Grignard reagent preparing, reactions, physical properties

Grignard is used to extend carbon chain. Griganard ( R-MgX , here R= alkyl group, X= Cl,Br,I) is prepared by reaction of alkyl halides (halo alkanes) and magnesium in dry ether medium. Grignard reagent reacts with many organic compounds and give different organic compounds with extended carbon chains.

grignard examples

Lithium also can be used to prepare Grignard reagent. But most of times magnesium is used.

This tutorial, we discuss following sections of grignard reagent.

  1. Preparing grignard reagent
  2. Physical properties of grignard reagent and nomenclature
  3. Reactions of grignard reagent and other organic compounds

Preparing Grignard reagent by alkyl halides

Magnesium and dry ether is mixed with alkyl halides to prepare Grignard reagent.
For keep Grignard stable, dry ether is essential. In aqueous medium, grignard reagent slowly hydrolyses and give hydrocarbons.

grignard preparing from alkyl halides

Nomenclature of Grignard

CH3MgBr - methyl magnesium bromide
CH3CH2MgCl - ethyl magnesium chloride

Physical properties of grignard reagent

  • Grignard reagent are strong alkalis. Therefore they react with compounds which can supply H+ ions.
  • Strong nucleophiles.
  • Grignard reagent should be kept in dry environment (in dry ether). If moisture or water is contact with grignard, grignard is destroyed.

Reactions of Grignard reactions

Grignard reagent reacts with different organic compounds and give different products such as alcohols, alkanes, alkynes with extended carbon chain. Usually grignard reagent is used to extend the carbon chain in organic chemistry.

Compounds reacting with grignard reagent

Following organic compounds react with grignard reagent and form compounds which have extended carbon chain. In some of these reactions one grignard molecules ( one alkyl group in grignard ) is attached, sometimes two grignard molecules are attached. Therefore you should learn mechanism of these reactions very well. Luckily, most of these reactions mechanisms are same.

Grignard reagent also reacts with alcohols, carboxylic acids, amines and more. But those reactions products are not much useful and important.

What are the products of grignard reagent reactions

Alkanes, alkynes, alcohols can be given by grignard reagent reactions. Each product formation and grignard regent reactions are discussed below.

Grignard reagent and alkyl halides reactions

Grignard react with alkyl halides and give hydrocarbons. Here, Grignard reagent acts as a nucleophile. This reaction extends the carbon chain.

grignard and alkyl halides.jpg

Grignard reagent and water reaction

Grignard reagent reacts with water and give hydrocarbons. Alkyl group of grignard reagent takes a hydrogen atom from water molecule to produce the hydrocarbon. Most times this hydrocarbon will be an alkane.
CH3CH2MgBr + H2O → CH3CH3 + MgBrOH
CH3MgCl + H2O → CH4 + MgClOH

Grignard reagent with aldehydes, ketones

Aldehydes react with grignard reagent and give secondary alcohols. Only formaldehyde (simplest aldehyde compound) gives a primary alcohol. Also ketones give titary alcohols when they react with grignard reagent.

Ethanal and grignard reaction

Ethanal and grignard reaction

Prapanone and grignard reaction

Prapanone and grignard reaction

Formaldehyde and grignard reaction

Simplest aldehyde, Formaldehyde ( methanal | HCHO ) reacts with ethylmagnesium bromide ( CH3CH2MgBr ) to form propanol. which is a primary alcohol. Water should be added as a reagent for the reaction to provide H+ ions.

First, alkyl group of grignard attacks the carbonyl carbon atom. Then a new bond between, carbonyl carbon and carbon atom of grignard (carbon atom which has high electrons density) is formed. With that, electrons of one bond of C=O bond, goes towards O atom. Finally, a hydrogen atom from a water molecule is taken by that O atom to make OH bond to give the alcohol.

Formaldehyde and ethyl magnesium bromide reaction mechanism

Formaldehyde and grignard reaction

Acid chloride and grignard reagent reaction

Acid chloride react with excess grignard reagent and give tertiary alcohols. Two alkyl groups are substituted to the carbon atom which is attached to the Cl and O atoms. Water should be added after grignard adding. In this reaction, grignard reagent is acted as an nucleophile.

Ethanoyl chloride and grignard reaction

acid halide and grignard reaction

Esters and grignard reagent reaction

Esters and excess grignard reagent reaction will give a tertiary alcohols. First, grignard reagent react with ester and give a ketone. Due to presence of excess Grignard reagent, again ketone reacts with grignard reagent. Finally we add a diute acid(H+) or H2O to get the tertiary alcohol.

ethyl ethanoate and grignard reaction

ethyl ethanoate and grignard reaction

Phenyl magnesium bromide

Bromobenzene does not react easily like aliphatic alkyl halides. Phenyl magnesium bromide is an important grignard compound which is used to synthesis so many benzene substitutes organic compounds. Phenyl magnesium bromide can be prepared by reaction of bromobenzene with Mg and dry ether. phenyl magnesium bromide preparing
Phenyl magnesium bromide react with water and give benzene. phenyl magnesium bromide with water give benzeneg
Also it reacts with ethyl bromide and give phenylethane.

Phenyl magnesium bromide and Ethanoyl chloride

When excess phenyl magnesium chloride is added to ethnoyl chloride and then H2O is added. We can get a tritary alcohol. phenyl magnesium bromide with acetyle chloride

Preparing carboxylic acids by carbon dioxide and grignard reagent reaction

When Grignard, CO2 and H2O react, carboxylic acids are given. The number of carbon atoms in the grignard reagent is increased by one when final product is prepared.

methyl magnesium chloride, CO2 and water reaction

ethanoic acid given.

methyl-magnesium-chloride, CO2 and water reaction

ethyl magnesium bromide, CO2 and water reaction

propanoic acid is given.

ethyl magnesium bromide, CO2 and water reaction

phenyl magnesium bromide, CO2 and water reaction

benzoic acid is given.

phenyl magnesium bromide, CO2 and water reaction

CO2, grignard reagent and H2O reaction mechanism

C atom in the CO2 is polarized as positive. Alkyl group in the grignard reagent has negative charge.

step 1: C atom in the CO2 is attacked by :CH3- in the grignard reagent.

step 2: A H atom in the water molecule is attacked by O-.


Grignard and alcohols reaction

Grignard reagent and alcohol react and give hydrocarbon as the product.

Learn more about grignard reagent and alcohol reaction, mechanism

Examples and Questions - Grignard reagent

Alkane synthesis from grignard reagent

1. Complete this chart indicating reagents and compounds for synthesis alkane(butane).

In this reaction, we start from ethene. To prepare grignard reagent, first we should synthesis alkyl halide. For that ethene react with HBr. It give bromoethane. Then, Mg and dry ether is added to bromoethane. Her we gets grignard reagent. Now we can synthesis butane(alkane) by adding CH3CH2Br into grignard reagent.

alkane synthesis from grignard

Summary of Grignard reactions

reagents react with grignard

Questions of Grignard reagent

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With what compounds grignard reagent can't be prepared?

We know Grignard reagent react with alcohols, aldehyde, ketone, esters, acid chloride, carboxylic acids, alkyl halides, alkynes with acidic H. We need a alkyl halide compound to prepare Grignard compound. If alkyl halide compound consists active group of above compound series, we are unable to synthesis Grignard reagent from thar alkyl halide compound.

Compounds which grignard reagent can't be prepared

Compounds which grignard reagent can't be prepared

Is it possible to prepare Grignard reagent from 3-Nitrochlorobenzene?

aromatic Grignard NO2 group exist

It is not possible to synthesis Grignard reagent when from 3-Nitrochlorobenzene. A reaction occurs through -NO2 group with Grignard reagent. Therefore we cant prepare Grignard reagent when from 3-Nitrochlorobenzene .