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Organometallic compounds


Organometallic compounds are chemical compounds in which at least one metal atom like Li, Mg, Fe , etc. is bonded with carbon atom of organic molecule. Examples: C2H5MgBr, CH3Li, (C2H5)2Zn, (C2H5)3Al, etc.

[Note: Compounds like Ni(CO)4, NaCN, RCOONa, (C3H7O)4Ti, etc. are not organometallic compounds as is not metal-carbon bond.]

The study of compounds containing metal-carbon bonds and their reaction s is called organometallic chemistry.

Henry Gilman was an American organic chemist known as the father of organometallic chemistry.He, discovered Gilman reagent , diorganolithium copper (R2CuLi).

The first synthetic organometallic compound is Zeise’s salt , i.e. K[PtCl3(C2H4)].


Classification of Organometallic compounds


On the basis of nature of the metal to carbon bond, organometallic compounds are classified as:

1. Sigma(σ) bonded organometallic compounds: Organometallic compounds having metal-carbon σ –bond are called sigma bonded organometallic compounds. Examples: Ethylmagnesium bromide (C2H5MgBr), Ethyl lithium (C2H5 Li), etc.

2. Pi(π) bonded organometallic compounds: Organometallic compounds having metal-carbon π –bond are called pi-bonded organometallic compounds. Examples: Zeise’s salt, Ferrocene i.e. Fe(C5H5)2, etc. Examples:


Nature of metal-carbon bond


The relative electronegativities of carbon and metal suggests that the C-M bond will be highly polar. Electropositive metal atom gives up electrons to the carbon atom, resulting in a polar C-M bond with a partial positive charge on the metal and negative charge on the carbon.

Partial negative charge of an organic group bonded to a highly reactive metal results in a special reactivity which is as nucleophilic character.


Preparation of organometallic compounds


1. Organolithium compound (alkyl lithium) can be prepared as:

2. Organocopper compound (lithium dialkylcuprate) can be prepared as:

3. Organo cadmium compound (dialkyl cadmium) can be prepared as:


Grignard reagent


Grignard reagent is the alkyl magnesium halide. It is organometallic compound which is represented by the general formula RMgX. Eg.

CH3MgBr ( Methyl magnesium bromide)

CH3CH2MgI ( Ethyl magnesium iodide)

Preparation : Grignard reagent can be prepared by heating haloalkanes or haloarenes with magnesium in the presence of dry ether.

Precautions :

⇒Grignard reagent is very sensitive to water. When it comes in contact with water, it converts to alkane.

Therefore, during preparation of Grignard reagent there should not be the presence of water molecules i.e. all the reagents should be anhydrous and apparatus oven dried.

⇒There should not be naked flames nearer.


Reactions of Grignard reagents


1. With water: When Grignard reagent is hydrolyzed with water, alkanes are obtained.

2. With alcohol: Alcohols react with Grignard reagent to form alkane.

3. With aldehydes and ketones: Aldehydes and ketones (i.e carbonyl compounds) when treated with Grignard reagent gives addition product, which upon acidic hydrolysis give alcohols.

a. Formaldehyde gives primary alcohol. Eg.

b. Aldehydes other than formaldehyde give secondary alcohol. Eg.

c. Ketones give tertiary alcohol. Eg.

4. With carbon dioxide: Carboxylation(carbonation) reaction:

Grignard reagent reacts with carbon dioxide in presence of dry ether to give addition product and hydrolysis of addition product in presence of dilute acid gives carboxylic acid. Eg.

5. With acid chlorides:

Acid chlorides react with Grignard’s reagent to give ketones, which further react with Grignard’s reagent to give tertiary alcohols. Eg.

6. With esters:

Esters (except those of formic acid) react with Grignard’s reagent to give ketones, which further react with Grignard’s reagent to give tertiary alcohols. Eg.

7. With hydrogen cyanide (HCN):

Reaction of Grignard reagent with HCN followed by acid hydrolysis gives aldehydes. Eg.

8. With alkane nitrile (RCN):

Reaction of Grignard reagent with RCN followed by acid hydrolysis gives ketones. Eg.


EXERCISE


1. Organometallic compounds are chemical compounds in which at least one metal atom like Li, Mg, Fe, etc. is bonded with carbon atom of organic molecule.

a. Write two examples of organometallic compounds.

b. What is the nature of carbon-metal bond of organometallic compounds?

c. Write one example each of sigma and pi bonded organometallic compound.

d. Potassium acetate and sodium ethoxide are not organometallic compounds, why?

2. The carbon-magnesium bond in a Grignard reagent is polar covalent with carbon being the negative end of the dipole, which explains its nucleophilicity and the magnesium-halogen bond is largely ionic.

a. Define Grignard reagent. How aliphatic and aromatic Grignard reagent is prepared?

b. Mention the precautions for its preparation.

c. What is the role of ether in Grignard reaction?

d. Ether should be pure and dry in Grignard reaction, why?

e. Write the products obtained by reacting methyl magnesium bromide with (a)HCHO (b)CO2 (c)CH3CHO (d)CH3CN (e)CH3COOCH3 (f)CH3COCl. Write their complete reactions.

3. Convert:

a. Hydrogen cyanide to acetaldehyde

b. Ethanenitrile to acetone

c. Phenyl magnesium bromide to benzoic acid

d. Benzonitrile to acetophenone

e. Ethyl methanoate to propan-2-ol

f. Ethanoyl chloride to 2-methyl propan-2-ol

4. Grignard reagent is an organometallic compound which is used to synthesize various organic compounds. Starting from CH3MgBr, how would you prepare:

a. Methane

b. Ethanoic acid

c. Ethanol

d. Propan-2-ol

e. 2-methylpropan-2-ol

5. An aromatic hydrocarbon ‘X’ is treated with halogen in dark to give compound ‘Y’. the compound ‘Y’ on heating with magnesium metal gives compound ‘Z’. How can you convert benzoic acid and acetophenone from compound ‘Z’. Write a suitable reaction.