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.
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.
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.
CH3MgBr - methyl magnesium bromide
CH3CH2MgCl - ethyl magnesium chloride
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.
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.
Alkanes, alkynes, alcohols can be given by grignard reagent reactions. Each product formation and grignard regent reactions are discussed below.
Grignard react with alkyl halides and give hydrocarbons. Here, Grignard reagent acts as a nucleophile. This reaction extends the carbon chain.
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
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.
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.
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.
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.
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 react with water and give benzene.
Also it reacts with ethyl bromide and give phenylethane.
When excess phenyl magnesium chloride is added to ethnoyl chloride and then H2O is added. We can get a tritary alcohol.
ethanoic acid given.
propanoic acid is given.
benzoic acid is given.
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 reagent and alcohol react and give hydrocarbon as the product.Learn more about grignard reagent and alcohol reaction, mechanism
First thing you should know is, formaldehyde reacts with grignard reagent. SO we cannot prepare formaldehyde from grognard reagent in a single reaction.
Add methyl magnesium bromide solution to the water. Methane is given as the product.
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.
A tertiary alcohol is given as the product if there is excess ethyl magnesium bromide. Otherwise, a ketone will give as the product. Mechanism and steps are explained below.
Grignard reagent cannot be prepared when acidic hydrogen is in the compound. In the given alkyne, there is an acidic hydrogen atom which is connected to the carbon atom which is at end of the chain.
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.
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 .
Alcohols, carboxylic acids are given from grignard and other organic compound reactions.
Following reagents and reactions can lengthen carbon chain.