GRIGNARDS REAGENT NEW CHAPTER R-Mg-X

GRIGNARD’S REAGENT F. A. Victor Grignard (1871-1935) was a Nobel Prize winning French chemist. In 1900, V. Grignard reported that an alkyl halide (RX) reacts with magnesium metal (Mg) in diethyl ether to give a cloudy solution of an organomagnesium compound (RMgX). He was awarded the Nobel Prize in Chemistry in 1912. The Grignard reagent is usually described with the general chemical formula R-Mg-X. Iodides, bromides, and chlorides can be used, including aryl and alkyl halides.

GRIGNARDS REAGENT The Grignard reaction became one of the most versatile C-C bond forming tools. It involves an insertion of magnesium into the new carbon–halogen bond.

GRIGNARDS REAGENT PREPARATION There is also a change in oxidation state of the magnesium, from Mg(0) to Mg(II). The reaction is therefore known as an oxidative insertion or oxidative addition.

PREPARATION GRIGNARDS REAGENT The C-Mg bond is very polar and the partial negative charge resides on the carbon atom, so Grignard reagents are excellent carbon nucleophiles.

R-Mg-X Formation (Radical Mechanism) It also involves radical intermediates. Grignard formation does not involve a radical chain mechanism. It is a non-chain radical reaction.

Ether Solvent Diethyl ether is an especially good solvent for the formation of Grignard reagents because ethers are non-acidic (aprotic). Water or alcohols would protonate and thus destroy the Grignard reagent, and would form a hydrocarbon. But Grignard reagents are stable in ethers.

Ether Solvent Ethers are good at solvating cations, because the C-O bond is relatively polar, thus allowing the oxygen end of the ether dipole to solvate and stabilize the magnesium ion.

REACTIONS OF GRIGNARD REAGENTS The Mg-C bond in a Grignard reagent is polarized, e.g. methylmagnesium bromide. The carbon attached to Mg bears a partial negative charge. This carbon is Nucleophilic, and is subject to attack by electrophiles. An Electrophile is a chemical species which seeks electrons.

Reactions of Grignard reagents Grignard reagents and water Grignard reagents react with water to produce alkanes. So, everything has to be very dry during the preparation above. Grignard reagents and carbon dioxide Grignard reagents react with carbon dioxide in two stages. In the first, you get an addition of the Grignard reagent to the carbon dioxide.

Mechanism

Reaction between Grignard reagents and carbonyl compounds The reactions between the various types of carbonyl compounds and Grignard reagents can look quite complicated, but in fact they all react in the same way, only changes are the groups attached to the carbon-oxygen double bond. In the first stage, the Grignard reagent adds across the carbon-oxygen double bond: Grignard reagents and methanal Methanal has two hydrogen substituents. Methanal is the simplest possible aldehyde. A primary alcohol is formed. A primary alcohol has only one alkyl group attached to the carbon atom with the -OH group on it. Primary Alcohol

The reaction between Grignard reagents and other aldehydes The next biggest aldehyde is ethanal. One of the R group is CH3. Ethanal Secondary alcohol You could 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.

Mechanism In the first stage, the Grignard reagent adds across the carbon-oxygen double bond. Dilute acid is then added to this to hydrolyse it.

The reaction between Grignard reagents and ketones Ketones have two alkyl groups attached to the carbon-oxygen double bond. The simplest one is propanone and it gives tertiary alcohol. Reaction with Ethylene oxide or epoxide

Reation with Ester Two molecules of Grignard's reagents react with ester to form Tertiary alcohol. Tertiary alcohol

R and S Configuration (REVISION) Arrange hard Line substituent opposite to dotted line substituent and then assign R or S according to priority order.

R and S Configuration

Resonance Effects

negative inductive effect positive resonance effect halogens - weak deactivating - negative inductive effect > positive mesomeric (resonance effect) Halogens are an exception of the deactivating group that directs the ortho or para substitution. The halogens deactivate the ring by inductive effect. In halogens inductive effect (-I) is greater than resonance effect (+M).

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