| Organometallic Grignard Reagent |
| Chapter Organometallic compounds are compounds that contains a carbon metal bond. You are already familiar with some organometallic compound like sodium acetylide (NaC==CH) and alkyl lithium which has an ionic bond between carbon and sodium or carbon and lithium. But simply having a metal and carbon in the same compound is not sufficient for classification as organometallic. Sodium methoxide (NaOCH3), for example, is not an organometallic compound ; sodium is not bonded directly with carbon atom.
Preparation of organomagnesium : Grignard Reagents
The most important organometallic reagents in organic chemistry are organomagnesium compounds. They are called Grignard reagents. Grignard developed efficient methods for the preparation of organic derivatives of magnesium and their application in the synthesis of many organic compounds.
Grignard reagents are prepared directly from organic halides by reaction with magnesium, a group II metal in the presence of ether as solvent.
Properties
Grignard reagent shows following reactions.
(i) Substitution reactions
When grignard reagent reacts with compounds containing active ?H? or RX. It shows substitution reaction.
(ii) Addition reaction
Grignard reagent reacts with compounds containing multiple bonds between atoms with electronegativity difference. It shows addition reaction between two atoms.
(i) > C = O (ii) > C = S (iii) ? C º N (iv) > S = O, etc. |
| Organometallic Grignard Reagent |
| Chapter Organometallic compounds are compounds that contains a carbon metal bond. You are already familiar with some organometallic compound like sodium acetylide (NaC==CH) and alkyl lithium which has an ionic bond between carbon and sodium or carbon and lithium. But simply having a metal and carbon in the same compound is not sufficient for classification as organometallic. Sodium methoxide (NaOCH3), for example, is not an organometallic compound ; sodium is not bonded directly with carbon atom.
Preparation of organomagnesium : Grignard Reagents
The most important organometallic reagents in organic chemistry are organomagnesium compounds. They are called Grignard reagents. Grignard developed efficient methods for the preparation of organic derivatives of magnesium and their application in the synthesis of many organic compounds.
Grignard reagents are prepared directly from organic halides by reaction with magnesium, a group II metal in the presence of ether as solvent.
Properties
Grignard reagent shows following reactions.
(i) Substitution reactions
When grignard reagent reacts with compounds containing active ?H? or RX. It shows substitution reaction.
(ii) Addition reaction
Grignard reagent reacts with compounds containing multiple bonds between atoms with electronegativity difference. It shows addition reaction between two atoms.
(i) > C = O (ii) > C = S (iii) ? C º N (iv) > S = O, etc. |
Chapter
In case of G.R always substitution reactions are preferred over addition reactions
(x) Reaction with Cyanogen and Cyanogen chloride
Alkyl cyanides are produced when Grignard reagents react with cyanogen and cyanogen chloride .
(xi) Reaction with Iodine
When an alkyl magnesium chloride or bromide is treated with iodine, alkyl iodides are formed.
A good method for preparing an alkyl iodide from the corresponding alkyl chloride or bromide.
(xii) Reaction with Inorganic halides
Organometallic and organo - nonmetallic compounds result when Grignard reagents react with inorganic halides. For example :
Synthesis of cyclopropane by an organic reagent
Zinc reacts with alkyl halides in a manner similar to that of magnesium.
Organozinc reagents are not nearly so reactive toward aldehydes and ketones as Grignard reagents and organolithium compounds but are intermediates in a number of reactions with alkyl halides and dihalides.
An important organozinc compound in organic synthesis is iodomethylzinc iodide (ICH2ZnI), prepared by the reaction of zinc-copper couple [Zn(Cu), zinc that has had its surface activated with little copper] with di- iodomethane in ether.
What makes iodomethylzinc iodide such a useful reagent is that it reacts with alkenes to give cyclopropanes.
This reaction is called the Simmons-Smith reaction and mechanistically, the Simmons-Smith reaction seems to proceed by a single-step cycloaddition of a methylene (CH2) unit from iodomethylzinc iodide to the alkene :
Methylene transfer from iodomthylzinc iodide is stereospecific, reacting species which are cis in the alkene remain cis in the cyclopropane also. For example
Carbenes and carbenoids
Iodomethylzinc iodide is often referred to as carbenoid, because it resembles a carbene in its chemical reactions. Carbenes are neutral molecules in which one of the carbon atom has six valence electrons. Such carbons are divalent; they are directly bonded to only two other atoms and have no multiple bonds. Iodomethylzinc iodide reacts as if it was a source of the carbene H ?? C ?? H.
It is clear that free [:CH2] is not involved in the Simmons ? Smith reaction, but there are many evidence which indicate that carbenes are formed as reactive intermediates in certain other reactions that convert alkenes to cyclopropanes. The most studied examples of these reactions involve dichlorocarbene (:CCl2) and dibromocarbene (:CBr2)
Carbenes are too reactive to be isolated and stored, so these are trapped in frozen argon for spectroscopic study at very low temperatures.
Dihalocarbenes are formed when tri halomethanes are treated with a strong base, such as potassium tert-butoxide. The tri halomethyl anion produced on proton abstraction dissociates into di halocarbene and a halide anion.
When generated in the presence of an alkene, dihalocarbenes undergo cycloaddition to the double bond to give di halo cyclopropanes :
The reaction of dihalocarbenes with alkenes is stereospecific, where syn addition is observed. The process in which a di halocarbene is formed from a tri halo methane corresponds to an elimination in which a proton and a halide are lost from the same carbon. It is an example of
proceeding via the organometallic intermediate K+[:CX3]? while most of elimination are included in 
for example dehydration of alcohol.
There is a big difference of a reaction when alkenes reacts with haloform in alcoholic bases or weak bases for exampleIn first reaction nucleophilic substitution reaction occurs while second reaction is an example of where synthesis of dichlorocarbene takes place.
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