Organometallic Compounds

Organometallic compounds

Organometallic compounds are the compounds containing at least one chemical bond between a carbon atom of an organic molecule and a metal including alkali, alkaline earth and transition metals (sometimes bonded with metalloids like boron, silicon and selenium as well). It is defined as a compound containing a metal-carbon bond. The bonding is ionic, covalent or delocalized between an organic group and a metal atom. They are divided into:

a. Symmetrical: Example. (C₂H₅)₂Hg (Diethyl mercury)
b. Unsymmetrical: Example. CH₃-Hg-C₂H₅ (Ethyl methyl mercury)

Mixed organometallic compounds are those in which a metal atom is bonded with more than one identity of organic or inorganic constituents. Example: C₂H₅MgBr (Ethyl magnesium bromide).

Organolithium compounds

They contain carbon-lithium bonds. Their general formula is R-Li(R is an alkyl group) or Ar-Li (Ar is an aryl group). The bond is covalent but strongly polar in nature.
Examples: CH₃-Li (methyl lithium), C₂H₅-Li (ethyl lithium), C₆H₅-Li (phenyl lithium), etc.

Organocopper compounds

They contain carbon-copper bonds. There are two types of organocopper compounds.
i. Organo copper compounds (R-Cu): eg. CH₃Cu (methyl copper), C₂H₅Cu (ethyl copper), etc.
ii. Lithium organocuprates (R2CuLi) : Gilman’s reagent: eg. (CH₃)₂CuLi (Lithium dimethyl cuprate).

Organocadmium compounds

They contain carbon-cadmium bonds. Their general formula is R₂Cd. eg. (CH3)₂Cd (dimethyl cadmium), (CH₃CH₂)₂Cd (diethyl cadmium), etc.

Grignard’s Reagent

Grignard reagents are organometallic compounds having a carbon to magnesium covalent bond and magnesium to halogen ionic bond. They are named as alkyl magnesium halide (RMgX) where R may be alkyl or aryl group. They are highly reactive in nature and used in the synthesis of many important organic compounds like alkanes, alcohols, carboxylic acids, etc.
eg. CH₃MgCl (methyl magnesium chloride), C₂H₅MgBr(Ethyl magnesium bromide).

Preparation of Grignard Reagent

When haloalkane or haloarenes are reacted with magnesium metal in presence of dry ether, Grignard’s reagent is prepared.

\begin{aligned} \underset{Haloalkane}{R-X}+Mg\ &\xrightarrow[ether]{dry} \underset{\substack{Alkyl\ magnesium\\ halide\\ (Grignard's\\ reagent)}}{RMgX}\\
\text{X = Cl, Br, I}\\
\underset{Chloromethane}{CH_{3}Cl}+Mg\ &\xrightarrow[ether]{dry} \underset{\substack{Methyl\ magnesium
\\ chloride}}{CH_{3}MgCl}\\
\underset{Bromoethane}{CH_{3}CH_{2}Br}+Mg\ &\xrightarrow[ether]{dry} \underset{\substack{Ethyl\ magnesium
\\ bromide}}{CH_{3}CH_{2}MgBr}\\
\end{aligned}

Bromobenzene and iodobenzene form Grignard’s reagent treated with magnesium metal in dry ether but fluorobenzene does not.
In the preparation of Grignard’s reagent, THF(tetrahydrofuran, a cyclic ether) or dry ether is used to avoid air, moisture, etc. and to provide an inert atmosphere. It is because Grignard’s reagent is highly reactive. It can react with H₂O, CO₂, O₂, etc.

Chemical Properties of Grignard Reagent

1. Action with water: Grignard’s reagent reacts with water to form alkane.

\begin{aligned} \underset{\substack{Grignard's\\ reagent}}{RMgX}+H_{2}O\ &\rightarrow \underset{Alkane}{RH}+Mg(OH)X\\
CH_{3}MgBr+H_{2}O\ &\rightarrow CH_{4}+Mg(OH)Br\\
CH_{3}CH_{2}MgI+H_{2}O\ &\rightarrow CH_{3}CH_{3}+Mg(OH)I
\end{aligned}

2. Action with aldehyde and ketone: Grignard’s reagent reacts with carbonyl compounds (aldehyde and ketone) to form an addition compound (adduct) which on acidic hydrolysis gives alcohol.

i. Addition of formaldehyde to Grignard’s reagent followed by acidic hydrolysis of adduct gives primary alcohol.

ii. Addition of aldehydes other than formaldehyde to Grignard’s reagent followed by acidic hydrolysis of adduct gives secondary alcohol.

iii. Addition of ketone to Grignard’s reagent followed by acidic hydrolysis of adduct gives tertiary alcohol.

3. Action with carbon dioxide (carbonation): Grignard’s reagent reacts with carbon dioxide followed by acidic hydrolysis gives carboxylic acid.

4. Action with HCN: Grignard’s reagent reacts with HCN followed by acidic hydrolysis of adduct gives aldehyde.

5. Action with RCN: Grignard’s reagent reacts with RCN followed by acidic hydrolysis of adduct gives ketone.

6. Action with ester: All esters except those of formic acid reacts with Grignard’s reagent to form tertiary alcohols. Esters of formic acid gives secondary alcohols.

7. Action with acid chloride: Acid chloride reacts with Grignard’s reagent to give ketone which further reacts with Grignard’s reagent to give tertiary alcohol.

Some Important Questions

1. Write two examples each of organolithium, organocopper and organocadmium compounds.
2. What happens when i. Bromoethane ii. Isopropyl iodide iii. Bromobenzene are treated with magnesium in presence of dry ether?
3. Obtain primary, secondary and tertiary alcohol using ethyl magnesium iodide.
4. How can you convert Grignard’s reagent into Pentanal and Butan-2-one by using cyanide?
5. Give the reaction of:
   i. Ethyl acetate is reacted with methyl magnesium bromide.
   ii. Acetyl chloride is reacted with ethyl magnesium iodide.
6. What is Grignard’s reagent? How can you obtain ethane and Propanoic acid by using a suitable
   Grignard’s reagent?
7. How could you convert primary alcohol into Grignard’s reagent?
8. Compound A reacts with C₂H₅MgI to form an adduct which on hydrolysis gives 3-methyl
   pentan-3-ol. What is A?
9. How is phenol obtained from phenyl magnesium bromide?
10. A compound X reacts with PCl₃ to get Y.Y reacts with magnesium to form Z which is treated with
    acetone and the product is hydrolysed to give 2-methyl butan-2-ol. Identify X, Y and Z.