Have you ever wondered how a whole molecule is formed just by starting with something as simple as an alkyl halide? The answer to this question will be known after reading this article on the Grignard reagent. Grignard reagent is an Organometallic compound written as RMgX, where R is an alkyl or aryl group and X is a halogen. Just as salt is inseparable from food, so is the Grignard reagent from Organic chemistry.
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A French Chemist named Francois Grignard discovered this reagent in 1900. He was investigating the organic reactions of halides with metals, and while reacting different metals with organic halides, he noticed that the reaction of magnesium with organic halides in the presence of dry ether forms a very reactive organo-magnesium compound. Due to this discovery, he was awarded the Nobel Prize in 1912. In this article, we will cover the topic of the Grignard Reagent. This topic falls under the broader category of Haloalkanes And Haloarenes, which is a crucial chapter in (Class 12 Chemistry). It is not only essential for board exams but also for competitive exams like the JEE Mains Exam , National Eligibility Entrance Test (NEET), and other entrance exams such as SRMJEE, BITSAT, WBJEE, BCECE, and more
The structure of Grignard’s reagent involves a polar carbon-magnesium bond, with magnesium having a partially positive charge and carbon having a partially negative charge. The polarisation enables Grignard’s reagent to act as a strong nucleophile that can attack an electrophilic centre. Grignard’s reagent exists as a monomer and dimer and is stabilized by an ether molecule coordinated with the magnesium atom.
Grignard reagents are prepared from magnesium metal by treating it with an organic halide. For stabilizing these organomagnesium compounds ethers are required. These compounds require air-free conditions, and the use of protic solvents may not be used as it will create protonolysis or oxidation may destroy these compounds.
However, this compound can be formed in solution by the use of ultrasound since it activates the magnesium and, thereby, consumes water present in the solution. An anhydrous condition is more suitable for the preparation of the Grignard reagent. Following the correct procedures results in the formation of the Grignard reagent. The following figure shows the preparation of the Grignard reagent from an organic halide.
Grignard reagent, on reaction with any compound containing an active H atom, produces an alkane.
$\mathrm{MgRX}+\mathrm{HZ} \rightarrow \mathrm{MgXZ}+\mathrm{RH}$
Unsaturated halide with Grignard reagent gives alkene
$\mathrm{CH}_3 \mathrm{MgI}+\mathrm{CH}_2=\mathrm{CHI} \longrightarrow \mathrm{CH}_2=\mathrm{CHCH}_3+\mathrm{MgI}_2$
Lower alkyne $+G R \rightarrow$ Product $\xrightarrow[\text { haldide }]{\text { alks }}$ Higher alkyne
$\mathrm{R}-\mathrm{C} \equiv \mathrm{C}-\mathrm{H}+\mathrm{CH}_3 \mathrm{MgBr} \xrightarrow[-\mathrm{CH}_4]{ } \mathrm{R}-\mathrm{C} \equiv \mathrm{C}-\mathrm{MgBr}$
(a) Primary alcohol: Obtained from :
(i) Dry oxygen
(ii) Epoxy ethane
(iii) Formaldehyde
(b)Secondary Alcohol: obtained from:
(i) all aldehydes except formaldehyde
(ii) Ethyl formate + 2 moles of RMgX
(c) Tertiary Alcohol: Obtained from:
(i) All esters except ethyl formate
(ii) All ketones
(iii) (RCOCl) + 2 moles of RMgX
(i) From HCN
(ii) From ethyl formate + 1 mole of RMgX
(i) Alkyl Cyanide
$\mathrm{CH}_3 \mathrm{CN}+\mathrm{C}_2 \mathrm{H}_5 \mathrm{MgBr} \rightarrow \mathrm{CH}_3 \mathrm{COC}_2 \mathrm{H}_5+\mathrm{NH}_3+\mathrm{MgBrOH}$
(ii) Acetyl Chloride
$\mathrm{CH}_3 \mathrm{COCl}+\mathrm{CH}_3 \mathrm{MgX} \rightarrow \mathrm{CH}_3 \mathrm{COCH}_3+\mathrm{MgXCl}$
Grignard reagents react with carbonyl compounds like ketones and aldehydes to form corresponding alcohols. The nature of the substituent that gets attached to the carbonyl compound determines the product. When methanal is used as an aldehyde, the obtained alcohol will be primary, and if any aldehyde other than this is used, a secondary aldehyde is obtained. It can also be used for the alkylation of aldehydes and ketones. The Grignard reagent acts as a nucleophile, and thereby nucleophilic substitution reactions take place. The figure below shows the reaction of the Grignard reagent to form Benzyl alcohol.
The reaction of a carbonyl compound with the Grignard reagent.
Grignard reagents are basic compounds and they react with phenol alcohol acceptor to give their corresponding alkoxides that is ROMgBr.
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Grignard reagents react with the metal to form their related compound. For example, when the Grignard reagent reacts with cadmium chloride, it forms a dialkyl cadmium by the transmethylation reaction. The following reaction explains this.
$2 \mathrm{RMgX}+\mathrm{CdCl}_2 \mathrm{R}_2 \mathrm{Cd}\rightarrow 2 \mathrm{MgXCl}$
Grignard reagent reaction with dioxane to give diorganomagnesium compounds, and the reaction involving is known as the Schlenk equilibrium. And the reaction is conducted in a solvent that is diethyl ether and tetrahydrofuran.
$2 \mathrm{RMgX}+$ dioxane $\rightarrow \mathrm{R}_2 \mathrm{Mg}+\mathrm{MgX} \mathrm{X}_2$
Grignard reagent reacts with oxygen and forms magnesium organic peroxide. The further hydrolysis of the compound obtained hydroperoxides or alcohol. The following reaction shows the formation of this, and it proceeds in radical intermediates.
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Question.1 When phenyl magnesium bromide reacts with tert. Butanol, which of the following is formed?
1)Tert. butyl methyl ether
2) (correct)Benzene
3)Tert. butyl benzene
4)Phenol
Solution
As we learned,
Zerewitinoff Method -
The reaction of the alcohol with the Grignard reagent.
- wherein
$R^{\prime} OH+RMgX \rightarrow RH+R^{\prime} OMgX$
C6H5MgBr + (CH3)3C-OH $\rightarrow$ C6H6 + [(CH3)3Co]MgBr
Hence, the answer is option (2).
Question 2 Reaction of ROH with R'MgX produces:
1)RH
2) (correct)R'H
3)R-R
4)R-R'
Solution
Alkyl magnesium halides(RMgX) are called Grignard reagents. These undergo double decomposition reactions with water, ammonia, alcohol or amine having active H atom(attached to strongly electronegative O, N, S, or F and triple bond, etc.) to give alkane corresponding to an alkyl group of Grignard reagent. The reaction occurs as follows:
So,
R'MgX+ROH$\rightarrow$R'H+Mg(OR)X
Therefore, option (2) is correct.
Question.3 Which of the following compounds will form a hydrocarbon on reaction with a Grignard reagent?
1) (correct)CH3CH2OH
2)CH3CHO
3)CH3COCH3
4)CH3CO2CH3
Solution
Reaction of Grignard reagent with H2O -
Alkane / Benzene is obtained
- wherein
Reaction of Grignard reagent with Alcohol -
Alkane is obtained.
- wherein
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Frequently Asked Questions (FAQs)
Grignard reagents can react with boronic esters in a transmetalation process. This reaction exchanges the organic group from magnesium to boron, forming a new organoboron compound. This is useful in organic synthesis, particularly in preparing reagents for Suzuki coupling reactions.
Grignard reagents can be used to prepare organometallic compounds of other metals through transmetalation reactions. For example, they can react with salts of copper, zinc, or mercury to form the corresponding organometallic compounds. This is useful for preparing reagents with different reactivities and selectivities.
Grignard reagents can be used to introduce deuterium by using D2O in the quenching step instead of H2O. The magnesium-carbon bond is cleaved by D2O, introducing a deuterium atom. This method is useful for isotopic labeling in mechanistic studies or for preparing deuterated compounds for spectroscopic analysis.
The Grignard version of the Reformatsky reaction involves the reaction of a Grignard reagent with α-halo esters or ketones. This forms β-hydroxy esters or ketones, similar to the traditional Reformatsky reaction but often with better yields and under milder conditions.
In carbometalation, Grignard reagents can add across carbon-carbon multiple bonds (like alkynes or alkenes) in the presence of suitable catalysts. This results in the formation of a new carbon-carbon bond and a carbon-magnesium bond, which can be further functionalized. It's useful for synthesizing complex organic molecules.
The Grignard reduction refers to the reduction of ketones or aldehydes to alcohols using Grignard reagents derived from alkyl halides with β-hydrogens. The β-hydrogen is transferred to the carbonyl, resulting in reduction instead of addition. This is useful when a simple reduction is needed without introducing new carbon atoms.
Grignard reagents can react with various sulfur compounds. For example, they can add to sulfoxides to form sulfides, or react with disulfides to form thioethers. These reactions are useful in the synthesis of organosulfur compounds, which have applications in organic and medicinal chemistry.
While Grignard reagents aren't directly used in the Wurtz-Fittig reaction, they share similarities in mechanism. Both involve organomagnesium intermediates. Understanding Grignard chemistry helps in comprehending the Wurtz-Fittig reaction, which couples aryl halides with alkyl halides using sodium.
Grignard reagents can react with imines or their derivatives (like iminium ions) to form tertiary amines after workup. This reaction is analogous to their addition to carbonyls but results in C-N bond formation instead of C-O. It's a useful method for introducing alkyl or aryl groups onto nitrogen.
The Fourneau-Tiffeneau rearrangement is a reaction where α-haloketones are treated with Grignard reagents to form aldehydes or ketones with one fewer carbon. The Grignard reagent initiates a rearrangement, resulting in the loss of a carbon atom from the original ketone.