Org. Synth. 1999, 76, 221
DOI: 10.15227/orgsyn.076.0221
6-CHLORO-1-HEXENE AND 8-CHLORO-1-OCTENE
[
1-Hexene, 6-chloro- and 1-Octene, 8-chloro-
]
Submitted by Pierre Mazerolles, Paul Boussaguet, and Vincent Huc
1
.
Checked by Frédéric Denonne and Léon Ghosez.
1. Procedure
A.
Allylmagnesium bromide
(ethereal complex solution). A dry, 3-L, three-necked, round-bottomed flask is equipped with a sealed mechanical stirrer
(Note 1), 500-mL, pressure-equalizing, dropping funnel and a reflux condenser, the top of which is connected to a calcium chloride drying tube. The flask is charged with a large excess of
magnesium turnings (90.00 g, 3.75 g-atom)
(Note 2) and
150 mL of dry diethyl ether
. To the stirred mixture is added dropwise a solution of
allyl bromide
(Note 3) (181.60 g, 1.50 mol) in
1.5 L of dry diethyl ether
. At the end of the addition (8 hr) the mixture is stirred for 1 hr, whereupon the Grignard reagent solution is transferred under nitrogen, with a cannula, into a 2-L, two-necked, round-bottomed flask equipped with a 500-mL dropping funnel and a reflux condenser fitted with a swan neck (distillation) adapter connected with a descending condenser
(Note 4).
B.
Allylmagnesium bromide
(THF complex solution). After removal of the water in the vertical reflux condenser, the solution of the Grignard reagent is heated with a water-bath (45°C, then progressively up to 65°C) with stirring to remove the uncomplexed diethyl ether (2 hr). Dry
tetrahydrofuran (THF, 500 mL) is added rapidly (10 min) to the gray pasty residue. A vigorous reaction occurs, and the decomplexed diethyl ether is easily removed by distillation. The reaction is completed by heating the mixture with stirring for 1 hr to give a fluid gray THF solution of allylmagnesium bromide
(Note 5).
C.
6-Chloro-1-hexene
. The swan neck and the descending condenser are removed, the vertical reflux condenser is fitted with a drying tube, and water is again circulated through the condenser. To the stirred mixture heated at 50-60°C, pure
1-bromo-3-chloropropane
(Note 6) (150.00 g, 0.95 mol) is added at a rate sufficient to maintain a good reflux in the condenser. At the end of the addition (0.5 hr), the mixture is boiled for 1 hr and then cooled. Excess Grignard reagent is destroyed by slow addition of water, while cooling the mixture, to produce two clear layers. The aqueous phase is extracted with
pentane (3 × 300 mL each), and the organic phase is dried over
calcium chloride
. Removal of the solvent (water bath at 45°C, then progressively to 95°C, Vigreux column 2'') gives 183.40 g of oily residue. The remainder of the solvent is removed under vacuum (up to 45°C/147 mm). Distillation of the residue in a Claisen flask gives the following fractions: Head fraction, from 50°C to 74°C/130 mm (21.16 g) containing 60% of chlorohexene
(Note 7); central fraction, 74-75°C/130 mm (93.00 g, 0.78 mol, 82% yield) consisting of almost pure chlorohexene
(Note 8); residue, clear yellow, about 1 mL.
8-Chloro-1-octene
. Similarly, the reaction of
1-bromo-5-chloropentane
(Note 9) (88.37 g, 0.47 mol) with a solution of allylmagnesium bromide prepared as above from
allyl bromide (90.75 g, 0.75 mol) and
magnesium turnings (45.00 g, 1.87 mol), gives, after the usual treatment, 60.14 g of
8-chloro-1-octene
(86% yield), bp 75-76°C/20 mm
(Note 10).
2. Notes
1.
Stirring with a
large magnetic bar and an
efficient magnetic stirrer (IKA mod. MAG. RET) is possible, but the magnetic bar turns with some difficulty at the beginning of the reaction.
2.
A large excess of
magnesium turnings is necessary to prevent the formation of
biallyl (1,5-hexadiene) by a Würtz-coupling reaction. Small
magnesium turnings (3-4-mm size, 0.5-mm thickness) were obtained with a lathe from a pure magnesium bar (A. Weber Métaux). The checkers used
Aldrich
magnesium turnings (98%).
3.
The reaction starts easily; the addition of a crystal of
iodine is not necessary. To minimize the formation of
hexadiene, the addition of
allyl bromide must be slow enough to maintain the temperature of the flask below the boiling point of ether.
Allyl bromide (Fluka "purum") even when stored in the dark in the presence of
silver wool contains some high-boiling material and must be purified by distillation. Pure product (bp
71°C) is used just after distillation.
4.
Unreacted
magnesium, washed with water, alcohol and ether and dried in an
oven, weighs
56.16 g (
94% of the theoretical yield).
5.
Such a large amount of THF is not necessary to remove the complexed
ether, but if a smaller quantity is used, the mixture solidifies on cooling.
6.
1-Bromo-3-chloropropane (Fluka "purum") was dried and distilled before use (bp
56°C/ 34 mm).
7.
The amount was estimated by VPC. As the principal impurity is THF, the yield of
chlorohexene may be increased to 90% by careful distillation.
8.
The product obtained contains traces of THF and a small amount (3-5%) of
6-bromo-1-hexene (identified by mass spectroscopy). Usually this by-product is not an impurity, especially when
6-chloro-1-hexene is used to prepare the corresponding Grignard reagent. The spectral data are as follows:
1H NMR (80 MHz, CDCl
3) δ: 1.79 (m, 6 H), 3.51 (t, 2 H, J = 8.0), 4.98 (m, 2 H), 5.71 (m, 1 H)
;
13C NMR (20 MHz, CDCl
3) δ: 26.1, 32.0, 32.9, 44.8, 114.8, 136.1
.
9.
1-Bromo-5-chloropentane is easily prepared from
tetrahydropyran according to the process described by Newman and Wotiz,
2 bp
89°C/17 mm; n
20
D
1.4842. The checkers purchased
1-bromo-5-chloropentane from Aldrich Chemical Company, Inc.
10.
Like
6-chloro-1-hexene,
8-chloro-1-octene contains a small amount (3-5%) of the corresponding bromide (
8-bromo-1-octene), which is not a problem in some reactions.
Handling and Disposal of Hazardous Chemicals
The procedures in this article are intended for use only by persons with prior training in experimental organic chemistry. All hazardous materials should be handled using the standard procedures for work with chemicals described in references such as "Prudent Practices in the Laboratory" (The National Academies Press, Washington, D.C., 2011 www.nap.edu). All chemical waste should be disposed of in accordance with local regulations. For general guidelines for the management of chemical waste, see Chapter 8 of Prudent Practices.
These procedures must be conducted at one's own risk. Organic Syntheses, Inc., its Editors, and its Board of Directors do not warrant or guarantee the safety of individuals using these procedures and hereby disclaim any liability for any injuries or damages claimed to have resulted from or related in any way to the procedures herein.
3. Discussion
Haloalkenes, CH
2=CH-(CH
2)
nX (n = 3-7; × X = Cl, Br), are interesting synthons for use: in the preparation of long chain alkenols and alkenoic acids;
3 in cyclization reactions,
4,5,6,7,8,9,10 as intermediates in the synthesis of pheromones;
11,12 in the preparation of silica gel having a functional surface;
13 and for the synthesis of ω-iodochloroalkanes,
14 and organogermanium dendrimers.
15 They are usually prepared from the corresponding alkenols.
An interesting synthesis of this type of compound is the reaction of
allyl bromide with a mono Grignard reagent of a dihalide, especially a chlorobromide:
16 However this synthesis is only suitable for long chains, n ≥ 4: n = 4, 45% yield; n = 6, 60% yield.
On the other hand, bromoalkanes react with saturated
17 or ethylenic
18 Grignard reagents in
tetrahydrofuran in the presence of a suitable catalyst (dilithium tetrachlorocuprate
19,20):
It was shown that alkyl bromides react without a catalyst with allylmagnesium bromide in THF. However, because allyl bromide reacts quantitatively with magnesium in THF to give 1,5-hexadiene by a Würtz-coupling reaction, allylmagnesium bromide can be obtained in this solvent only by solvent exchange:
The reaction with the readily available
1-bromo-3-chloropropane is a direct, inexpensive method for preparing of
6-chlorohexene in high yields (
80-90%); this reaction may be readily extended to
1-bromo-5-chloropentane
2 and to other accessible ω-bromochloroalkanes.
21
With dibromoalkanes the corresponding dienes are formed in good yield:
With aralkyl dibromides, only alkyl bromides react:
Appendix
Chemical Abstracts Nomenclature (Collective Index Number);
(Registry Number)
6-Chloro-1-hexene:
1-Hexene, 6-chloro- (8,9); (928-89-2)
8-Chloro-1-octene:
1-Octene, 8-chloro- (8,9); (871-90-9)
Allylmagnesium bromide:
Magnesium, bromo-2-propenyl- (8,9); (1730-25-2)
Magnesium (8,9); (7439-95-4)
Allyl bromide:
1-Propene, 3-bromo- (8,9); (106-95-6)
1-Bromo-3-chloropropane:
Propane, 1-bromo-3-chloro- (8,9); (109-70-6)
1-Bromo-5-chloropentane:
Pentane, 1-bromo-5-chloro- (9); (54512-75-3)
6-Bromo-1-hexene:
1-Hexene, 6-bromo- (8,9); (2695-47-8)
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