Reacts 1 bromopropane with Grignard reagent

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Synthesis of Grignard compounds

Grignard compounds can be synthesized from the respective organic halogen compounds (=,,; =,) and elemental magnesium in anhydrous solvents, usually ethers such as diethyl ether or THF (Fig. 1). In this reaction, the reactivity of the halogen compounds decreases from iodine via bromine to chlorine. The magnesium is mostly used in the form of chips (larger surface for the reaction). Since magnesium is often passivated, the reaction may only start very slowly. In this case, for example, by adding a few grains of elemental iodine, by careful heating and / or using ultrasound, an attempt can be made to remove the passivation and to get the reaction to start. The reaction of the halogen compound with the magnesium is usually exothermic, so that it is often necessary to work with cooling.

The synthesis of Grignard compounds is illustrated by the following experimental video (Fig. 2) using the example of the conversion of 2-iodopropane with magnesium to 2-propyl magnesium iodide (Fig. 3):

Often the Grignard connections in situ generated and directly implemented. However, various common Grignard reagents are also commercially available as a solution in ether (e.g. or), which also underlines the great importance of the Grignard reaction in synthetic organic chemistry.

Grignard compounds in solution

In solution, Grignard compounds are in equilibrium with the respective diorganyl magnesium compounds and the magnesium dihalides (Fig. 4). This equilibrium is also known as the Schlenk equilibrium after its discoverer Wilhelm Schlenk. The equilibrium position depends on the solvent used.

The equilibrium position and its solvent dependency are based, among other things, on the property of the Grignard compounds in solution to bind two ether molecules in a coordinative manner (Fig. 5) or to form dimers (depending on the concentration) (Fig. 6). This can be explained by the electronic structure of the Grignard compounds: The magnesium in is surrounded by only four electrons. For example, by coordinating two ether molecules, an electron octet is achieved, which contributes to stabilization. In diethyl ether and THF, the Schlenk equilibrium is on the side of the species, which is why these two solvents are among the most suitable for the synthesis of Grignard compounds and for carrying out Grignard reactions. In contrast, in 1,4-dioxane as solvent, precipitation occurs and the equilibrium is shifted towards.

Properties of Grignard compounds

The bond in is a covalent bond, but it is strongly polarized. The properties of the organic radical are therefore very similar to those of carbanions, e.g. the tendency to nucleophilic attack or the easy protonation. The latter property is made use of when using Grignard reagents as bases in various reactions. But it also shows that one must pay attention to the absence of water and other protic solvents in the synthesis of the reagents and their use. This property can also be used to exchange halogens for hydrogen via the synthesis of the Grignard compound and subsequent hydrolysis, e.g. for the production of deuterated compounds using (Fig. 7).

Grignard reagents also enter into transmetalations, which is used, for example, in the Kumada cross-coupling.