 2006 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2006; 19: 664–675

Zn-BENZYL HALIDE REACTION
665
1
was used as the sorbent; a 5:1 hexane–diethyl ether
mixture was used as the eluent. The reaction products in
gaseous phase were analyzed by GC with Tsvet-800
instrument equipped with a thermal conductivity detector
and a steel column (l ¼ 2 m, d ¼ 3 mm), packed with
molecular sieves 4A (0.25 mm fraction). The column
temperature was 50 8C; argon was used as the carrier gas
b.p. 60–62 8C/3 mm Hg). H NMR d (ppm): 4.19 (s, 2H,
CH ), 7.03 (m, 5H, Ph).
Synthesis of (þ)-R-1-chloro-1-phenylethane was car-
2
ried out by interaction of (ꢁ)-S-1-phenylethanol with
9
POCl in the presence of pyridine in pentane. Yield 76%.
3
2
5
B.p. ¼ 80–81 8C/17 mm Hg, a þ 94.18 (l ¼ 1). Lit. data:
91
D
2
5
b.p. ¼ 78–82 8C/17 mm Hg, a þ 125.48 (l ¼ 1, 100%).
D
H NMR (CDCl ):1.68 (d, 3H, CH ), 4.86 (dd, 1H, CH),
ꢁ
1
(
the flow rate was 100 mL min ).
3
3
Organic reaction products were analyzed on a Hewlett–
7.14 (m, 5H, Ph).
Synthesis of (þ)-R-1-bromo-phenylethane was carried
Packard GC MS instrument (HP 5972 mass-selective
detector, HP 5890 chromatograph) using a capillary
column (l ¼ 30 m, d ¼ 0.25 mm) with a diphenyl (5%)
stationary phase supported on polydimethylsiloxane. The
column temperature was 40–250 8C; the heating rate was
out by interaction of (ꢁ)-S-1-phenylethanol with POBr
3
10
in the presence of pyridine in pentane. Yield 74%.
B.p. ¼ 86–87 8C/11 mm Hg, [a] þ 73.38 (l ¼ 1) (From
2
5
D
2
5
Ref. 10, b.p. ¼ 86–87 8C/11 mm Hg, [a] þ 96.38 (l ¼ 1,
D
100%)). H NMR (CDCl ): 1.68 (d, 3H, CH ), 4.86 (dd,
1
3
08/min. Helium was a carrier gas (the carrier-gas flow
3
3
ꢁ
1
rate was 1 mL min ). The injector temperature was
1H,CH), 7.14 (m, 5H, Ph).
Synthesis of (ꢁ)-S-1-phenylethanol was carried out
2
50 8C, and the detector temperature was 280 8C.
Inorganic reaction products (Zn(I) and Zn(II) cations)
11,12
as described in literature.
Yield 55%. B.p. ¼ 94–95 8C/
121
25
were determined by ion chromatography on a Tsvet-3006
chromatograph by using Diacat-3 columns (Elsiko,
Moscow; l ¼ 150 mm, d ¼ 3 mm). An aqueous 4 mM
ethylenediamine, 5 mM citric acid, and 5 mM tartaric acid
solution was used as an eluent. The rate of elution was
14 mm Hg, [a] ꢁ 37.658 (l¼ 1). Lit. data: b.p. ¼ 94–95 8C/
D
2
5
14 mm Hg, [a] ꢁ 44.28 (l¼ 1). H NMR (CDCl ):1.21 (d,
D
3H, CH ), 3.78 (q, 1H,CH), 7.06 (m, 5H, Ph).
3
3
Synthesis of gaseous DCl was carried out by
interaction of sulfuric acid-d (Aldrich Chemicals Co.
2
ꢁ
1
1
1
5 mL min . The sample volume was 100 mL (after
:1000 dilution with water). The anions were analyzed on
Ltd., 98%) with NaCl in the presence of DCl (37%
solution in D O, Aldrich Chemicals Co. Ltd.).
All solvents were purified according to standard pro-
2
a steel column (l ¼ 2 m, d ¼ 4 mm). Khiks-1 (Institute of
Chemistry of the Academy of Sciences of Estonia,
particle size 15 mm) was used as the sorbent. A 0.03 M
Na CO solution in water was used as the eluent (flow rate
1
3
cedures. All organic compounds were freed from
dissolved gases by repeatedly freezing and thawing at re-
duced pressure and stored in ampoules in the absence of air.
2
3
ꢁ
1
was 2 mL min ). The volume of sample was 10 mL. Zinc
sample purities were verified by atomic absorption
spectrometry on a GBC-908 AA (Australia) equipment
according to standard procedure.
Reaction of benzyl halides with zinc,
general procedure
A solution of 21.5 mmol PhCH Hal (Hal ¼ Br, I) in
2
Reagents
11 mL DMF was slowly added (1/each 5 sec) to 1.7 g
(
26 mmol) of zinc powder at 0 8C, which had been
14
Zinc powder (Aldrich Chemicals Co Ltd. of >99.998%
purity, 100 mesh) was commercially obtained. In order to
investigate low temperature reaction of benzyl halides
with atomic zinc, the metal was purified by a sevenfold
sublimation in vacuo (10 mm Hg) at 920–930 K.
A zinc wire (Goodfellow Corporation, ZN005090 zinc,
d ¼ 0.05 mm, Zn content of 99.0% and ZN005115 zinc,
d ¼ 0.25 mm, Zn content of 99.99%) was exposed to
concentrated nitric acid for 5–10 sec, and washed with
water, acetone, and then DMF. The entire operation was
carried out in oxygen-free argon.
activated according to procedure. After 2 h of stirring at
5 8C the mixture was filtered. The filtrate was treated with
50 mL benzene. Precipitated white crystals were filtered
off, washed by hexane and then evacuated. The reaction
ꢁ3
of zinc with PhCH Cl in DMF was carried out by a similar
2
procedure, except that the reaction mixture was stirred at
40 8C for 12 h.
1
5
[Zn(DMF) Cl ]. Yield 48%, m.p. 117–118 8C (lit
2
2
m.p. ¼ 118 8C). Analysis: C H N ZnCl O calculated: C
6
14
2
2 2
25.51, H 5.00, Cl 25.10, N 9.92, O 11.33, Zn 23.15%.
Found: C 25.53, H 5.02, Cl 25.07, N 9.98, O 11.29, Zn
—
All organic compounds were commercially obtained.
The purity of commercial samples (Aldrich Chemicals
Co. Ltd.) of benzyl chloride and benzyl bromide was
checked by GC compounds which contained toluene or
were less than 99% pure were purified by low-
temperature fractional recrystallization or by fractional
distillation.
23.11%. IR (KBr), n ¼ 1670 (C—O), 1510 (C—N), 1430
ꢁ
1
(CH ), 1255 (C—N), 1122 (C—N), 695 (OCN) cm . IR
(mineral oil), n ¼ 384 (Zn—O), 335 (Zn—Cl), 299 (Zn—
3
ꢁ
1
Cl) cm .
15
[Zn(DMF) Br ]. Yield 47%, m.p. 119–120 8C (lit
2
2
m.p. ¼ 120 8C). Analysis: C H N ZnBr O calculated: C
6
14
2
2 2
19.41, H 3.80, Br 43.03, N 7.54, O 8.62, Zn 17.60%. Found:
C 19.43, H 3.77, Br 43.05, N 7.51, O 8.60, Zn 17.64%. IR
Benzyl iodide was prepared according to a known
8
8
—
procedure, the yield was 90%. B.p. 61–628/3 mm Hg (lit
(KBr), n¼ 1670 (C—O), 1500 (C—N), 1430 (CH ), 1255
3
Copyright # 2006 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2006; 19: 664–675
DOI: 10.1002/poc

6
66
A. M. EGOROV
ꢁ
1
C—N), 1123 (C—N), 698 (OCN) cm . IR (mineral oil),
(
Study of the reaction kinetics
ꢁ
1
n ¼ 384 (Zn—O), 258 (Zn—Br), 299 (Zn—Br) cm .
The residue was poured into a 20% solution of HCl.
Organic products of reaction were extracted with benzene
The reactions of zinc with benzyl halides in DMF were
studied by the resistometric method according to
20
the published procedure in a water- and oxygen-free
19
0
(
50 mL). Toluene, 1,2-diphenylethane and 4,4 -dimethyl-
biphenyl were found as products and benzyl halides and
DMF were found as remaining starting materials. The
yields of resulting compounds are recorded in Table 1.
atmosphere of argon. The kinetics of the reaction were
studied by monitoring the electrical resistance of species
during experiments. Species were zinc wire (diameters
were 0.25 and 0.05 mm and lengths were 100 and
6.25 mm). Consequently, an increase in species electrical
resistance reflects a decrease in the thickness of the wire.
To obtain kinetic and thermodynamic parameters of the
process, three series of experiments were carried out. The
2
0
13
Toluene. b.p. ¼ 109–110 8C, n ¼ 1.4969. (lit m.p. ¼
D
2
D
0
1
9
(
[
10 8C, n ¼ 1.4969). MS (EI, 70 eV): calculated m/z ¼
þ
þ
2.06 (M), found m/z ¼ 92 [M] (71.8), 91 [M—H]
þ þ
100), 90 [M—2H] (5.1), 65 [M—C H ] (12.8), 63
2 3
ꢁ1
þ
first one was carried out at initial CRHal ¼ 0.5 mol L and
M—C H ] (10.3).
2
4
ꢁ1
initial CDMF was changed from 0.2 to 7 mol L (Fig. 1,
curve 1). The second series was realized at initial
1
6
1
,2-Diphenylethane. m.p. ¼ 51–52 8C (lit m.p. ¼
ꢁ
ꢁ
1
1
CDMF ¼ 0.5 mol L and initial CRHal was changed also
51–52 8C). H NMR (CDCl ): d ¼ 2.82 (s, 4H, —
CH —), 7.02 (m, 10H, -Ph.) ppm. MS (EI, 70 eV):
calculated m/z ¼ 182.11 (M), found m/z ¼ 182 [M] (23),
3
1
from 0.2 to 7 mol L (Fig. 1, curve 2). We made the third
2
ꢁ
1
þ
series at initial CDMF ¼ 2 mol L . In this case the initial
ꢁ
1
þ
CRHal was changed from 0.2 to 7 mol L (Fig. 1, curve
). Benzene was used as a neutral solvent for
determination of the kinetic characteristics of the reaction
9
1 [M/2] (100).
3
0
4
,4 -Dimethylbiphenyl. MS (EI, 70 eV): calculated
1
8
þ
of zinc with benzyl chloride in the presence of DMF.
m/z ¼ 182.11 (M), found m/z ¼ 182 [M] (100), 167
þ þ
M—CH ] (56), 152 [M—2CH ] (15).
3 3
The reaction was studied in the kinetic mode, as
evidenced by the independence of the rate of zinc
dissolution in the test medium from the rate of stirring.
Benzyl iodide reacts with zinc at a transport-controlled
rate. Tables 2 and 3 summarize the results of this study.
[
Identification of radical species in solution
The reaction in a fivefold excess of a radical trap was
studied similarly to the general procedure. Dicyclohexyl
deuterophosphine (DCPD) was used as the radical
17,18
trap.
The residues of benzyl chloride and DMF, as
Studies of the reaction of atomic zinc with
benzyl halides at low temperature
well as 1,2-diphenylethane and a-deuterotoluene, were
detected in the benzene solutions. Table 1 summarizes the
yields of organic reaction products.
Co-condensation of zinc with benzyl halides.
Studies of the low-temperature reaction of zinc with
benzyl halides were carried out in a vacuum apparatus
1
a-Deuterotoluene. H NMR (CDCl ): d ¼ 2.32 (m,
3
21
2
H, —CH —), 7.15 (m, 5H, -Ph.) ppm. MS (EI, 70 eV):
2
similar to that reported previously. The reagents were
ꢁ4
þ
calculated m/z ¼ 93.07 (M), found m/z ¼ 93 [M] (100),
evaporated in an evacuated (10 mm Hg) reactor
(V ¼ 10 L) and condensed on the reactor surface cooled
þ
þ
þ
2 [M—H] (93), 91 [M—D] (46), 66 [M—H—C H ]
2
9
(
2
þ
9), 65 [M—C H D] (11).
2
with liquid N . Atomic zinc was prepared by evaporating
2
2
Table 1. Product composition in the oxidative dissolution of zinc in benzyl chloride-DMF systems in the presence (and in the
absence) of radical trap
Yield, %
a
b
c
N
RHal
RCl
DCPD/RHal
T (K)
Time (h)
R–H
R–D
R–R
1
2
3
4
5
6
7
0:1
5:1
5:1
0:1
0:1
5:1
0:1
313
313
313
278
313
313
278
12
12
15
2
0.5
0.5
2
96
44
44
95
78
32
85
—
56
56
—
—
68
—
4
—
—
5
22
—
15
RBr
d
RI
a
impurity of dicyclohexyl phospine 1%.
impurity of toluene 1%.
impurity of 4,4 -dimethylbiphenyl < 0.1%.
Benzyl iodide decomposes at higher temperature to form iodine.
b
c
d
0
Copyright # 2006 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2006; 19: 664–675
DOI: 10.1002/poc

Zn-BENZYL HALIDE REACTION
667
6
-2
-1
kept at this temperature for 20 min. The excess of DCl
was removed at 190–200 K (100 mm Hg). The reaction
mixtures were heated to 298 K. The liquid (298 K) and
gaseous (160 K) phases were analyzed by GC. Reaction
products were isolated by preparative LC. Yields are
shown in Table 4.
W•10 , g•cm •min
50
40
30
20
10
0
1
Acidolysis of reaction products at 298 K. Once co-
condensation of zinc with benzyl chloride and DMF was
completed and evacuation terminated, the reactor was
filled with dry pure Ar (760 mm Hg). The samples were
heated to 298 K. At this temperature the film melted and
decolorized. The reaction mixtures were treated with 20%
3
2
0
2
4
6
8
solution of DCl in D O (99.5 atom. % D, Aldrich) at
2
-
1
C, mol•l
298 K in pure, dry Ar. The liquid and gaseous phases were
analyzed by GC. The reaction products were isolated by
preparative LC. The results are given in Table 4.
Figure 1. Rates (w) of the zinc reaction with benzyl
chloride in DMF as a function of initial concentrations (C)
of mixture components in benzene at 283 K. W-dependence
ꢁ
1
on: (1) CDMF, CRHal ¼ 0.5 mol L (the initial CDMF was ranged
Detection of benzylpolyzinc hydrides. After com-
pletion of co-condensation of zinc with benzyl halides, the
samples were heated to 160 K, kept at this temperature
for 20 min, and then cooled down to 77 K. CCl was con-
4
densed on their surfaces. The CCl -containing samples were
4
ꢁ
1
from 0 to 7 mol L , the initial C
is kept constant); (2)
RHal
ꢁ
1
CRHal, CDMF ¼ 0.5 mol L (the initial CRHal was ranged from 0
ꢁ1
to 7 mol L , the initial C
is kept constant); (3) CRHal,
DMF
ꢁ
1
CDMF ¼ 2 mol L (the initial CRHal was ranged from 0 to
ꢁ
1
mol L , the initial C
7
is kept constant)
DMF
kept at 160 K for 1 h. Acidolysis of the samples was carried
out at 160 and 298 K. The liquid and gaseous phases were
analyzed by GC. Reaction products were isolated by
preparative LC. Yields are shown in Table 4.
the purified metal from a corundum crucible at 570–
22
6
0
2
00 K.
.2 mmol min . Benzyl halides were evaporated at
The rate of evaporation of Zn was
ꢁ1
73–308 K (PhCH Hal:Zn ¼ 50–100:1). The duration
2
of co-condensation was 2–4 h.
Chloroform. MS (EI, 70 eV): calculated m/z ¼ 117.91
þ
þ
(
[
[
M), found m/z ¼ 118 [M] (1), 117 [M—H] (0.7), 83
þ þ þ
2
M—Cl] (100), 82 [M—H—Cl] (4), 70 [Cl ] (1), 47
M—H—2Cl] (36), 36 [HCl] (1).
Acidolysis of reaction products at 160 K. After
completion of the co-condensation of zinc with benzyl
halides, the samples were heated to 160 K, kept at this
temperature for 20 min, and then cooled down to 77 K.
DCl (50 g) was condensed on their surfaces. After the end
of the evacuation, the reactor was filled with dry and pure
Ar (760 mm Hg). The samples were heated to 160 K (at
this temperature the film melted and decolorized) and
þ
þ
Study of stereochemistry
A solution of 50 mmol (þ)-R-1-bromo-1-phenylethane in
50 mL DMF was slowly added at ꢁ15 8C to 100 mmol
zinc powder, which had been activated according to the
Table 2. Kinetic and thermodynamic parameters of the oxidative dissolution of zinc in the benzyl chloride–DMF system
calculated using the Langmuir–Hinshelwood mechanism with adsorption of the reagent and the solvent at identical active
centers of the metal
3
ꢁ2
ꢁ1
ꢁ1
ꢁ1
K (L mol )
Parameter
T (K)
k ꢀ 10 (g cm min
)
K (L mol
1
)
2
2
2
3
3
3
3
83
0.413 ꢂ 0.004
1.13 ꢂ 0.01
2.55 ꢂ 0.02
6.02 ꢂ 0.05
12.2 ꢂ 0.1
27.4 ꢂ 0.2
0.999
1.20 ꢂ 0.01
0.864 ꢂ 0.008
0.639 ꢂ 0.005
0.471 ꢂ 0.004
0.369 ꢂ 0.002
0.274 ꢂ 0.002
0.999
0.273 ꢂ 0.002
0.195 ꢂ 0.002
0.138 ꢂ 0.001
0.0979 ꢂ 0.0008
0.0715 ꢂ 0.0006
0.0548 ꢂ 0.0005
0.999
93
03
13
23
33
a
r
E (kJ mol
ꢁ
1
)
64.9 ꢂ 2.0
A
DH8
(kJ mol
(kJ mol
(J mol
(J mol
ꢁ1
)
ꢁ22.9 ꢂ 0.8
ꢁ79 ꢂ 4
RHal
8
DMF
ꢁ1
DH
DS
DS
)
ꢁ25.5 ꢂ 0.8
ꢁ101 ꢂ 7
8
ꢁ1 ꢁ1
ꢁ1 ꢁ1
K )
K )
RHal
8
DMF
a
Sample correlation coefficient.
Copyright # 2006 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2006; 19: 664–675
DOI: 10.1002/poc

6
68
A. M. EGOROV
Table 3. Kinetic and thermodynamic parameters of the oxidative dissolution of zinc in the benzyl bromide–DMF system
calculated using the Langmuir–Hinshelwood mechanism with adsorption of the reagent and the solvent at identical active
centers of the metal
2
ꢁ2
ꢁ1
ꢁ1
ꢁ1
K (L mol )
Parameter
T (K)
k ꢀ 10 (g cm min
)
K (L mol
1
)
2
2
2
3
3
3
3
83
1.05 ꢂ 0.01
1.90 ꢂ 0.01
3.72 ꢂ 0.02
6.41 ꢂ 0.05
11.5 ꢂ 0.1
18.8 ꢂ 0.1
0.999
1.85 ꢂ 0.01
1.21 ꢂ 0.01
0.893 ꢂ 0.007
0.646 ꢂ 0.006
0.477 ꢂ 0.003
0.355 ꢂ 0.003
0.999
0.272 ꢂ 0.002
0.194 ꢂ 0.002
0.132 ꢂ 0.001
0.0986 ꢂ 0.0008
0.0719 ꢂ 0.0007
0.0548 ꢂ 0.0004
0.999
93
03
13
23
33
a
r
E (kJ mol
ꢁ
1
)
45.6 ꢂ 1.4
A
8
ꢁ1
)
DH
DH
DS
DS
(kJ mol
(kJ mol
(J mol
(J mol
ꢁ25.5 ꢂ 1.0
ꢁ85 ꢂ 6
RHal
8
DMF
ꢁ1
)
ꢁ25.3 ꢂ 0.8
ꢁ100 ꢂ 7
8
ꢁ1 ꢁ1
ꢁ1 ꢁ1
K )
K )
RHal
8
DMF
a
Sample correlation coefficient.
procedure described in Ref. 14. After 3.5 h of stirring at
15 8C the mixture was filtered. The filtrate was treated
with 50 mL benzene. Precipitated white crystals were
filtered off, washed by hexane and then evacuated. The
reaction of zinc with (þ)-R-1-chloro-1-phenylethane in
DMF was carried out by a similar procedure, except that
the reaction mixture was stirred at 30 8C for 10 h. The
ppm (From Refs. 23 and 24 data: b.p. 130–132 8C/
2
0
ꢁ
7 mm Hg, n ¼ 1.5557.).
D
RS,RS-2,3-Diphenylbutane (3). B.p. 144–149 8C/
1
1
2 mm Hg, m.p. 126–127 8C. H NMR (CDCl3):
d ¼ 1.03 (d, 6H, 2CH ), 2.75 (m, 2H, CH—CH), 7.24
3
(m, 10H, 2Ph.) ppm (From Refs. 23 and 24, b.p. 144–
residue was poured into 20% solution DCl in D O (99.5%
2
149 8C/12 mm Hg, m.p. 126–127 8C.).
D). Organic products of reaction were extracted with
benzene (50 mL). Yields are shown in Table 5. The
reaction products were as follows:
1
Styrene (6). H NMR (CDCl ): d ¼ 5.20 (m. 1H,
3
—
CH —), 5.70 (m. 1H, CH —), 6.69 (m. 1H, —CH—),
—
2
—
2
þ
Mixtures of RS-1-phenylethane-1D and pheny-
6.90 (m. 5H, Ph). MS, m/z (%): 104 [M] (100), 103 [M—
1
þ þ
H] (40), 102 [M—H] (12), 78 [M—C H ] (32), 77
2
þ
lethane (1 and 4). H NMR (CDCl ): d ¼ 1.20 (d, 3H,
3
2
þ þ
[M—C H ] (24), 51 [M—C H ] (28).
2 3 4 5
CH ) (1), 1.24 (t, 3H, CH ) (4), 2.62 (q, 1H, CH) (4), 2.66
3
3
(
m, 2H, CH ) (1), 7.19 (m, 5H, -Ph.) ppm.
2
The co-condensation of (þ)-R-1-halogen-1-pheny-
lethane with zinc (10:1) was carried out according to
co-condensation of zinc with benzyl halides. Acidolyses
of products at 160 and 298 K were similar to those of
products in the case of unsubstituted benzyl halides.
RR,SS-2,3-Diphenylbutane (2). B.p. 130–132 8C/
7
6
20
1
mm Hg, n ¼ 1.5557. H NMR (CDCl ): d ¼ 1.17 (d,
3
D
H, 2CH ), 2.76 (m, 2H, CH—CH), 7.24 (m, 10H, 2Ph.)
3
Table 4. Composition of reaction products obtained by acidolyses of zinc co-condensates with benzyl halides
Yield, % mol
a
b
c
RHal
RCl
RHal/Zn
50
T (K)
R–R
R–D
D2
CHCl3
Zn/RD
Zn/D2
n
160
298
160
298
160
298
160
298
160
298
16.7
35.6
34.5
44.2
32.7
40.0
44.4
49.0
58.8
73.5
75.7
64.4
62.4
55.8
67.3
60.0
55.6
51.0
41.2
26.5
7.6
—
3.1
—
—
—
—
—
—
—
0.05
—
0.01
—
0.001
—
—
—
—
1.10
1.00
1.05
1.00
1.00
1.00
1.00
1.00
1.00
1.00
11
—
21
—
—
—
—
—
—
—
1.10
1.00
1.05
1.00
1.00
1.00
1.00
1.00
1.00
1.00
5
0
1
1
00
00
50
RBr
5
0
1
1
00
00
50
RI
5
0
—
a
The acidolysis was carried out at temperatures listed.
b
c
D
2
contains trace amounts HD.
n is the amount of Zn atoms in PhCH
Zn
2 n
Cl ꢀ DMF.
Copyright # 2006 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2006; 19: 664–675
DOI: 10.1002/poc

Zn-BENZYL HALIDE REACTION
Table 5. Yields of products 1–5 (%) in the stereochemical study of the Zn-benzyl halide reaction
Yields of products, % (optical purity, %)
669
RHal
Cl
T (K)
Solvent
1
2
3
4
5
303
DMF
—
—
—
—
DMF
—
—
—
—
90.4
52.8 (18.5)
34.8 (7.4)
40.5
15.8
78.3
35.7 (3.7)
22.0
28.9
4.1
23.8
31.0
29.5
40.4
10.3
32.5
37.2
35.1
44.4
4.0
23.1
30.0
28.6
39.1
10.0
31.2
36.1
34.1
43.1
1
0.5
—
a
1
2
1
2
60
0.3
2.2
0.8
2.4
0.9
0.4
2.4
1
a
98
60
98
258
2.0
0.6
2.3
0.5
0.2
2.3
0.9
2.5
Br
a
1
2
1
2
60
a
98
60
98
7.4
2.6
a
Benzyl halide was condensed on zinc film surfaces.
2
D
0
20
D
RR,SS-, RS,RS-diphenylbutanes, styrene, (þ)-S-1-phe-
(þ)-S-1-Phenylethane-1D (1). [a] þ 0.15 and [a] þ
2
D
0
nylethane-1-D were found as products.
0.03 (l ¼ 0.1) (From Refs. 23 and 24 [a] þ 0.81 (l ¼ 0.1)).
2
D
0
(
1
þ)-S-1-Phenylethane-1D (1). B.p. 135–136 8C, n ¼
20
1
.4954, [a] þ 0.06 (l ¼ 0.1). H NMR (CDCl ): d ¼ 1.23
D
d, 3H, CH ), 2.62 (q, 1H, CH), 7.20 (m, 5H, -Ph.) ppm
3
(
(
[
RESULTS AND DISCUSSION
3
2
0
From Refs. 23 and 24, b.p. 135–1368C, n ¼ 1.4919,
D
2
0
a] þ 0.81 (l ¼ 0.1).).
The reaction of zinc with benzyl halides and DMF is
outlined in Scheme 1.
D
Studies of the low-temperature reactions of compact
zinc with (þ)-R-1-halogen-1-phenylethane were carried
While benzene is added to the reaction mixture, white
crystals of [Zn(DMF) Hal ] (47–48%) are formed:
21
out in an apparatus similar to that reported previously.
2
2
Hal ¼ Cl, Br, I
ꢁ
4
Zinc was evaporated in an evacuated (10 mm Hg)
reactor (10 L) from corundum crucible at 570–600 K and
condensed on the reactor surface cooled with liquid N2.
The residues are treated with 20% solution HCl in H O.
2
Thus the yield of toluene reflects that of benzylzinc
halides. Hydrogen was not observed in the gaseous
phase.
(þ)-R-1-Halogen-1-phenylethane was condensed on the
zinc film surfaces (RHal:Zn ¼ 1:1). Acidolysis of
products at 160 and 298 K were similar to those of the
products in the case of unsubstituted benzyl halides.
RR,SS-, RS,RS-diphenylbutanes, styrene, (þ)-S-1-phe-
nylethane-1-D were found as products.
The analysis of water solutions demonstrated that the
2þ
samples contained Zn cations and halide anions, whose
ratio corresponded to the formula ZnHal (Hal ¼ Cl, Br,
2
or I). Zn (I) was not found. The evaporation of solvents in
vacuo gives colorless crystals of ZnHal ꢀ 2H O. Organic
2
2
Scheme 1. Hal ¼ Cl, Br, I
Copyright # 2006 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2006; 19: 664–675
DOI: 10.1002/poc

6
70
A. M. EGOROV
products yields are given in Table 1. The total amount of
organic products corresponds to 100%. As expected, the
order of decreasing halide reactivity is I, Br, Cl. The
reaction of zinc with benzyl iodide is facile at low
temperature. However, benzyl iodide provides lower
yield of organozinc compounds (Table 1) than benzyl
bromide and benzyl chloride do. As illustrated in entries 4
and 5, the yield of benzylzinc halides is temperature
dependent. Significant amounts of organozinc com-
pounds are found in entries 1 and 4. 1,2-Diphenylethane
completely disappears at 110 K. At temperatures higher
than 130–150 K, the resolution of the ESR-spectra is
substantially impaired, apparently due to the overlap of
the signals of benzyl and 4-tolyl radicals, as well as of the
signals of the corresponding radical pairs. The ESR signal
of zinc co-condensates with benzyl chloride disappears at
170 K. In that of benzyl bromide and benzyl iodide the
ESR signal disappears at 185 and 210 K, respectively.
While excess of benzyl halide is settling on to the zinc
ꢁ
4
film surface (the thickness of the film is about 10 mm),
UHF power dissipation increases. The consequent
decreases of ESR spectrum resolution can be attributed
to increases in the zinc film electroconductivity. The
paramagnetic particles appearing in benzyl halide
compact zinc systems are identical to the particles
formed when atomic zinc films are used. The ESR
spectrum of benzyl chloride-compact zinc film is similar
to the low-intensity spectrum of zinc co-condensate with
benzyl chloride, with the ratio singlet:triplet of quartets
being increased by two- to threefold. The main cause of
this increase is the stabilization of ion-radical pairs by
charge distribution among the whole group of zinc atoms.
Mono- and polyorganomagnesium compounds can be
formed as a result of the decay of ion-radical pairs
0
and 4,4 -dimethylbiphenyl were found as side products
and 4-benzyl-1-methylbenzene was not detected. It may
be reasonable to assume that the reaction proceeds
according to the radical mechanism. In this case
isomerization of benzyl radical occurs only in the radical
25
pair.
The paramagnetic particles that formed in the reaction
of zinc with benzyl halides were studied by ESR-
spectroscopy. The ESR spectrum of the co-condensate of
zinc with benzyl bromide at 77 K shows a triplet of
quartets with width of about 50 G and g factor of
H
CH
H
o
H
p
2
.002 ꢂ 0.002, a
of 16.6 ꢂ 1.0 G, a of 5.5 ꢂ 0.6 G, a
2
of 5.5 ꢂ 0.6 G. The ESR spectrum of the co-condensate of
zinc with benzyl iodide is analogous to that of the co-
condensate of zinc with benzyl bromide. However, the
former is more poorly resolved. The comparison of the
parameters of the typical ESR spectrum obtained in this
research with those of benzyl radicals in solid matrices
allows the assignment of the signals of this spectrum to
ꢁ
ꢀ
þ
ꢀ
RX ꢁ Mg . The mechanism of this reaction also
n
depends on the bond-breaking energy of the carbon–
The spectra
7,28
halogen bond of the starting benzyl halide.
obtained show that ion-radical pairs are possible
intermediates in the formation of organozinc halides
from benzyl derivatives. The reaction mechanism seems
to be dependent on the type of halogen in the benzyl
halide. The composition of organozinc compounds was
determined by acidolysis co-condensates of zinc with
benzyl halides (1:50–100). The ratio metal:matrix of
1:100 leads to approximately 85% of atomic and 15% of
dimeric metal. The ratio metal:matrix 1:10 000 results in
7
,18,26
benzyl radical.
The ESR spectrum of the co-condensate of zinc with
benzyl chloride is a superposition of the triplet quartets
(which is similar to the spectrum of zinc co-condensate
with benzyl bromide and benzyl iodide) and a singlet with
a half-width of 8 ꢂ 2 G. The ESR spectra are similar for
7
reactions of benzyl chloride with magnesium and zinc.
We may conclude that the singlet with a half-width of
7,22
atomic metal.
The low melting point of DCl made it
8
ꢂ 2 G corresponds to the ion-radical pair of the
possible to detect the reaction products both before
(159 K) and after (298 K) defrosting of the sample.
Organozinc compounds have been observed reacting with
DCl according to the scheme:
ꢁ
ꢀ
þ
ꢀ
RX ꢁ Zn type and the absence of hyperfine structure
7
is related to exchange processes. According to ESR
spectra, the total fraction of paramagnetic species in the
samples at 77 K amounts to 9, 12, 22% for Cl, Br, I of the
amount of precipitated zinc atoms and depends on the
energy of the carbon–halogen bond in the starting benzyl
As represented in Table 4 the benzyl chloride shows
high yield of benzylzinc halides (75.7%) at 160 K. When
the RHal/Zn ratio was increased to 100, the yield of
organozinc compounds was reduced to 62.4%. For the
reaction of zinc-benzyl bromide, observations are similar
to those for benzyl chloride: 67.3 and 55.6%. The
comparison of the results in Table 4 shows that the rise of
temperature causes an increase in the yield of homo-
coupled product and a decrease that of benzylzinc halides.
The formation of organopolymagnesium hydrides was
observed in the reactions of Mg with organic halides
ꢁ
1 27
halide (95, 69 and 45 kcal mol ). Examination of the
ESR spectra recorded at different temperatures provides
data on the stability and conversion of the reaction
intermediates. The intensity of the ESR signal gradually
decreases as the co-condensate of Zn with benzyl halides
are heated to 100 K. In the case of Zn co-condensates with
benzyl chloride the signal of the ion-radical pair (singlet)
decreases more rapidly than that of benzyl radical and
Copyright # 2006 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2006; 19: 664–675
DOI: 10.1002/poc

Zn-BENZYL HALIDE REACTION
671
which were characterized by high energies of the
benzyl iodide. Thus we can conclude that ion-radical
pairs are not suitable intermediates in reaction of zinc
with benzyl halides. Outer-sphere electron transfer can
appear partly in the case of benzyl chloride at low
temperature.
28
cleavage of the carbon–halogen bond while these
products were not detected in the reactions of Mg with
7
benzyl halides. Benzylpolyzinc halides and hydrides
react with DCl to form D2:
It can be seen from Table 4 that detectable amounts of
D were found only in the reaction of zinc with benzyl
chloride. The appearance of D2 in small amounts
indicates that benzylpolyzinc halides and benzylpolyzinc
hydrides are absent in the reaction mixture or present in
We have studied the reaction of optically active (þ)-R-
1-haloghen-1-phenylethane with zinc in DMF hoping to
obtain experimental evidence for optical activity reten-
tion at the asymmetry center in the course of formation of
organozinc compounds:
2
Hal ¼ Cl, Br
insignificant amount at 298 K. A minor amount (<15%)
of the cluster structures and their disappearance after
acidolysis at 298 K can easily be explained: benzylpo-
lyzinc halides can decompose already at 160 K to form
zinc in the reaction:
Table 5 shows that this reaction results in the formation
of optically inactive RS-1-phenylethane-1-D and four other
unlabeled hydrocarbons 2–5. The formation of optically
inactive 2, 3 with the ratio of 2:3 equal to 1.03:1 along with
traces of 4 and 5, shows that the reaction occurs according
to the radical mechanism by which recombination and
disproportionation of 1-phenylethyl radical can proceed
after they enter the solution. However, organozinc halide
racemization may proceed just after its formation. To
confirm this assumption, we have studied the low-
temperature reaction of zinc with (þ)-R-1-halogen-1-
phenylethane. The co-condensation of a zinc vapor with
(þ)-R-1-halogen-1-phenylethane (1:10) which were pre-
cipitated in the molecular beam mode on the surface cooled
The amount of hydride complexes was estimated by
CCl condensation on to the sample surface at 77 K before
4
acidolysis. According to GC data listed in Table 4 the
amount of chloroform in acidolysis products was
about 0.05% for the case of zinc-benzyl chloride co-
condensate and less than 0.01% and 0.001% for the
case of systems containing benzyl bromide and
with liquid N produced white films. The acidolysis was
2
carried out by DCl at 160 and 298 K:
Copyright # 2006 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2006; 19: 664–675
DOI: 10.1002/poc

6
72
A. M. EGOROV
Hal ¼ Cl, Br
It is shown in Table 5 that isolation of 52.8% the (þ)-S-
heterogeneous reactions; it is characterized by high
In order to
1
8,19,29
1
-phenylethane-1-D (Hal ¼ Cl, 160K) with optical purity
accuracy and reproducibility of results.
determine the kinetic characteristics of the process, the
reaction is performed in a neutral solvent of benzene
of 7.4% may be evidence for partial formation of
benzylzinc chlorides on the zinc surface or by decay of
ion-radical pair. The condensation of (þ)-R-1-halogen-1-
phenylethane on zinc film surface results in dark films.
After completion of condensation of (þ)-R-1-halogen-1-
phenylethane the sample were heated to 160 K, kept at this
temperature for 3 h and then cooled down to 77K. The
acidolysis of the samples was carried out by DCl at 160 and
ꢁ
1 3
ðDN
¼ 0:42 kJ ꢀ mol Þ . The kinetic experiments
SbCl
5
were performed in pure benzene (Figs. 1 and 2, point
CDMF ¼ 0 of curves 2, 3). The rate of the reaction in pure
benzene was smaller than the sensitivity of the equipment
under our conditions. Benzyl iodide reacts with zinc at a
transport-controlled rate: it increases with increasing
stirring rate (1000–2500 rpm). The solution exhibits new
absorption band of l ¼ 520 nm. It may be concluded that
2
1
98 K. The result of this reaction was formation of (þ)-S-
-phenylethane-1-D with optical purity 3.7% (Hal¼ Br).
30
Such optical purity of 1-phenylethane-1D does not exceed
that for 1-phenylethyl radical recombination within a
solvent cage. The high optical purity of (þ)-S-1-
phenylethane-1-D 18.5% (Hal ¼ Cl) is associated with
partial formation of ion-radical pairs in reaction of (þ)-R-
benzyl iodide decomposes to form iodine. Benzyl
chloride and bromide react with zinc in DMF at a
nontransport-controlled rate.
Figures 1 and 2 exhibit kinetic curves that have
maxima. An increase in the concentration of DMF from
ꢁ
1
0.5 to 2 mol L did not change the shape of the curves,
1
-chloro-1-phenylethane with zinc at low temperatures
(
160 K, Table 5). Comparison of the optical purities of
reaction products outlined in Table 5 and the proportion of
deuterium in 1-phenylethane-1-D both lead us to conclude
that the reaction of benzyl halides with Zn occurs on the
metal surface. Racemization of benzylzinc halides takes
place in the solution through fast structure inversion.
After having obtained evidence for radical intermedi-
ates in the reaction of zinc with benzyl halides, next we
sought for an agent to trap these radicals in the solution.
4
-2
-1
W•10 , g•cm •min
14
12
1
1
0
8
6
4
2
0
18,26
17
3
We have shown
that DCPD can be utilized as
trapping agent for benzyl radicals. Table 1 shows the
results of reaction of benzyl halides with zinc in DMF
when 5 equi. of DCPD were added. Entries 2 and 3 show
that DCPD does not react with benzylzinc halides during
the reaction or after it was completed. Comparison of the
data from entries 1,4 and 2,6 show that the radicals that
escaped the zinc surface are trapped by DCPD. Thus in
this case it appears that among the benzylzinc halides
formed in this reaction, a minimum of 52% results from
radicals that diffuse into the solution and then return to the
zinc surface.
2
0
2
4
6
8
-1
C, mol•l
Figure 2. Rates (w) of the zinc reaction with benzyl bromide
in DMF as a function of initial concentrations (C) of mixture
components in benzene at 283 K. W-dependence on: (1)
ꢁ
1
CDMF, CRHal ¼ 0.5 mol L (the initial CDMF was ranged from 0
ꢁ
1
to 7 mol L , the initial C
is kept constant); (2) CRHal
,
RHal
The reaction kinetics of the oxidative dissolution of Zn
in the benzyl halide-DMF system has been studied using
the resistometric method. This method provides an
opportunity to study various kinetic features of fast
ꢁ1
CDMF ¼ 0.5 mol L (the initial CRHal was ranged from 0 to
ꢁ1
7
mol L , the initial C
is kept constant); (3) CRHal
,
DMF
ꢁ
1
CDMF ¼ 2 mol L (the initial CRHal was ranged from 0 to
ꢁ
1
7 mol L , the initial CDMF is kept constant)
Copyright # 2006 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2006; 19: 664–675
DOI: 10.1002/poc

Zn-BENZYL HALIDE REACTION
673
showing the dependence of the reaction rate on the
and
concentration of benzyl halide. This indicates that the
process proceeds by the Langmuir–Hinshelwood mech-
anism with adsorption of the reagent and the solvent at
1
þ K ½PhCH Halꢃ
1=2
1
2
¼
b
ðkK1K2½PhCH2HalꢃÞ
18,31
identical active centers of the metal surface
:
Then
K1
1
=2
½
Solvꢃ
PhCH Hal þ S Ð ðPhCH HalÞS
(1)
(2)
2
2
¼
a½Solvꢃ þ b
1=2
w
K2
The similar operations for [Solv] ¼ constant lead to
Solv þ S Ð ðSolvÞS
K1
¼
¼
a1
1
=2
k3
ðkK K ½SolvꢃÞ
1
2
ðPhCH HalÞS þ ðSolvÞS ꢁ! products
ð3Þ
2
and
Scheme 2:
1
þ K ½Solvꢃ
ðkK K ½SolvꢃÞ
2
where Solv is DMF; Hal ¼ Cl or Br, K and K are the
b1
1
2
1=2
1
2
equilibrium constants of benzyl halide and DMF
adsorption, respectively; k is the rate constant of the
3
a ða½Solvꢃ þ bÞ
1
limiting step and it corresponds to the formation of
reaction products; S are the active centers at which the
adsorption of benzyl chloride and DMF takes place. In
this case, surface coverages derived from the Langmuir
isotherms for adsorption of individual components appear
in the rate equation, and the reaction rate can be expressed
by Eqn (4):
K1 ¼
bb ꢁ aa ½PhCH Halꢃ½Solvꢃ
1
1
2
a
K ¼ ð1 þ K ½PhCH HalꢃÞ
2
1
2
b
K2
k ¼
2
a K ½Solvꢃ
1
The study of the reaction kinetics at different
temperatures allowed to obtain E of the chemical
kK K ½PhCH Halꢃ½DMFꢃ
1
2
2
w ¼
(4)
A
2
ð1 þ K ½PhCH Halꢃ þ K ½DMFꢃÞ
1
2
2
reaction and enthalpies and entropies of adsorption of the
reagents on the zinc surface. As Tables 2 and 3 show, the
Arrhenius plots of the rate or equilibrium constants versus
1/T gave good correlations (r ¼ 0.999). The high accuracy
of results obtained favors the mechanism (Scheme 2).
Therefore, the adsorption of benzyl halides and DMF
should be the first steps of the reaction. The limiting step
is inner- or outer-sphere electron transfer. The equi-
2
where k ¼ k ꢀ N ; N is the amount of active centers of the
3
metal surface at which the adsorption of benzyl halide and
DMF takes place. The Langmuir–Hinshelwood scheme
for the test process suggests that the interaction of
adsorbed reactant molecules with the metal surface, viz. a
surface chemical reaction, is a rate-limiting step of the
reaction.
librium constants K , enthalpy and entropy for adsorption
2
The treatment of the experimental relations (Figs. 1
and 2) using the set of Eqns (1)–(3) allowed to determine
the equilibrium constants of benzyl halide and DMF
adsorption on the surface of zinc (K and K , respectively)
and the rate constant k of the chemical reaction.
The Eqn (4) can be written as
of DMF have the same values in reactions of benzyl
bromide and chloride (Tables 2 and 3). The enthalpies and
entropies of adsorption values obtained indicate that DMF
and benzyl halides do not dissociate on zinc surface
during adsorption.
Nechaev et al. have shown that the adsorption of
1
2
32
organic compounds on metal surface depends on the
3
3–
1
1 þ K ½PhCH Halꢃ þ K ½Solvꢃ
1
2
2
ionization potential of these organic compounds only.
36
PhCH Hal and DMF have similar ionization potentials
¼
w1
=2
1=2
ðkK K ½PhCH Halꢃ½SolvꢃÞ
2
1
2
2
and this is a reason why they have similar DHads values.
The rate constant ratio, kRBr/kRCl, may provide an
independent information for the reaction mechanism. The
When [PhCH Hal] ¼ const:
2
1
=2
½
Solvꢃ
1 þ K ½PhCH Halꢃ
1
2
¼
free-radical reaction would lead to kRBr/kRCl ¼ 3000–
1
=2
1=2
w
37
000
ðkK K ½PhCH HalꢃÞ
1
2
2
5
and S 2-mechanism would lead to k
N
/
RBr
38
kRCl ¼ 100. Benzyl bromide reacts much faster than
benzyl chloride with zinc in DMF. The surface size of
zinc wire for reaction with benzyl chloride was 80 times
as much as for that of benzyl bromide. The relative rate of
zinc reaction with benzyl halides in DMF kRBr/kRCl ¼ 773
(318 K) and kRBr/kRCl ¼ 369 (353 K) as calculated from
temperature relations agree well with the order reported
K ½Solvꢃ
2
þ
1
=2
ðkK K ½PhCH HalꢃÞ
1
2
2
where
K2
ðkK K ½PhCH HalꢃÞ
¼
a
1=2
1
2
2
Copyright # 2006 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2006; 19: 664–675
DOI: 10.1002/poc

6
74
A. M. EGOROV
for halogen atom transfer rate-limited radical mechanism
28
CONCLUSIONS
3
9
(
760 at 318 K and 496 ꢂ 128 at 363 K ). The result
obtained agree well with data reported in Rieke’s paper.
The author proposes that the electron transfer between Zn
and substrates possesses an inner sphere transfer
The results of this work indicate that the benzyl halide
reaction with zinc in DMF takes place at the metal surface
by halogen atom transfer mechanism (inner sphere
electron transfer) via formation of benzyl radicals, which
undergo recombination and isomerization mainly in
solution. The reaction occurs by Langmuir–Hinshelwood
mechanism (adsorption of reagent and solvent takes place
at identical active centers of the metal surface) according
to the following scheme.
40
character.
3
Comparison of the results with the published data on the
oxidative dissolution of zinc in the EtI-DMF system
demonstrated that DHads DMF at the surface of zinc
remained almost unchanged (ꢁ25.5 and ꢁ25.5 ꢂ
ꢁ
.8 kJ mol ) with replacement of EtI by PhCH Hal,
2
1
0
whereas the corresponding values for a organic halide
changed considerably (from ꢁ3.3 to ꢁ22.9 ꢂ 0.8 and
The limiting step of the reaction of benzyl halides
with zinc is a halogen atom transfer to metal surface.
ꢁ
1
ꢁ
25.5 ꢂ 1.0 kJ mol ). This fact indicates the selective
adsorption of a dipolar aprotic solvent, which participates in
the reaction, on the surface of zinc.
react quickly with benzyl halide :
Recombination and isomerization of benzyl radicals
proceed mainly in solution according to the scheme:
Copyright # 2006 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2006; 19: 664–675
DOI: 10.1002/poc

Zn-BENZYL HALIDE REACTION
675
The coordination compounds of zinc(II) with organic
ligands in the solution are formed via the following
sequence:
13. Gordon AJ, Ford RA. The Chemist’s Companion: A Handbook of
Practical Data, Techniques and References. Wiley: New York,
972.
1
1
4. Baker KV, Brown JM, Hughes N, Skarnulis AU, Sexton A. J. Org.
Chem. 1991; 56: 698–703.
ZnHal DMF þ DMF ! ZnHal ꢀ 2DMF
15. Imanakunov BI, Kim TP, Baidinov T. Reaktsii formamida i
dimetilformamida s neorganicheskimi solyami (The reactions of
formamide and dimethylformamide with inorganic salts). –Frunze,
Ilim, 1986.
2
2
PhCH ZnHal DMF þ DMF ! PhCH ZnHal ꢀ 2DMF
2
2
16. Caubere P, Moreau J. Tetrahedron. 1969; 25: 2469–2486.
17. Ashby EC, Oswald J. J. Org. Chem. 1988; 53: 6068–6076.
18. Egorov AM, Matyukhova SA, Anisimov AV. J. Phys. Org. Chem.
2005; 18: 1023–1031.
where Hal ¼ Cl, Br, I.
Acknowledgements
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9. Nifontova GA, Echmaev SB, Sikorenko YuB, Lavrent’ev IP. Zh.
Fiz. Khim. 1998; 72: 147–151.
The work has been done with financial support of the
Russian Federation’s Ministry of Education under Rus-
sia’s Higher School Fundamental Studies in Natural
Sciences and Humanities program, grant UR 05.01.012
and UR 05.01.419.
2
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VP. Zh. Obshch. Khim. 1990; 60: 2268–2272.
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Copyright # 2006 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2006; 19: 664–675
DOI: 10.1002/poc