Organometallic Compounds
Class 12 Chemistry Notes • NEB Syllabus 2083
Syllabus, 8 solved multiple choice questions, the full textbook exercise with solutions and diagrams, handwritten and typed PDF notes and frequently asked questions on organometallic compounds and Grignard reagent.
1. Syllabus
Unit 17: Organometallic Compounds
These notes follow the NEB Class 12 Chemistry syllabus (+2 Chemistry, Nepal). Practise the multiple choice questions first, then work through the solved numerical and subjective questions, and finish with the PDF notes for quick revision.
2. Multiple Choice Questions
Question 1. Which of the following reagents is suitable to obtain ethane from Grignard’s reagent?
- a) Ethanol
- b) Ethanal
- c) Ethanoyl chloride
- d) Ethyl magnesium bromide
Correct Answer: a) Ethanol
Reason: Grignard’s reagent is a strong base and takes a proton from any compound with an acidic hydrogen. Ethyl magnesium bromide with ethanol gives ethane: \(\mathrm{CH_3CH_2MgBr+CH_3CH_2OH\longrightarrow CH_3CH_3+CH_3CH_2OMgBr}\). Ethanal and ethanoyl chloride add to the reagent to give alcohols, and ethyl magnesium bromide does not react with itself.
Question 2. What is Z in the following reaction sequence? \(\mathrm{CH_3-Mg-Br+CH_3COCH_3}\) (dry ether) gives Y; Y (\(\mathrm{H^+/H_2O}\)) gives Z
- a) 2-Methylpropan-2-ol
- b) 2-Hydroxypropane
- c) 2-Methylpropanal
- d) 3-Methylpropan-1-ol
Correct Answer: a) 2-Methylpropan-2-ol
Reason: The reagent adds to the carbonyl carbon of propanone: \(\mathrm{CH_3COCH_3+CH_3MgBr\longrightarrow(CH_3)_3C-OMgBr\ (Y)}\). Hydrolysis gives \(\mathrm{(CH_3)_3C-OMgBr+H_2O\longrightarrow(CH_3)_3C-OH\ (Z)+Mg(OH)Br}\), a tertiary alcohol: 2-methylpropan-2-ol.
Question 3. Which of the following is the molecular formula of Zeise’s salt anion?
Figure: Zeise’s salt, K[PtCl₃(C₂H₄)]·H₂O. The C=C π bond of ethene is bonded sideways to platinum(II)
- a) [PtCl₂(C₂H₄)]
- b) [PtCl₃(C₂H₄)]
- c) [PtCl₃(C₂H₆)]
- d) [PtCl₃(C₂H₅)]
Correct Answer: b) [PtCl₃(C₂H₄)]
Reason: Zeise’s salt is \(\mathrm{K[PtCl_3(C_2H_4)]\cdot H_2O}\). The anion has three chloride ions and one ethene (\(\mathrm{C_2H_4}\)) molecule bonded to platinum(II): \((+2)+3(-1)=-1\), which balances the charge of one \(\mathrm{K^+}\).
Question 4. What would be the product when methyl magnesium bromide is treated with HCN followed by hydrolysis?
- a) Methanoic acid
- b) Ethanoic acid
- c) Acetaldehyde
- d) Formaldehyde
Correct Answer: c) Acetaldehyde
Reason: \(\mathrm{CH_3MgBr+HC\equiv N\longrightarrow CH_3CH=NMgBr}\). Hydrolysis: \(\mathrm{CH_3CH=NMgBr+2H_2O\longrightarrow CH_3CHO+NH_3+Mg(OH)Br}\). Grignard’s reagent with HCN gives an aldehyde that has one more carbon atom than the alkyl group: ethanal (acetaldehyde).
Question 5. Choose the best alternative from the following table.
| Reactants | Condition | Products |
|---|---|---|
| M | Dry ether/Δ | \(\mathrm{LiCH_3+LiCl}\) |
| \(\mathrm{CH_3COCl+CH_3MgBr}\) | \(\mathrm{H_2O/H^+}\) | N |
| Ester + Grignard’s reagent | O | Secondary alcohol |
- a) M – Li + CH₃Cl, N – 2-methylpropan-2-ol, O – H₂O/H⁺
- b) M – LiCH₃ + Cl₂, N – 3-methylpropan-2-ol, O – H₂O/H⁺
- c) M – Li + CH₃Cl, N – 1-methylpropan-1-ol, O – H₂O/H⁺
- d) M – Li + CH₃Cl, N – ethylpropan-1-ol, O – H₂O/H⁺
Correct Answer: a) M – Li + CH₃Cl, N – 2-methylpropan-2-ol, O – H₂O/H⁺
Reason: M: \(\mathrm{CH_3Cl+2Li\xrightarrow{dry\ ether}CH_3Li+LiCl}\). N: ethanoyl chloride takes two molecules of the reagent: \(\mathrm{CH_3COCl+2CH_3MgBr\longrightarrow(CH_3)_3C-OMgBr+MgBrCl}\), then \(\mathrm{(CH_3)_3C-OMgBr\xrightarrow{H_2O/H^+}(CH_3)_3C-OH}\) (2-methylpropan-2-ol). O: an ester of methanoic acid with Grignard’s reagent, followed by hydrolysis with water and acid, gives a secondary alcohol, so O is \(\mathrm{H_2O/H^+}\) (other esters give tertiary alcohols).
Question 6. What is Z in the following chemical equation? \(\mathrm{R-X+Mg}\) (dry ether/Δ) gives Z
- a) Alkyl magnesium
- b) Magnesium halide
- c) Alkyl magnesium halide
- d) Alkane
Correct Answer: c) Alkyl magnesium halide
Reason: Magnesium inserts into the carbon-halogen bond: \(\mathrm{R-X+Mg\xrightarrow{dry\ ether}R-Mg-X}\). The product \(\mathrm{RMgX}\) is an alkyl magnesium halide, called Grignard’s reagent.
Question 7. What is the chemical name of Zeise’s salt?
- a) Potassium dichloro(ethylene) platinate(I) monohydrate
- b) Potassium trichloro(ethylene) platinate(I) monohydrate
- c) Potassium trichloro(ethylene) platinate(II) monohydrate
- d) Potassium dichloro(ethylene) platinate(II) monohydrate
Correct Answer: c) Potassium trichloro(ethylene) platinate(II) monohydrate
Reason: The formula is \(\mathrm{K[PtCl_3(C_2H_4)]\cdot H_2O}\): three chloro ligands, one ethylene (neutral). Oxidation state of Pt: \(x+3(-1)+0=-1\Rightarrow x=+2\), so platinate(II), and one water of crystallisation gives monohydrate.
Question 8. What happens when alkyl cyanide is treated with Grignard’s reagent?
- a) Aldehyde is formed
- b) Alcohol is formed
- c) Ketone is formed
- d) Carboxylic acid is formed
Correct Answer: c) Ketone is formed
Reason: \(\mathrm{R’C\equiv N+RMgX\longrightarrow R’C(R)=NMgX\xrightarrow{H_2O/H^+}R’COR+NH_3+Mg(OH)X}\). The imine salt is hydrolysed to a ketone (only HCN itself gives an aldehyde).
3. Numerical and Subjective Questions
Short Answer Questions
a. How is Grignard’s reagent prepared? Explain with an example
Magnesium turnings are heated with an alkyl or aryl halide in dry ether. Magnesium donates one electron to the C-X bond, giving a radical pair which combines to form the alkyl magnesium halide.
b. Can we prepare Grignard’s reagent from ethane?
Not directly, because the C-H bond of an alkane does not react with magnesium. But it can be done in two steps: ethane is first converted into a halide, and the halide then gives the reagent.
c. Why can’t we use water instead of ether?
Grignard’s reagent is a very strong base and water has acidic hydrogen, so the reagent is destroyed at once and an alkane forms instead of the reagent:
Dry ether has no acidic hydrogen and its oxygen lone pairs coordinate with magnesium, which dissolves and stabilises the reagent.
Zeise’s salt
Why organometallic. The pi electrons of the C=C bond of ethene are bonded to platinum, so the compound has a bond between a metal and carbon.
Name. Potassium trichloro(ethylene)platinate(II) monohydrate, \(\mathrm{K[PtCl_3(C_2H_4)]\cdot H_2O}\). It is prepared from potassium tetrachloroplatinate(II) and ethene:
Use. It is one of the first organometallic compounds known and is used as a model compound to study the bonding between a metal and an alkene, and as a starting material for other platinum-alkene complexes.
Similarity with Grignard’s reagent. Both contain a direct metal-carbon bond (Pt-C in Zeise’s salt, Mg-C in R-MgX), so both are organometallic compounds. In both the carbon is bonded to a metal, which makes the organic group reactive.
Figure: Zeise’s salt, K[PtCl₃(C₂H₄)]·H₂O. The C=C π bond of ethene is bonded sideways to platinum(II)
Definition, examples and the nature of the bond
Definition. Organometallic compounds are compounds that contain at least one direct bond between a metal atom and a carbon atom of an organic group.
| Type | Example | Preparation |
|---|---|---|
| Organolithium | Methyllithium, \(\mathrm{CH_3Li}\) | \(\mathrm{CH_3Cl+2Li\xrightarrow{dry\ ether}CH_3Li+LiCl}\) |
| Organocadmium | Dimethylcadmium, \(\mathrm{(CH_3)_2Cd}\) | \(\mathrm{2CH_3MgBr+CdCl_2\longrightarrow(CH_3)_2Cd+2MgBrCl}\) |
| Organocopper | Lithium dimethylcuprate, \(\mathrm{(CH_3)_2CuLi}\) (Gilman reagent) | \(\mathrm{2CH_3Li+CuI\longrightarrow(CH_3)_2CuLi+LiI}\) |
Nature of the bond. Carbon (electronegativity 2.5) is more electronegative than the metals, so the bond is polarised \(\mathrm{C^{\delta-}-M^{\delta+}}\). With strongly electropositive metals (Li, Mg) the bond is highly polar, the carbon acts like a carbanion and the compound is a strong nucleophile and base. With Cd and Cu the bond is less polar and more covalent, so these reagents are milder. In compounds of transition metals, such as Zeise’s salt, the sigma donation of the ligand to the metal is accompanied by pi back-donation from filled d orbitals of the metal into the empty antibonding pi orbital of the ligand.
a. Grignard’s reagent is treated with water
An alkane is formed, with magnesium hydroxy halide.
b. Ethyl ethanoate is treated with methyl magnesium chloride
Two molecules of the reagent react with the ester and a tertiary alcohol, 2-methylpropan-2-ol, is formed after hydrolysis. Propanone is formed as an intermediate.
c. Methyl magnesium chloride is heated with ethanoyl chloride
Ethanoyl chloride also takes two molecules of the reagent and gives the same tertiary alcohol, 2-methylpropan-2-ol.
a. Methane
Treat the reagent with water (or any compound with acidic hydrogen).
b. Ethyl alcohol
Treat with methanal, then hydrolyse.
c. Acetic acid
Pass carbon dioxide into the reagent, then hydrolyse.
d. Isopropyl alcohol
Treat with ethanal, then hydrolyse.
e. Propanol (propan-1-ol)
Treat with ethylene oxide (oxirane), then hydrolyse. The chain grows by two carbon atoms.
a. Grignard’s reagent from an aryl halide
An aryl halide is treated with magnesium in dry ether or tetrahydrofuran (THF), usually with heating. Aryl halides react more slowly than alkyl halides, so THF is often used.
Figure: Grignard reagent from an aryl halide
b. Alcohols of formula C₃H₈O
| Structural formula | IUPAC name | |
|---|---|---|
| Primary alcohol | \(\mathrm{CH_3CH_2CH_2OH}\) | Propan-1-ol |
| Secondary alcohol | \(\mathrm{CH_3CH(OH)CH_3}\) | Propan-2-ol |
The third isomer of \(\mathrm{C_3H_8O}\), methoxyethane \(\mathrm{CH_3OCH_2CH_3}\), is an ether and is not an alcohol.
c. Preparation of these alcohols from Grignard’s reagent
Propan-1-ol (primary): ethyl magnesium bromide with methanal.
Propan-2-ol (secondary): methyl magnesium bromide with ethanal.
Differences in the reactions
| SN | HCN + RMgX | RCN + RMgX |
|---|---|---|
| 1. | The intermediate adduct is \(\mathrm{R-CH=N-MgX}\) | The intermediate adduct is \(\mathrm{R’-C(R)=N-MgX}\) |
| 2. | Hydrolysis gives an aldehyde, \(\mathrm{RCHO}\) | Hydrolysis gives a ketone, \(\mathrm{R’COR}\) |
| 3. | Example: \(\mathrm{CH_3MgCl+HCN}\) gives ethanal | Example: \(\mathrm{CH_3MgCl+CH_3CN}\) gives propanone |
Differences in the reactions
| SN | RCOOR’ + RMgX | RCOCl + RMgX |
|---|---|---|
| 1. | The leaving group is the alkoxide \(\mathrm{R’O^-}\) | The leaving group is the chloride \(\mathrm{Cl^-}\) |
| 2. | Gives a tertiary alcohol, together with R’OH (a secondary alcohol from methanoate esters) | Gives a tertiary alcohol, together with the magnesium salt \(\mathrm{Mg(OH)Cl}\) |
| 3. | Example: ethyl ethanoate gives 2-methylpropan-2-ol | Example: ethanoyl chloride gives 2-methylpropan-2-ol |
Project work table
| Compound | Formula and structure | Characteristics and use |
|---|---|---|
| Zeise’s salt | \(\mathrm{K[PtCl_3(C_2H_4)]\cdot H_2O}\): platinum(II) bonded to three Cl and to the C=C bond of ethene | Yellow crystals; first transition metal-alkene complex; model for metal-alkene bonding |
| Tetraethyl lead | \(\mathrm{(C_2H_5)_4Pb}\): four ethyl groups bonded to lead | Anti-knock additive in petrol, phased out because of its toxicity |
| Gilman reagent | \(\mathrm{R_2CuLi}\), for example \(\mathrm{(CH_3)_2CuLi}\) | Forms C-C bonds with alkyl halides (coupling reactions) |
| Ziegler-Natta catalyst | \(\mathrm{TiCl_4}\) with \(\mathrm{Al(C_2H_5)_3}\) | Polymerisation of ethene and propene to polyethene and polypropene |
| Ferrocene | \(\mathrm{Fe(C_5H_5)_2}\): an Fe²⁺ ion between two cyclopentadienyl rings (sandwich) | Orange, stable, aromatic rings; used as a fuel additive and in research |
Figure: Ferrocene, Fe(C₅H₅)₂, a sandwich compound (each ring is bonded to iron through all five carbon atoms)
4. View PDF Notes
i. Handwritten Notes
ii. Typed PDF Notes
5. Frequently Asked Questions
What are organometallic compounds? Give the definition and examples. (Organometallic compounds Class 12 Notes)
Give a list of organometallic compounds with their formulas. (Organometallic compounds list and examples)
What is the nature of the metal-carbon bond in organometallic compounds?
How is Grignard’s reagent prepared and why is dry ether used?
What are the reactions of Grignard’s reagent with aldehydes, ketones, carbon dioxide, esters and acid chlorides?
What are the products of Grignard’s reagent with HCN and with alkyl cyanide (RCN)?
What is Zeise’s salt? Write its formula, name and structure.
What are Gilman reagent, tetraethyl lead, ferrocene and the Ziegler-Natta catalyst used for?
Where can I get Organometallic compounds notes pdf, handwritten notes and the book pdf free download?
What topics are included in Unit 17 Organometallic Compounds of NEB Class 12 Chemistry?
Where can I get Class 12 Chemistry notes of NEB (handwritten, PDF, Nepal)?
Students also search for: