What is an Organometallic Compound?

These are the compounds containing at least one carbon-metal bond in their molecule. It consists of metal and organic parts. These metals may be lithium, sodium, magnesium, zinc, and lead whereas the organic part can be the alkyl, aryl, alkenyl, or alkynyl group.

Naming of Organometallic Compound

The organic part of the chain is named before the metal. For example: $C H_{3} L i$ is termed methyl lithium and $C_{6} H_{5} L i$ is termed phenyl lithium.

Grignard Reagent

Discovered by Victor Grignard, organometallic (magnesium) halides are known as Grignard reagents with a general form of $R M g X$ when R is an organic group and X is a halide.

Synthesis of Grignard Reagent

The reaction of an alkyl halide with magnesium in the presence of dry ether gives us the Grignard reagent.

$\begin{array}{l} R - X + M g \overset{D r y E t h e r}{\to} R M g X \\ C_{2} H_{5} B r + M g \overset{D r y E t h e r}{\to} C_{2} H_{5} M g B r \end{array}$

Tetra hydro furan (THF) is considered to be the best solvent because it increases the reactivity of alkyl or aryl halides towards magnesium. Grignard reagents of alkyl and aryl halides are preferred to form using THF.

The ease of formation of Grignard reagent is in the following order:

$R I > R B r > R C l$ .

However, for a particular halogen, the ease of formation of Grignard reagent decreases with an increase in the number of carbon atoms in the alkyl group.

Points to remember while preparing Grignard reagent are:

Apparatus should be moisture-free.
All reactants and solvents should be taken in pure form for the reaction to be carried out.
Dry ether used should be free of alcohol.
The atmosphere of the lab should be taken into consideration. The atmosphere of nitrogen-free oxygen and carbon dioxide should be formed. The solution prepared will be used for various reactions.

Physical Properties of Grignard Reagent

Non-volatile
Colorless solids
It has an explosive nature in the Free State
Used in the solution form in which they are prepared

Chemical Properties of Grignard Reagent

They undergo Nucleophilic Substitution and Addition Reactions Reaction with Active Hydrogen

Active hydrogen means more acidic hydrogen as compared to hydrogen in alkanes.

$R M g X + H A \to R H + M g A X$

This is the general reaction.
Hydrocarbons formed here are in good quantity. This forms the basis of the Zerewitinoff method of estimation of hydrogens.

Higher alkynes are produced. For example, alkenyl magnesium halide reacting with alkyl iodide gives alkynes.

$C H_{3} - C \equiv C M g I + C_{2} H_{5} I \to C H_{3} - C \equiv C - C_{2} H_{5} + M g I_{2}$

Here 2-Pentyne is formed.
Aryl magnesium halides undergo such reactions to form aromatic hydrocarbons.
Dry ether is used as a reagent to carry out the desired chemical reaction followed by hydrolysis in this part of organometallic chemistry.

Nucleophilic Substitution Reaction in Organometallic Chemistry

Formation of alkanes: reaction with alkyl halides- tert-butyl chloride reacting with methyl magnesium halide gives neo-pentane.

C H_{3} - M g I + C {(C H_{3})}_{3} C l \to C {(C H_{3})}_{4} + M g (C l) I

Formation of alkenes: reaction with unsaturated halides-Allyl halide reacting with methyl magnesium halide gives Butene.

C H_{3} - M g I + I C H_{2} - C H = C H_{2} \to C H_{3} - C H_{2} - C H = C H_{2} + M g I_{2}

Formation of higher ethers: reaction with halogenated ethers- reaction of chloroethyl ether with ethyl magnesium iodide to give methyl propyl ether.

C H_{3} C H_{2} - M g I + C l C H_{2} - O - C H_{3} \to C H_{3} C H_{2} C H_{2} - O - C H_{3} + M g (C l) I

Formation of other reactions with organic halides-Cd is a transition metal cadmium. This transition metal has -2, +1, +2 oxidation state. It belongs to group12. These transition metals if present for a longer duration in the body weakens bone and leads to kidney disease. Other organometallic compounds containing transition metals are nickel tetra carbonyl, chromium hexacarbonyl.

4 C_{2} H_{5} - M g I + 2 P b C l_{2} \to {(C_{2} H_{5})}_{4} P b + P b + 4 M g (C l) I

2 C_{2} H_{5} - M g B r + C d C l_{2} \to {(C_{2} H_{5})}_{2} C d + P b + 2 M g (C l) B r

2 C_{2} H_{5} - M g B r + H g C l_{2} \to {(C_{2} H_{5})}_{2} H g + P b + 2 M g (C l) B r

4 C_{2} H_{5} - M g B r + 2 S n C l_{2} \to {(C_{2} H_{5})}_{4} S n + 4 M g (C l) B r

Nucleophilic Addition Reactions

Grignard reagents undergo nucleophilic addition reactions with compounds containing multiple bonds (such as $> C = O, > C = S, - C \equiv N, > S = O$ ). The additional products formed are easily hydrolyzed in an acidic medium to furnish a large variety of products. The nucleophilic group of organometallic compounds RMgX (i.e., alkyl or aryl group) gets attached to the electropositive end of the multiple bonds whereas the MgX group gets bonded to the electronegative end.

Formation of Alcohols

Reaction of Grignard reagent with carbonyl compounds, followed by hydrolysis in an acidic medium to form alcohols.

Formation of primary alcohols: The reaction of Grignard reagent, organometallic compound with formaldehyde to form 1° alcohol.
Formation of secondary alcohols: The reaction of Grignard reagent, organometallic compound with aldehyde other than formaldehyde to form 2° alcohol.
Formation of tertiary alcohols: The reaction of Grignard reagent, organometallic compound with ketones to form 3° alcohol.
Formation of secondary and tertiary alcohols: Reaction of Grignard reagent with esters. This is a two-step reaction.

“Reaction of Grignard reagent with esters”

Formation of tertiary alcohols: The reaction of carbonyl compounds with organometallic reagent results in the formation of a tertiary alcohol.

2 moles of reagent react with 1 mole of acid chloride or anhydride to yield tertiary alcohol. Initially, an aldehyde or ketone is initially formed and finally forms alcohol.

Abnormal behavior of Grignard reagent (in organometallic chemistry)

Reaction with ketones: When isopropyl magnesium bromide is added to di-isopropyl ketone, the expected tertiary alcohol is not formed. On the other hand, di-isopropyl carbinol and propene are formed.
MPV synthesis: Meerwein-ponndorf-verley reduction of ketones generates six-membered cyclic transitions.
1, 2-addition: this chemical synthesis is favored due to Increased substitution at the ethylenic bond favors 1, 2-addition.
1, 4-addition: this chemical synthesis is favored due to the presence of bulky groups on the carbonyl carbon favors 1,4-addition.

Limitations of using Grignard Reagent

They are highly sensitive, thus difficult to handle.
No active substituent group should be there.
Due to steric hindrance, Grignard reagent does not react with bulky groups.
Nitro compounds cannot be used with Grignard reagent as they oxidize the Grignard reagent.

Organolithium Compounds

Organometallic compounds such as organolithium are the most important derived from alkali metals.

The most important organometallic compound general formula is: R-M = R^-M⁺
The carbon lithium bond is covalent but strongly polar in character.

Synthesis of Organolithium Compounds

By reaction with lithium and alkyl halide: The reaction of lithium and alkyl halide is carried out at a very low temperature. Benzene or dry hexane are used as a solvent and nitrogen gas is used to provide inert atmosphere.

C H_{3} - B r + 2 L i \to_{263 K, N_{2}}^{D r y H e x a n e} C H_{3} - L i + L i B r

By metal exchange reaction: The reaction of Lithium with a less reactive metal-containing compound yields an organolithium compound.

{(C H_{3} C H_{2})}_{2} H g + 2 L i \to 2 C H_{3} C H_{2} L i + H g

By exchange reaction: Aryl lithium are prepared by the following reaction:

C_{6} H_{5} B r + n C_{4} H_{9} L i \to C_{6} H_{5} L i + n C_{4} H_{9} B r

Physical properties of Organometallic Compounds containing Lithium

In chemistry, the organometallic compound exists as dimers or higher as an aggregated molecule due to the strong polarization of bond.
They are colorless liquids or solids with low melting points and highly solubility in ether.

Chemical Properties of Organometallic Compounds in Chemistry

Formation of Hydrocarbons

Organometallic lithium reacts with substances containing active hydrogen to form hydrocarbons.

$R - L i + H_{2} O \to R H + L i O H$

Formation of Alcohols

Organometallic lithium reacts with carbonyl compounds to give alcohols.

Formation of primary alcohols:

H C H O + R - L i \to R C H_{2} O H + L i O H

Formation of secondary alcohols:

R^{'} C H O + R - L i \to R R^{'} C H O H + L i O H

Formation of tertiary alcohols:

R^{'} C R " O + R - L i \to R R^{'} C R " O H + L i O H

Formation of alcohol using Epoxides

Organometallic lithium reacts with ethylene epoxide to give primary alcohol.

$C_{4} H_{9} - L i + C_{2} H_{4} O \to C_{4} H_{9} C H_{2} - C H_{2} O H$

Formation of nitriles using Cyanogen Chloride:

$C H_{3} - L i + C l - C N \to C H_{3} - C N + L i C l$

The reaction of phenyl lithium with chloronaphthalene undergoes an elimination addition reaction.

$C_{6} H_{5} L i + C_{10} H_{7} C l \overset{- L i C l}{\to} C_{10} H_{7} (C_{6} H_{5}) + C_{10} H_{7} (C_{6} H_{5})$

Organozinc Compounds

Organozinc are the milder reagents that were the first organometallic prepared in the chemistry laboratory.

Synthesis of Organozinc Compounds

From alkyl halides: zinc fillings are treated with alkyl halides (mainly alkyl iodide) to form alkyl zinc iodides. The disproportionation reaction occurs to yield di-alkyl zinc.

$2 R I + 2 Z n \to 2 R Z n I \to R_{2} Z n + I_{2}$

$2 C_{2} H_{5} I + 2 Z n \to 2 C_{2} H_{5} Z n I \to {(C_{2} H_{5})}_{2} Z n + I_{2}$

From organometallic (containing aluminum) compounds:

$2 {(C_{2} H_{5})}_{3} A l + Z n C l_{2} \to {(C_{2} H_{5})}_{2} Z n + {(C_{2} H_{5})}_{2} A l C l$

Here, triethyl aluminum is treated with zinc chloride to give diethyl zinc, an organometallic (zinc) compound in the chemistry.

Physical Properties

Possess Linear geometry in chemistry.
Non-polar low boiling liquids.
Sometimes exist as solids and are low melting solids.
Soluble in organic solvents.
Sp hybridization of zinc atoms.
Highly inflammable, that is why synthesis is done in the atmosphere of carbon dioxide and is used in the same atmosphere.

Chemical Properties of Organozinc Compounds

Formation of hydrocarbons: Organometallic zinc is treated with water to give hydrocarbons.
Formation of hydrocarbons: Organometallic zinc is treated with water to give hydrocarbons.

$C H_{3} - Z n - C H_{3} + C H_{3} C O C l \to C H_{3} C O C H_{3} + C H_{3} Z n C l$

Reformatsky reaction: This chemical reaction in the chemistry corresponds to $β - h y d r o x y e s t e r s .$ In this, an aldehyde or ketone is treated with $α - b r o m o e s t e r s .$ and metallic zinc in the presence of dry ether or benzene. An organozinc compound is formed. This when added to the carbonyl group gives an additional product. The addition product thus formed is treated with a dilute mineral acid to give $β - h y d r o x y e s t e r s .$

Common Mistakes

When attempting to draw a molecule in a different configuration, the R/S stereochemistry was flipped.
A meso compound's "enantiomer" is depicted in chemistry.
A reagent was forgotten, such as adding NaOH to the hydroboration's oxidation stage.

Context and Applications

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for

B.Sc in Chemistry
B.Sc in Molecular Chemistry
M.Sc in Chemistry
M.Sc in Analytical Chemistry

Bromobenzene
Electrophile
Organometallic Chemistry
Hydroboration

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