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 $R_{2}Mg$.
• 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,

$Rx+Mg\stackrel{}{\to }RMgX$

In general, the $\mathrm{RX}$ associate over $\mathrm{Mg}$ suspended within the solvent. $E{t}_{2}O \& C_4{H}_{8}O (\mathrm{THF})$ are usually used as solvents, alternative polar non-protic solvents that are appropriate for the preparations are$(\mathrm{NE}{t}_3), (M{e}_{2}S), (M{e}_2\mathrm{Se}), \& (M{e}_2\mathrm{Te})$. In most cases, it's necessary to activate $\mathrm{Mg}$, by the addition of $I_2$, 1,2-dibromoethane, or sonication.

After the initial addition of $\mathrm{RX}$, 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 $\mathrm{RX}$. If the interaction is once initiated, the addition of $\mathrm{RX}$ is maintained at an acceptable rate to make sure the interaction is maintained till the entire $\mathrm{RX}$ is consumed. The left over $\mathrm{Mg}$ is removed by filtration. It is not needed to use a liquid or solid $X_2$dissolved within the solvent. Effervescent $\mathrm{RX}$ through associate $E{t}_{2}O$ suspension of $\mathrm{Mg}$ yields $\mathrm{MeMgCl}$. The advantage of a vaporific $\mathrm{RX}$ is that the interaction is extremely clean as all the $\mathrm{Mg}$ is consumed & also the excess $\mathrm{RX}$ has bubbled away.

 Alkyl fluorides are unreactive so extremely coordinating polar solvents like $C_4{H}_{8}O or C_3{H}_{7}NO$ (DMF) should be used for synthesis.

$RMgX+THF\to RH+H_{2}C=C{H}_2+H_{2}C=C\left(H\right)MgX$

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

$RMgX+RX\to R-R+Mg{X}_2$

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 2^nd involves aryl radical formation.

Structure

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 $C_4{H}_{8}O$ 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.

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.

It is ascertained that $RMgX$ are slightly conducting, & $Mg$ is deposited at each anode & cathode suggesting the formation of $RM{g}^{+} \& {\left[RMg{X}_2\right]}^{-}$.

Dialkyl magnesium compounds

Synthesis of dialkyl magnesium compounds

The most common preparation of $R_{2}Mg$ is by the interaction of an $RMgX$ with $C_4{H}_8{O}_2$. This technique is helpful for the production of cyclic compounds.

Another method of preparation that doesn't need $C_4{H}_8{O}_2$ involves the metal exchange interaction between $Mg$ metal & $R_{2}Hg$ compound. In certain cases, $Mg$ can react with acidic hydrocarbons like $C_5{H}_5$ at high temperatures.

$R_{2}Hg+Mg\to R_{2}Mg+Hg$

Structure

In the vapor section, $R_{2}Mg$ units are usually monomeric linear compounds. In the absence of coordinating solvents, $R_{2}Mg$ 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.

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.

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 $C_6{H}_{5}MgBr$?

a) 2-methyl oxirane

b) Olefin compound

c) Alkyl formate

d) $C{O}_2$

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 $C{H}_{3}MgBr$?

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 $C{H}_{3}MgBr$, 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.