Grignard Reagent - Structure, Preparation, Application, Uses, FAQs

Grignard reagent is an Organometallic compound. Just as salt is inseparable from food, so is the Grignard reagent from Organic chemistry. If you want to transform a simple molecule into a complex one, then the Grignard reagent will help to carry out the reaction in a very simple way. 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 with Nobel Prize in 1912.

The formula of an organometallic compound known as a Grignard reagent is R-Mg-X.

• Where R in R-Mg-X represents an alkyl group
• X represents Halides.

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

Structure of Grignard reagent

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.

Preparation of Grignard reagent

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 correct procedures results in the formation of the Grignard reagent. The following figure shows the preparation of Grignard regent from an organic halide.

Applications of grignard reagents:

Reaction with carbonyl compounds-

Grignard reagents react with carbonyl compounds like ketone 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. Grignard reagent acts as a nucleophile and thereby nucleophilic substitution reactions are taking place. The below figure shows the reaction of the Grignard reagent to form Benzyl alcohol.

Reaction of Grignard reagent with water

The reaction of a carbonyl compound with the Grignard reagent.

Reactions acting as a base

Grignard reagents are basic compounds and they react with phenol alcohol acceptor to give their corresponding alkoxides that is ROMgBr.

Grignard agents can be used for the alkylation of metals

Grignard reagents react with the metal to form their related compound. For example, when the Grignard reagent reacts with cadmium chloride it forms a di alkyl cadmium by the transmethylation reaction. The following reaction explains this.

2RMgX+CdCl₂R₂Cd -- 2MgXCl

Schlenk equilibrium

Grignard reagent reaction with dioxane to give di organomagnesium compounds and the reaction involving is known as Schlenk equilibrium. And the reaction is conducted in a solvent that is diethyl ether and tetrahydrofuran.

2RMgX+dioxane→R₂Mg+MgX₂

Oxidation

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.

Also Read:

• NCERT Solutions for Class 12 Chemistry Chapter 10 Haloalkanes and Haloarenes
• NCERT Exemplar Class 12 Chemistry Solutions Chapter 10 Haloalkanes and Haloarenes
• NCERT notes Class 12 Chemistry Chapter 10 Haloalkanes and Haloarenes

Uses of Grignard reagent

• Grignard reagents are a very important organic compound that has many applications in the chemical field mainly the organic chemistry field.
• Grignard reagents have industrial applications also. The very important use of the Grignard reagent is the production of tamoxifen a medicine used for the treatment of breast cancer. The below reaction shows the preparation of tamoxifen.

Preparation of tamoxifen

• Grignard reagents can be used to produce alcohol from epoxides.
• Grignard reagent reaction with aldehyde ketone and esters to form alcoholic compounds.
• It can be used for the degradation reaction of Grignard reagent used for the chemical analysis of triacylglycerol and also for some cross-coupling reactions that are involved in the formation of carbon-carbon and carbon atom bonds.
• It can be used for the synthesis of many organometallic compounds.
• Grignard reagents are best for the preparation of amides, acetals, amino compounds, organosulphur compounds, ethers, ketones, aldehydes, etc.
• It can be used for the production of several compounds that have a very important application in the pharmaceutical, perfume, and specialty chemicals fields.
• Grignard reagents are used for the production of optically active compounds by the reaction of secondary alcohol with the alkyl halides in the presence of a chiral phosphine metal as a catalyst.

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Examples

Q.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 the option (2).

Q.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.

Q.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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