Would you like to change to the Indonesia site? Richey Editor. Undetected location. NO YES. Grignard Reagents: New Developments. Selected type: Hardcover. Added to Your Shopping Cart. Out of stock. This is a dummy description. The Grignard reagent is one of the most useful and versatile reagents known to the organic chemist and years after its discovery, this book presents authoritative perspectives on the significant developments taking place using Grignard Reagents and related organomagnesium compounds.
About the Author H. Synthesis , The addition of the phenylbromide is thereafter continued. On several occasions problems occurred in the pilot plant with the initiation. Although more activator was added, the reaction did not initiate even with stirring at this high temperature for several hours.
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Over the last 3 to 4 years the regulations established by the authorities have strengthened the protection of employees handling carcinogenic or suspected carcinogens such as 1,2dibromoethane. To overcome the requirement for this carcinogenic initiator and the hazardous initiation conditions involved in the activation with 1,2-dibromoethane it was decided to look for a new method which could be performed in a multipurpose reactor. The ideal method for the chemical initiation should be reliable, the initiation should not be performed with carcinogenic reagents, the initiation should preferably be performed below the boiling point of the etheral solvent, and it should be robust against a varying moisture content of the solvent.
Reported Methods for Magnesium Activation. The first step was to screen the literature for various methods for the initiation of the formation of the Grignard reagent especially those that have been reported for use in large scale.
Different Forms of Magnesium. Magnesium can take a variety of different physical forms: turnings, powder chips, and finely divided metal. Each form offers advantages and disadvantages. Turnings are easy to handle but can cause abrasiveness to glass-lined reaction vessels if stirred over a longer time. Powdered magnesium is more reactive but is more quickly oxidized on the surface and can be pyrophoric. Magnesium chips have a high purity, but the surface area is smaller than that for turnings, which results in less reactivity. In Rieke and associates8 reported a general approach for preparing highly reactive metal powders by reducing metal salts with alkali metals.
The magnesium salt has to be anhydrous. Potassium metal is difficult to handle on large scale, and the prepared Rieke magnesium is also very pyrophoric. Removal of Oxide Layers.
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Since Grignard reactions with organic bromides and iodides are diffusion-controlled after their initiation, the reaction rate is directly proportional to 8 a Rieke, R. Although the removal of oxide layers by washing with a mineral acid will increase the active surface area, the removal of the acid and water is extremely difficult on industrial scale.
Dry Stirring of the Magnesium Turnings. Some reports have appeared in the literature on activation by stirring the magnesium turnings in an inert atmosphere. During stirring the oxide layer on the magnesium turnings is reduced, leaving the activated metal surface. For instance, the method was used for the synthesis of the Grignard reagent from p dimethylamino phenyl bromide 1. This method was never used at Schering for large-scale activation due to possible abrasiveness to the glass-lined reactor and because no reactor is fitted with a low stirrer.
On a laboratory scale, equipment drying does not present a significant challenge. Equipment drying is the largest impediment to the initiation of the reaction on large scale, given the large equipment and the complicated piping. In addition, it must be kept in mind that the surfaces of equipment, even if they appear to be completely dry, are often covered with a thin film of water. To ascertain a safe initiation, the equipment is usually dried through multiple solvent rinsings to a water content below 0. Prior to performing these drying operations, it is necessary that the ingoing solvent meet the final water specification.
DIBAH does not have the limitation from Vitride where the 2 mol equiv of released methoxyethanol react with the formed Grignard reagent. In our study we investigated the behavior of three different aryl bromides 4-bromobenzaldehyde dimethylacetal 2 , 4bromoacetophenone ethylene ketal 3 , and 4- dimethylamino bromobenzene 1 from different projects Scheme 2. For the study the water content of THF was measured and found to be 0. This water content is easily obtained through distillation of THF.
This was then actually the case even with 4-bromoN,N-dimethylaniline, previously reported as being notoriously difficult to initiate. During the investigation no external cooling was used.phon-er.com/js/iphone/iphone-measure-distance-on-map.php
Grignard reagents new developments download
The results of this investigation are summarized in Table 1. As these experiments were done using normal laboratory equipment, the obtained data are only indicative. For safety evaluation each formation of a new Grignard reagent should be investigated in a reaction calorimeter also when using this activation method, as the reaction calorimetry is independent of the activation method, and also when the formation is performed well below the boiling point of the solvent. With 5 vols of THF with regard to magnesium the initiation was in all cases more or less spontaneous.
In all studied cases the reaction started within 30 min except in the case with p- dimethylamino phenyl bromide 1 with 30 vols of THF. Furthermore the temperature rise is dependent on the amount of solvent. In every case also with 5 vols of THF with regard to magnesium the internal temperature in the reaction flask did not reach the boiling point of THF. The adiabatic temperature rise should be studied in a reaction calorimeter for each new Grignard reagent formation with different amounts of organic halide and solvent to ascertain a safe scale-up. In every case also with 30 vols of THF it was possible to easily detect the increase in temperature.
For 30 vols of THF the increase was 6 and 11 K. From each aryl bromide the yield of the corresponding Grignard reagent was determined through hydrolysis and GC. To ascertain that no accumulation of the aryl bromide occurs during the reaction, the addition of the remaining aryl bromide was started after the temperature from the initiation had reached its maximum. For the reliability of the method one experiment with 4-bromobenzaldehyde dimethyl acetal 2 in 20 vols of THF was performed five times see Table 1.
The effect of stirring the reaction mixture was also determined during the investigation. If the mixture is not stirred at all, the initiation takes much longer up to more than 1 h , which indicates the oxide layer on the metal surface is not reduced with DIBAH.
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The same experiment was also done with Vitride with the same result. If stirring is turned on and the turnings are slightly moved, the reaction starts immediately.
ISBN 13: 9780471999089
This makes the stirring of the mixture on large scale important, showing that a low stirrer is crucial for the initiation. The quality of the magnesium turnings was also studied. The time in which the surface remains active after the initiation was determined through storing the reaction mixture Table 1. Addition of 0. Initiation of the reaction was performed through addition of 0.
Scheme 3 Scheme 4 Scheme 5 overnight 16 h prior to the addition of the rest of the bromide solution.