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Halogen exchange reactions catalyzed by amines
357
Journal ofhlofecukiw Catalysis. 10 (2981) 357 - 360 0 Ekevier Sequoia S.A., Iausann e - Printed in Lhe NetherlPsds
HALOGEN
YOEL Wt!.i 9!000
EXCWGE
SASSON Institute (IsruelJ
(R-tied
REACTIONS
CATALYZED
BY AMINES
and MEND-4 YONOVKX-WEISS of Applied
Chemiehy,
The Hebrew
Uniuersi-9 of berusalcn.
Jemsakm
September 1,198O)
Halogen exchange reactions between alkyl bromides and alkyl chlorid.es were homogeneously catalyzed by tertiary amines at 110 %.
-In~oduction Halogen exchange reaction between metal salts and alhyl halides [I, 21 (Finkelstem reaction [ 31) is a convenient method for preparing al_4yl iodides and fluorides from the corresponding alIcy chlorides or bromides. The process is not as effective for converting a&y1 chlorides to alkyl bromides 14, 51, since no simple methods are available for shifting the equilibrium exchange reaction in the desired direction. Another type of exchange reaction was proposed by Tahni [S] ; in his process, organic halides exchange halogens in the gas phase in tie presence of place calcium salts as catalysts. For example, smooth exchange ties between L,Z-dkhloroethane and 1,2dibromoethane at 250 “c over calcium sulfate catalyst according to ClCHaCHaCi
+ BrCHaCHaBr
+ 2ClCH&HsBr
(1)
A similar reaction, under milder conditions and in the liquid phase, was reported by IViNy and collaborators [7]_ These authors used ethyl bromide as a bromination agent according to the following scheme: C=H,Br
+ RCl + CaHsCl
f RBr
(2)
The reaction was catalyzed by bromide ion and required the presence of a highly polar solvent, namely N-methyl-2-pyrolidone (NM!?), *noorder to maintain 2 homogeneous system. An excess (10 mol) of ethyl bromide was used to ensure complete conversion of &he a&y1 chloride. The major disadvantage in the proposed system is the bromide catalyst which requirthe presence of the polar solvent. This solvent, in addition to its high price and problems of recycling also tends to participate in side reactions with some substrates 171.
358
TABLE 1 Bromide-chloride Eaperiment
Starting
exchange
catalyzed
misturc
Products
CBHSCHzCl + nCJH7Br C6H&I12Cl + nC&Br CBH$H&I + nC&UBr nCsH&l + nC4HSBr nCsHwCl + n+H,Br .ClCH,CH,Cl + BrCH.&H,Br CI(CH2);Cl + Br(CH234Er C6H&H$l+ BrCH2CH,Br
9
C~HSCH#
+ Br(CH,),Br
by tri-n-butylaminem Time
C6HsCHzBr
+ nC3H$l
C&i&H@
+ n-C4H&i
CGH&HzBr + n-CGHl;a II-CgIilaBr + nCaH&l nC6HIaBr * nZaH$l ClCH&H2Br CI(CH2)4Br C6H&H,Br -C ClC!H+H2Br ClCH2CH2CI C6H&H,Br + Cl(CH2)+Br Cl(CH&CI
(h)
Yield (molS)
1.0 1.3 2.0 20.0 20.0 20.0 1.9
56.1 54 .o 50.0 49.2 22.4 38.6 47.0
7.5
59.gb
+ + 30
57-o=
=Reaction conditions: 0.1 mol chloride, O-1 mol bromide and O-002 mol tri-n-butylamine at 110 ‘C. b59_9 mol% of i,2 dibromoetbane were converted; 13.2 mol% to 1,2 dichloroethane and 46.7 molR tG l-bromo-2+zhIoruethane. cS?.O mol% of 1.4-dibromobuLane -+-ere converted, 22.6 mol% to 1;4xlichloroetbane and 34.4 mol% to l-bro&oA
N’e have found that simple amines (particularly tertiary amino) are very effective catalysts for the liquid phase homogeneous halogen exchange reaction. The high solubility of amines in various organic solvents allows the application of this type of catalyst in a Iarge variety of common solvents or even in the absence of any solvent except the substrates themselves. Results and discussion When stoichiometric quantities of a&y1 chloride and alhyl bromide were heated to 110 % in the presence of 0.02 mol-equivalent of tributylamine, equilibrium mixtures Fere obtained within 1 - 10 h depending on the nature of the halides. Examples of several rewtions are given in Table 1. It is quite clear from the ti!e that practically any akyl and benzyl halides and dihalides can be exchanged under these conditions. The usual equilibrium conversion is - 50%. Quantitative conversions are easily obtained by application of excess of the bromide and the chloride donor. When a volatile bromide is used, e.g. ethyl, n-propyl or n-but-j1 bromides; the product chloride is easily evaporated allowing simple separation of the pure product. Additional amines were test4 as cztalysts for the exchange of n-butyl bromide with benzyl chioride (eqn_ (3)). n-C4H9Br + CsHsCH&i
+ n-C4H&1 + C6HsCH2Er
(3)
-
359
Experiment
Time
C&dYst
trk.oc~lercine
di-n-butykmke di~-bctyIam&e dinac@zr&ne di-n+xtylamiue n-butykmine diethylamine tibet?nZykmine
10 11
'Reaction conditions:%1 amineat
molbenzyl
5B.l S-6 50.0 42.sb 27.1b 54Jab 20.4 51-O 19.8 42.Sb 12.7
1.0 1.0 1.0 3.3 10-o 3.0 IO.0 4.0 2.0 20.0
tiethylamine
i 8 9
Yield @zol%;!
1.0
ti-U-bPtphH!iUt tik-hexyhmine
1 2 3 4 5 6
(lx)
chloride,0.1molbutylbro~ide
and O-002 mol
110T.
bSolidpreeipitation~asobse~edduringthecourseofthereaction.
The results of this series of experiments are summarized in Table 2. The observed order of activity for n-bu*l amines is tertiary > secondary > primary. Tri-n-hexy;. and tri-n-o&$ amines were found to show similar activity to tri-n-butyl amine. Tribenzylamine had orAy minor activity and triethylamine promoted solid precipitation during the course of the catiysis. It is quite obvious that quaternary ammonium s&s play an important role in the mechanism of this exchange reaction. G&a’krnary salts .kve been separated from the reaction mixtures and in addition several quaternary salts were found to be catalykally ackive in the process. A possible mechanism for +&e exchange reaction R’Cl f R”Ek =+ R’Rr + R”C1 catalyzed by RaN would be via the following steps: (a) R’Cl f RzN = &R&l* (b) R”Br + RsN * R”kRIBr(c) R’&RsCl-
+ R”&RsBr-+
(d) R’kRaBr-
*
(e) R’rkR3C1-
= RRN -i- R”CI
R’$R$3r-
+ R”&&Cl-
RzN + R’Br
This mechanism was ruled out by the quaternary salts is not reversibie. 5fGca.l bromide and benzyki-n-butyl aklmonkm under the reaction conditions. Accordingly, the actual mechanism is (a) R’Cl f RsN -+ RZjRsCl(b) R”J3r f RsN --f It”&RsBr-
observation that the formation of salts, e.g. tek3bui;ylammofinium brcmide, were found to by stile apparently
as follows:
360
(c) RsR$l-
+ R”Br = &R3Br-
cd) R”&RJBr-
Experimental set
+ R’CI = R”&R&l-
+ R”Cl f R’Br
tion
Alkyl chlorides and bromides as xelI as the amines, were aII pu.rchas.zd from Aldrich Chemical Co, Milwaukw, and were distilled and degassd prior to use. Reactions were carried out in 100 ml flasks placed in a thermostatic silicon oil bath at 110 i 0.2 “c. The product mixtures were separated by GLC ar_alysis performed on a SS l/8 in X 6 ft column packed with 15% SE30 or 80 - 100 mesh chromosorb P. Prtiuct identification was made by ccmparison with authentic samples. -Procedure
for an exchange
reaction
In a typical experiment, 0.1 mol 1,2clibromoethane (18.8 g), 0.1 mol 1,2dichloroethane (9.8 g) and 2 X 10-j mol tri-n-butylamine were heated to 110 “C in a 100 ml flask for 2 h. After cooling, the mixture was analyzed and was found to contain 0.098 mol 1-bromo-2chloroethane, 0.051 mol 1,2CirYxoethane and 0451 mol of 1,2 dibromoethane. The mixture was ql:antitatively separated by fractional distillation_
References 1 FL T. Dillon, b. 2 H. A. C. McKay,
.Im. Chem Sot.. 54 (1932)
952. h’ature (London). I39 (1937) 283. 3 H. Pinkelstein, Eer., 43 (1910) 1528. 4 W. J_ Bailey and E. Fujiwara, J. Am. Chem. Sot., 77 (1955) 165. 5 E_ D. Hughes, C. K_ Ingold and J_ D. H_ Mackie, J. Chem. Sot.. (1955) 3173. 6 Israeli Patent 11,963 (1953) Lo A. Talmi. i A. E. Willy, D. R. McKean and 5. A. Barcia, Bull. Chen. Sot. Jpn_. 49 (1976)
L989.
Journal ofhlofecukiw Catalysis. 10 (2981) 357 - 360 0 Ekevier Sequoia S.A., Iausann e - Printed in Lhe NetherlPsds
HALOGEN
YOEL Wt!.i 9!000
EXCWGE
SASSON Institute (IsruelJ
(R-tied
REACTIONS
CATALYZED
BY AMINES
and MEND-4 YONOVKX-WEISS of Applied
Chemiehy,
The Hebrew
Uniuersi-9 of berusalcn.
Jemsakm
September 1,198O)
Halogen exchange reactions between alkyl bromides and alkyl chlorid.es were homogeneously catalyzed by tertiary amines at 110 %.
-In~oduction Halogen exchange reaction between metal salts and alhyl halides [I, 21 (Finkelstem reaction [ 31) is a convenient method for preparing al_4yl iodides and fluorides from the corresponding alIcy chlorides or bromides. The process is not as effective for converting a&y1 chlorides to alkyl bromides 14, 51, since no simple methods are available for shifting the equilibrium exchange reaction in the desired direction. Another type of exchange reaction was proposed by Tahni [S] ; in his process, organic halides exchange halogens in the gas phase in tie presence of place calcium salts as catalysts. For example, smooth exchange ties between L,Z-dkhloroethane and 1,2dibromoethane at 250 “c over calcium sulfate catalyst according to ClCHaCHaCi
+ BrCHaCHaBr
+ 2ClCH&HsBr
(1)
A similar reaction, under milder conditions and in the liquid phase, was reported by IViNy and collaborators [7]_ These authors used ethyl bromide as a bromination agent according to the following scheme: C=H,Br
+ RCl + CaHsCl
f RBr
(2)
The reaction was catalyzed by bromide ion and required the presence of a highly polar solvent, namely N-methyl-2-pyrolidone (NM!?), *noorder to maintain 2 homogeneous system. An excess (10 mol) of ethyl bromide was used to ensure complete conversion of &he a&y1 chloride. The major disadvantage in the proposed system is the bromide catalyst which requirthe presence of the polar solvent. This solvent, in addition to its high price and problems of recycling also tends to participate in side reactions with some substrates 171.
358
TABLE 1 Bromide-chloride Eaperiment
Starting
exchange
catalyzed
misturc
Products
CBHSCHzCl + nCJH7Br C6H&I12Cl + nC&Br CBH$H&I + nC&UBr nCsH&l + nC4HSBr nCsHwCl + n+H,Br .ClCH,CH,Cl + BrCH.&H,Br CI(CH2);Cl + Br(CH234Er C6H&H$l+ BrCH2CH,Br
9
C~HSCH#
+ Br(CH,),Br
by tri-n-butylaminem Time
C6HsCHzBr
+ nC3H$l
C&i&H@
+ n-C4H&i
CGH&HzBr + n-CGHl;a II-CgIilaBr + nCaH&l nC6HIaBr * nZaH$l ClCH&H2Br CI(CH2)4Br C6H&H,Br -C ClC!H+H2Br ClCH2CH2CI C6H&H,Br + Cl(CH2)+Br Cl(CH&CI
(h)
Yield (molS)
1.0 1.3 2.0 20.0 20.0 20.0 1.9
56.1 54 .o 50.0 49.2 22.4 38.6 47.0
7.5
59.gb
+ + 30
57-o=
=Reaction conditions: 0.1 mol chloride, O-1 mol bromide and O-002 mol tri-n-butylamine at 110 ‘C. b59_9 mol% of i,2 dibromoetbane were converted; 13.2 mol% to 1,2 dichloroethane and 46.7 molR tG l-bromo-2+zhIoruethane. cS?.O mol% of 1.4-dibromobuLane -+-ere converted, 22.6 mol% to 1;4xlichloroetbane and 34.4 mol% to l-bro&oA
N’e have found that simple amines (particularly tertiary amino) are very effective catalysts for the liquid phase homogeneous halogen exchange reaction. The high solubility of amines in various organic solvents allows the application of this type of catalyst in a Iarge variety of common solvents or even in the absence of any solvent except the substrates themselves. Results and discussion When stoichiometric quantities of a&y1 chloride and alhyl bromide were heated to 110 % in the presence of 0.02 mol-equivalent of tributylamine, equilibrium mixtures Fere obtained within 1 - 10 h depending on the nature of the halides. Examples of several rewtions are given in Table 1. It is quite clear from the ti!e that practically any akyl and benzyl halides and dihalides can be exchanged under these conditions. The usual equilibrium conversion is - 50%. Quantitative conversions are easily obtained by application of excess of the bromide and the chloride donor. When a volatile bromide is used, e.g. ethyl, n-propyl or n-but-j1 bromides; the product chloride is easily evaporated allowing simple separation of the pure product. Additional amines were test4 as cztalysts for the exchange of n-butyl bromide with benzyl chioride (eqn_ (3)). n-C4H9Br + CsHsCH&i
+ n-C4H&1 + C6HsCH2Er
(3)
-
359
Experiment
Time
C&dYst
trk.oc~lercine
di-n-butykmke di~-bctyIam&e dinac@zr&ne di-n+xtylamiue n-butykmine diethylamine tibet?nZykmine
10 11
'Reaction conditions:%1 amineat
molbenzyl
5B.l S-6 50.0 42.sb 27.1b 54Jab 20.4 51-O 19.8 42.Sb 12.7
1.0 1.0 1.0 3.3 10-o 3.0 IO.0 4.0 2.0 20.0
tiethylamine
i 8 9
Yield @zol%;!
1.0
ti-U-bPtphH!iUt tik-hexyhmine
1 2 3 4 5 6
(lx)
chloride,0.1molbutylbro~ide
and O-002 mol
110T.
bSolidpreeipitation~asobse~edduringthecourseofthereaction.
The results of this series of experiments are summarized in Table 2. The observed order of activity for n-bu*l amines is tertiary > secondary > primary. Tri-n-hexy;. and tri-n-o&$ amines were found to show similar activity to tri-n-butyl amine. Tribenzylamine had orAy minor activity and triethylamine promoted solid precipitation during the course of the catiysis. It is quite obvious that quaternary ammonium s&s play an important role in the mechanism of this exchange reaction. G&a’krnary salts .kve been separated from the reaction mixtures and in addition several quaternary salts were found to be catalykally ackive in the process. A possible mechanism for +&e exchange reaction R’Cl f R”Ek =+ R’Rr + R”C1 catalyzed by RaN would be via the following steps: (a) R’Cl f RzN = &R&l* (b) R”Br + RsN * R”kRIBr(c) R’&RsCl-
+ R”&RsBr-+
(d) R’kRaBr-
*
(e) R’rkR3C1-
= RRN -i- R”CI
R’$R$3r-
+ R”&&Cl-
RzN + R’Br
This mechanism was ruled out by the quaternary salts is not reversibie. 5fGca.l bromide and benzyki-n-butyl aklmonkm under the reaction conditions. Accordingly, the actual mechanism is (a) R’Cl f RsN -+ RZjRsCl(b) R”J3r f RsN --f It”&RsBr-
observation that the formation of salts, e.g. tek3bui;ylammofinium brcmide, were found to by stile apparently
as follows:
360
(c) RsR$l-
+ R”Br = &R3Br-
cd) R”&RJBr-
Experimental set
+ R’CI = R”&R&l-
+ R”Cl f R’Br
tion
Alkyl chlorides and bromides as xelI as the amines, were aII pu.rchas.zd from Aldrich Chemical Co, Milwaukw, and were distilled and degassd prior to use. Reactions were carried out in 100 ml flasks placed in a thermostatic silicon oil bath at 110 i 0.2 “c. The product mixtures were separated by GLC ar_alysis performed on a SS l/8 in X 6 ft column packed with 15% SE30 or 80 - 100 mesh chromosorb P. Prtiuct identification was made by ccmparison with authentic samples. -Procedure
for an exchange
reaction
In a typical experiment, 0.1 mol 1,2clibromoethane (18.8 g), 0.1 mol 1,2dichloroethane (9.8 g) and 2 X 10-j mol tri-n-butylamine were heated to 110 “C in a 100 ml flask for 2 h. After cooling, the mixture was analyzed and was found to contain 0.098 mol 1-bromo-2chloroethane, 0.051 mol 1,2CirYxoethane and 0451 mol of 1,2 dibromoethane. The mixture was ql:antitatively separated by fractional distillation_
References 1 FL T. Dillon, b. 2 H. A. C. McKay,
.Im. Chem Sot.. 54 (1932)
952. h’ature (London). I39 (1937) 283. 3 H. Pinkelstein, Eer., 43 (1910) 1528. 4 W. J_ Bailey and E. Fujiwara, J. Am. Chem. Sot., 77 (1955) 165. 5 E_ D. Hughes, C. K_ Ingold and J_ D. H_ Mackie, J. Chem. Sot.. (1955) 3173. 6 Israeli Patent 11,963 (1953) Lo A. Talmi. i A. E. Willy, D. R. McKean and 5. A. Barcia, Bull. Chen. Sot. Jpn_. 49 (1976)
L989.