What are organomagnesium compounds?

Compounds with a C-Mg bond are termed organomagnesium compounds. These compounds are of 2 types they are,

  • Complete compounds such as R2Mg.
  • Mixed compounds, such as RMgX.

Organomagnesium compounds

General methods of organomagnesium reagent synthesis

The interaction of alkyl halide with Mg metal in a suitable ether solvent is the common method for the synthesis of the Grignard reagent.

The chemical equation for synthesis is,


In general, the RX associate over Mg suspended within the solvent. Et2O & C4H8O (THF) are usually used as solvents, alternative polar non-protic solvents that are appropriate for the preparations are(NEt3), (Me2S), (Me2Se), & (Me2Te). In most cases, it's necessary to activate Mg, by the addition of I2, 1,2-dibromoethane, or sonication.

After the initial addition of RX, there is normally an induction period for an interaction to occur. But, the interaction is very exoergic & care ought to be taken to make sure that the interaction doesn't become out of control. Because of this reason, it is better to start an interaction by adding a little amount of the RX. If the interaction is once initiated, the addition of RX is maintained at an acceptable rate to make sure the interaction is maintained till the entire RX is consumed. The left over Mg is removed by filtration. It is not needed to use a liquid or solid X2dissolved within the solvent. Effervescent RX through associate Et2O suspension of Mg yields MeMgCl. The advantage of a vaporific RX is that the interaction is extremely clean as all the Mg is consumed & also the excess RX has bubbled away.

 Alkyl fluorides are unreactive so extremely coordinating polar solvents like C4H8O or C3H7NO (DMF) should be used for synthesis.


On the other hand, radical coupling interaction between the RMgX & unreacted RX is catalyzed by transition metal no matter the character of the solvent.


The mechanism for RMgX formation is assumed to be radical. Typically initiation happens at surface dislocations; however, the main interaction happens at a refined surface.

The interaction is found to be first order with relevance to the RX concentration; however, it's additionally been claimed to be first order with relevance to the solvent concentration. It’s thus been finished that the rate-determining step involves the metal-solvent interface.

The interaction of Mg with RX has been studied & is projected to occur by 2 interactions. The primary involves electron transfer between the RX & also the metal, whereas the 2nd involves aryl radical formation.

Schematic representation of the reaction of magnesium with alkyl halides


The structure of the RMgX reagent is controlled by the characteristics, size, and basicity of the solvent utilized in the preparation. The structure of EtMgBr crystallized from diethyl ether is a four-coordinate compound. It is represented in figure a. If sterically less hard C4H8O is used in the preparation, the final product has a five-coordinate monomeric structure and it is represented in figure b. If triethylamine is employed for synthesis, it yields a dimeric bromide bridged structure and is represented in figure c. The use of a chelate rough alkane provides a structure as in figure d which is similar to the structure of EtMgBr crystallized from diethyl ether.

Molecular structure of EtMgBr in (a) diethyl ether, (b) THF, (c) triethylamine, and (d) tetramethylethylenediamine (TMED)

In solution, organomagnesium compounds are fluxional such that no single outlined structure is present. The series of replacing interactions are referred to as associate degree extended Schlenk equilibrium.

Schematic representation of the extended Schlenk equilibrium observed for Grignard compounds in solution.

It is ascertained that RMgX are slightly conducting, & Mg is deposited at each anode & cathode suggesting the formation of RMg+ & [RMgX2]-.

Dialkyl magnesium compounds

Synthesis of dialkyl magnesium compounds

The most common preparation of R2Mg is by the interaction of an RMgX with C4H8O2. This technique is helpful for the production of cyclic compounds.

The most common synthesis of R2Mg is by the reaction of a Grignard with dioxane

Another method of preparation that doesn't need C4H8O2 involves the metal exchange interaction between Mg metal & R2Hg compound. In certain cases, Mg can react with acidic hydrocarbons like C5H5 at high temperatures.



In the vapor section, R2Mg units are usually monomeric linear compounds. In the absence of coordinating solvents, R2Mg forms a range of oligomers, which are represented in figure 1-3, are determined by mass measurements. Within the presence of coordinating solvents four-coordinate monomers predominate which is represented in the figure.



solution structure of R2Mg (R = Me, Et) in (1-3) non-coordinating solvents, and (4) diethyl ether

In solid-state a similar trend is ascertained, in which polymers are characterized within the absence of coordinating solvents which are represented in figure 1, whereas monomers or dimmer units are usually ascertained once crystallized from a coordinating solvent is represented in figure 2 & 3.

Solid state structure of R2Mg (R = Me, Et) crystallized in (1) the absence and (2 and 3) the presence of a coordinating solvents.

Context and Applications

This topic is significant for both undergraduate and postgraduate courses, especially for Bachelors and Masters in Chemistry.

Practice Problems

Question 1: Choose the term which describes the reactive nature of the organomagnesium reagent.

a) Carbocation

b) Free radical

c) Electrophile

d) Nucleophile

Answer: The correct answer is d.

Explanation: The bond between the carbon and metallic element is polarized toward carbon. This means that carbon is more electron-rich than metallic elements & is truly nucleophilic.

Question 2: Which of the subsequent statements concerning the RMgX chemical agent is false?

a) RMgX raises the radical of aldehydes & ketones.

b) Associate degree organosodium compound isn't reactive compared to an RMgX chemical agent.

c) RMgX is ready in ether or tetrahydrofuran.

d) RMgX is rotten by water & alcohol.

Answer: The correct answer is b.

Explanation: It is supposed that the Mg-C bond is powerfully polar valence, not ionic. RMgX measures less reactive than organosodium, K & Li compounds, that's the rationale why it's a lot of convenient to figure with them.

Question 3: Which of the subsequent compounds provides primary alcohol upon interaction with C6H5MgBr?

a) 2-methyl oxirane

b) Olefin compound

c) Alkyl formate

d) CO2

Answer: The correct answer is b.

Explanation: Olefin compound reacts with RMgX reagents to give primary alcohols with 2 more carbon atoms.

Question 4: Which of the subsequent compounds wouldn't form tert-butyl alcohol once treated with excess CH3MgBr?

a) Acyl halide

b) Aldehyde

c) Alkyl radical acetate

d) Anhydride

Answer: The correct answer is b.

Explanation: Aldehyde wouldn't form tert-butyl alcohol once treated with excess CH3MgBr, because in the reaction of  Grignard reagent the addition of organomagnesium salt to RX takes place, to form tertiary or secondary alcohol respectively.

Question 5: Which of the subsequent reagents, once treated with phenyl magnesium bromide followed by acid workup, can yield 2-phenyl ethanol?

a) Ethanol

b) Ether

c) Ethanal

d) Oxirane

Answer: The correct answer is d.

Explanation: 2-phenyl ethanol is obtained as the product when oxirane is reacted with diethyl ether and phenyl magnesium bromide in the presence of acid.

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