GRIGNARD REAGENTS
INTRODUCTION:
The Grignard reagents are organic derivatives of magnesium (Mg) commonly represented by the general formula RMgX (in which R is a hydrocarbon radical: CH3, C2H5, C6H5, etc.; and X is a halogen atom, usually chlorine, bromine, or iodine).
History:
Grignard reagents are named after Victor Grignard, a French chemist. In 1901 he discovered an important class of organic reagents now known as Grignard Reagents. For this work he received a Nobel Prize in 1912. He was searching for a catalyst for a Methylation reaction he was trying to induce; chemists had earlier tried to use Zinc in combination with various organic compounds and found it moderately successful. Grignard used magnesium mixed with organic halides in ether solution and obtained compounds of the type RMgX, where X is a halogen (Cl, Br, I) and R an organic group (alkyl or aryl group). These Grignard reagents are very important class of extremely reactive chemical compounds used in the synthesis of hydrocarbons, alcohols, carboxylic acids, and other compounds.Chemically, a Grignard reagent is an organic magnesium halide dissolved in a nonreactive solvent (typically dry ethyl ether). The substance is made up of an organic group, e.g., an alkyl or aryl group, joined by a highly polar covalent bond to magnesium, while the magnesium is joined by an ionic bond to a halogen ion, e.g., bromide or iodide. A Grignard reagent will react with water, oxygen, carbon dioxide, or almost any electrophilic organic compound. The reaction of Grignard reagents with aldehydes to form alcohols is of particular importance in the laboratory.
As Grignard reagents are so unstable, they are generally prepared just before use by reacting an organic halide, e.g., methyl iodide, with magnesium metal in a completely dry solvent; air is usually excluded from the reaction vessel, e.g., by flushing it with nitrogen.
Preparation of grignord
reagent
Grignard reagents are made by adding the halogenoalkane to small bits of magnesium in a flask containing ethoxyethane (commonly called diethyl ether or just "ether"). The flask is fitted with a reflux condenser, and the mixture is warmed over a water bath for 20 - 30 minutes.
Reactivity of G.R:
Reactivity of grignard reagent is due to C-Mg bond which is highly polar.
Mg is more electropositive than C and the covalent bond between C-Mg is highly polar, giving C the partial negative charge that makes alkyl group highly reactive towards electrophilic reactions.Its mostly reactions are exothermic in nature.
Mg is more electropositive than C and the covalent bond between C-Mg is highly polar, giving C the partial negative charge that makes alkyl group highly reactive towards electrophilic reactions.Its mostly reactions are exothermic in nature.
Reactions of Grignard Reagents
Grignard reagents and water:
Grignard reagents react with water to produce alkanes. This is the reason that everything has to be very dry during the preparation above.For example:
Grignard reagents and carbon dioxide:
Grignard reagents react with carbon dioxide in two stages. In the first, we get an addition of the Grignard reagent to the carbon dioxide.
Dry carbon dioxide is bubbled through a solution of the Grignard reagent in ethoxyethane.
The product is then hydrolysed (reacted with water) in the presence of a dilute acid. Typically, we add dilute sulphuric acid or dilute hydrochloric acid to the solution formed by the reaction with the CO2.
A carboxylic acid is produced with one more carbon than the original Grignard reagent.
Grignard reagents and
carbonyl compounds
What are Carbonyl compounds?
Carbonyl compounds contain the C=O double bond. The simplest ones have the form:
Aldehydes:
If one (or both) of the R groups of carbonyl group are hydrogens, the compounds are called Aldehyde For example:
Ketones:
If both of the R groups are alkyl groups, the compounds are called Ketones. Examples include:
The
general reaction between Grignard reagents and carbonyl compounds:
The reactions between the various sorts of carbonyl compounds and Grignard reagents can look quite complicated, but in fact they all react in the same way
When it is treated with any carbonyl compound, it produces primary, secondary and tertiary alcohols according to the nature of alkyl group attached to the carbonyl group. Reactions involves two steps
Dilute acid is then added to hydrolyse it.
when it is treated with grignord reagent it produces primary alcohols.
A primary alcohol has only one alkyl group attached to the carbon atom with the -OH group on it.
Grignard's reagent reacts with all other aldehydes except formaldehyde to produce secondary alcohols.
A secondary alcohol has two alkyl groups (the same or different) attached to the carbon with the -OH group on it.We can change the nature of the final secondary alcohol by either:
Ketones have two alkyl groups attached to the carbon-oxygen double bond. The simplest one is propanone.
Grignard's reagent reacts with ketones to produce tertiary alcohols.
A tertiary alcohol has three alkyl groups attached to the carbon with the -OH attached.
The simpilest epoxide is ethylene oxide.
Grignord reagent reacts with epoxide to form alcohols.
In
the first step,
the Grignard reagent adds across the carbon-oxygen double bond:
Reaction
with aldehyds:
It produces primary and secondary alcohols ,When it is reacted with aldehyds.
Aldehyde
Alcohol
For example Methanal (formaldehyde) is the simplest possible Aldehyde. Grignard's reagent reacts with all other aldehydes except formaldehyde to produce secondary alcohols.
A secondary alcohol has two alkyl groups (the same or different) attached to the carbon with the -OH group on it.We can change the nature of the final secondary alcohol by either:
- changing the nature of the Grignard reagent - which would change the CH3CH2 group into some other alkyl group;
- Changing the nature of the aldehyde - which would change the CH3 group into some other alkyl group.
The
reaction between Grignard reagents and Ketones:
Grignard's reagent reacts with ketones to produce tertiary alcohols.
A tertiary alcohol has three alkyl groups attached to the carbon with the -OH attached.
Addition of
Epoxies:
Epoxide:
Epoxides also known as oxiranes, are three-membered cyclic compounds. The ring consists of two carbon atoms and one oxygen atom.The simpilest epoxide is ethylene oxide.
Grignord reagent reacts with epoxide to form alcohols.
Epoxide Alcohol
Reaction with ester:
When esters are treated with grignord reagent, primary, and tertiary alcohols are produced.
