Class 12 Chemistry Organometallic Compounds Notes

Class 12 Chemistry Organometallic Compounds Notes
Unit 17
Organic Chemistry
Class 12 Chemistry

Organometallic Compounds

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NEB/CDC syllabus scope: Unit 17 is a 2-teaching-hour chapter covering the introduction, general formula and examples of organolithium, organocopper and organocadmium compounds; nature of the metal–carbon bond; definition and preparation of Grignard reagents from haloalkanes and haloarenes; and reactions of Grignard reagents with water, aldehydes, ketones, carbon dioxide, HCN, nitriles (RCN), esters and acid chlorides.

1. Introduction to Organometallic Compounds

Definition An organometallic compound contains at least one direct bond between a metal atom and a carbon atom belonging to an organic group.
General feature: C–M bond

Here M represents a metal such as Li, Mg, Cu or Cd.

Examples include methyllithium (CH₃Li), a Grignard reagent such as CH₃MgBr, organocopper compounds and organocadmium compounds.

What is not organometallic? A compound containing a metal and an organic group is not automatically organometallic. A direct metal–carbon bond is required. Therefore sodium ethoxide, C₂H₅ONa, and sodium acetate, CH₃COONa, are not classified as organometallic compounds because the metal is bonded through oxygen rather than directly to carbon.
Essential Feature of an Organometallic Compound Organometallic R–M direct carbon–metal bond Not organometallic R–O–M metal bonded through oxygen The C–M bond is the defining structural criterion.

Diagram 1: Direct C–M bond defines organometallic compounds

2. General Formula and Examples

ClassTypical general formulaExampleName
OrganolithiumRLiCH₃LiMethyllithium
OrganolithiumRLiC₄H₉LiButyllithium
OrganocopperR₂CuLi (common cuprate form)(CH₃)₂CuLiLithium dimethylcuprate
OrganocadmiumR₂Cd(C₂H₅)₂CdDiethylcadmium
OrganomagnesiumRMgXC₂H₅MgBrEthylmagnesium bromide

The syllabus emphasizes organolithium, organocopper and organocadmium examples briefly, then focuses mainly on organomagnesium halides—Grignard reagents.

Important Organometallic Families Organolithium R–Li CH₃Li Organocopper R₂CuLi (CH₃)₂CuLi Organocadmium R₂Cd (C₂H₅)₂Cd Grignard RMgX C₂H₅MgBr All contain a direct bond between carbon and a metal. R = alkyl/aryl group; X = halogen in a Grignard reagent.

Diagram 2: Major organometallic classes in the syllabus

3. Nature of the Metal–Carbon Bond

In many organometallic compounds of electropositive metals, carbon is more electronegative than the metal. The bond is therefore strongly polarized:

Cδ− — Mδ+

The carbon bonded to the metal behaves as though it has carbanion-like character. It is therefore strongly nucleophilic and strongly basic.

Polar Nature of the C–M Bond C δ− M δ+ electron density toward carbon Carbon behaves as a strong nucleophile/base and attacks electrophilic carbon atoms.

Diagram 3: Polarization of a metal–carbon bond

Why this matters for Grignard chemistry In RMgX, the organic group behaves approximately like R:⁻. This carbon nucleophile attacks the positively polarized carbon of C=O groups, which is why Grignard reagents are powerful carbon–carbon bond-forming reagents.

4. Grignard Reagent

Definition A Grignard reagent is an organomagnesium halide having the general formula RMgX, where R is an alkyl or aryl group and X is Cl, Br or I.
FormulaName
CH₃MgBrMethylmagnesium bromide
C₂H₅MgClEthylmagnesium chloride
C₆H₅MgBrPhenylmagnesium bromide
Moisture sensitivity Grignard reagents are destroyed by water, alcohols, acids and other substances containing sufficiently acidic hydrogen. All glassware and solvent must therefore be dry during preparation and use.

5. Preparation of Grignard Reagents

5.1 From Haloalkanes

A haloalkane is treated with magnesium metal in absolutely dry ether.

R–X + Mg → R–MgX   (dry ether)

Example

C₂H₅Br + Mg → C₂H₅MgBr   (dry ether)

5.2 From Haloarenes

Ar–X + Mg → Ar–MgX   (dry ether)

Example

C₆H₅Br + Mg → C₆H₅MgBr   (dry ether)
Preparation of Grignard Reagents R–X haloalkane Mg absolutely dry ether R–MgX alkyl Grignard reagent Ar–X haloarene Mg absolutely dry ether Ar–MgX aryl Grignard reagent Ether coordinates to Mg and helps stabilize the reagent in solution.

Diagram 4: Preparation from haloalkane and haloarene

6. Why Is Absolutely Dry Ether Used?

Dry ether performs two important roles:

  1. It acts as a solvent and coordinates to magnesium, stabilizing the Grignard reagent.
  2. It must be water-free because even traces of water destroy RMgX.
RMgX + H₂O → RH + Mg(OH)X
Exam sentence “Grignard reagent is prepared in absolutely dry ether because moisture decomposes it by protonating the carbon bonded to magnesium.”

7. Reaction of Grignard Reagent with Water

Grignard reagents are very strong bases and abstract a proton from water, forming a hydrocarbon.

RMgX + H₂O → RH + Mg(OH)X

Example

C₂H₅MgBr + H₂O → C₂H₆ + Mg(OH)Br
Grignard + Water → Hydrocarbon R–MgX strong base H₂O proton source R–H alkane + Mg(OH)X This reaction explains the need for completely dry conditions.

Diagram 5: Protonation of Grignard reagent by water

8. Reactions with Aldehydes and Ketones

Grignard reagents add to the carbonyl carbon. The first step forms a magnesium alkoxide; acidic hydrolysis then gives an alcohol.

8.1 With Methanal → Primary Alcohol

RMgX + HCHO → RCH₂OMgX → RCH₂OH   (H₃O⁺)
Carbon-count rule Methanal adds one carbon atom to the organic group R and gives a primary alcohol.

Example

CH₃MgBr + HCHO → CH₃CH₂OMgBr → CH₃CH₂OH

8.2 With Other Aldehydes → Secondary Alcohol

RMgX + R′CHO → R′CH(OMgX)R → R′CH(OH)R

Example

CH₃MgBr + CH₃CHO → (CH₃)₂CHOMgBr → (CH₃)₂CHOH

8.3 With Ketones → Tertiary Alcohol

RMgX + R′COR″ → R′C(OMgX)(R)(R″) → R′C(OH)(R)(R″)

Example

CH₃MgBr + (CH₃)₂CO → (CH₃)₃COMgBr → (CH₃)₃COH
Grignard + Carbonyl → Alcohol RMgX carbon nucleophile + HCHO alkoxide RCH₂OH 1° alcohol + R′CHO alkoxide R′CH(OH)R 2° alcohol + R′COR″ alkoxide R′C(OH)(R)(R″) 3° alcohol Final step in every case: acidic hydrolysis of the magnesium alkoxide. Methanal → 1° alcohol; other aldehyde → 2° alcohol; ketone → 3° alcohol.

Diagram 6: Alcohol classification from Grignard + carbonyl reactions

9. Reaction with Carbon Dioxide

Grignard reagent attacks CO₂ to form a magnesium carboxylate. Acidic hydrolysis gives a carboxylic acid.

RMgX + CO₂ → RCOOMgX RCOOMgX + H₃O⁺ → RCOOH + Mg-containing salts

Example

CH₃MgBr + CO₂ → CH₃COOMgBr → CH₃COOH
Carbon-count rule Reaction with CO₂ increases the carbon chain by one carbon atom.
Grignard + CO₂ → Carboxylic Acid RMgX CO₂ then H₃O⁺ RCOOH carboxylic acid CO₂ contributes the new carboxyl carbon.

Diagram 7: Carboxylation of a Grignard reagent

10. Reaction with HCN

HCN contains an acidic hydrogen. Because a Grignard reagent is a very strong base, the dominant school-level reaction is proton abstraction, producing the corresponding hydrocarbon.

RMgX + HCN → RH + MgXCN
Concept This reaction belongs to the same general pattern as reaction with water: RMgX + H–A → RH when H–A is sufficiently acidic.
Safety Hydrogen cyanide is acutely lethal. This equation is included only as syllabus chemistry and must never be treated as an experimental instruction.

11. Reaction with Nitriles (R′CN)

A Grignard reagent adds to the electrophilic carbon of a nitrile. The intermediate imine magnesium salt gives a ketone after acidic hydrolysis.

RMgX + R′C≡N → R′C(=N–MgX)R → R′COR   (H₃O⁺)

Example

CH₃MgBr + CH₃CN → intermediate → CH₃COCH₃
Grignard + Nitrile → Ketone RMgX nucleophile R′–C≡N nitrile R′–CO–R ketone after hydrolysis The nitrile carbon becomes the carbonyl carbon of the ketone.

Diagram 8: Ketone synthesis using a nitrile

12. Reaction with Esters

Esters normally react with two equivalents of a Grignard reagent. The first addition/elimination step gives a ketone intermediate; the ketone then reacts rapidly with a second equivalent. Acidic work-up gives a tertiary alcohol.

R′COOR″ + 2RMgX → R′C(OMgX)(R)₂ + R″OMgX → R′C(OH)(R)₂

Example: Ethyl Ethanoate + CH₃MgBr

CH₃COOC₂H₅ + 2CH₃MgBr → (CH₃)₃COMgBr → (CH₃)₃COH
Why two moles? The first equivalent converts the ester effectively into a ketone intermediate. Ketones are reactive toward Grignard reagents, so a second equivalent adds before final hydrolysis.
Ester + 2 RMgX → Tertiary Alcohol R′COOR″ ester 1st RMgX ketone intermediate 2nd RMgX tertiary alkoxide H₃O⁺ R′C(OH)(R)₂ tertiary alcohol Two C–C bond-forming additions normally occur before hydrolysis.

Diagram 9: Two Grignard additions to an ester

13. Reaction with Acid Chlorides

Acid chlorides also normally react with two equivalents of a Grignard reagent. The first equivalent gives a ketone after loss of chloride; the ketone then reacts with a second equivalent to form a tertiary alkoxide, which yields a tertiary alcohol after hydrolysis.

R′COCl + 2RMgX → R′C(OMgX)(R)₂ → R′C(OH)(R)₂

Example

CH₃COCl + 2CH₃MgBr → (CH₃)₃COMgBr → (CH₃)₃COH
Compare with nitrile A nitrile + one equivalent of Grignard gives a ketone after hydrolysis. An acid chloride generally does not stop at the ketone with ordinary Grignard conditions because the ketone intermediate reacts again, giving a tertiary alcohol.
Acid Chloride + 2 RMgX → Tertiary Alcohol R′COCl acid chloride ketone after first addition 3° alcohol after second RMgX + H₃O⁺ 1st RMgX 2nd RMgX The ketone intermediate is more reactive toward RMgX than the starting acid chloride product pathway allows isolation. Therefore ordinary Grignard conditions lead to tertiary alcohol after work-up.

Diagram 10: Double addition to an acid chloride

14. High-Yield Grignard Reaction Summary

Reactant with RMgXIntermediate / key processProduct after hydrolysis
H₂OProton transferRH (alkane)
HCHOCarbonyl addition1° alcohol, RCH₂OH
R′CHOCarbonyl addition2° alcohol
R′COR″Carbonyl addition3° alcohol
CO₂CarboxylationRCOOH
HCNAcid–base proton transferRH
R′CNAddition to nitrileKetone, R′COR
Ester, R′COOR″Two additionsUsually 3° alcohol
Acid chloride, R′COClTwo additions3° alcohol
Complete NEB Grignard Reaction Map RMgX Grignard reagent H₂O / HCN → RH HCHO → 1° alcohol R′CHO → 2° alcohol Ketone → 3° alcohol CO₂ → RCOOH R′CN → ketone Ester → 3° alcohol RCOCl → 3° alcohol Carbonyl/nitrile/acyl reactions require acidic work-up after the organomagnesium intermediate forms.

Diagram 11: Complete Grignard reaction map for Unit 17

15. Common Exam Mistakes

  • Calling any organic metal salt an organometallic compound. A direct C–M bond is required.
  • Writing Grignard reagent as R–Mg without the halogen. General formula is RMgX.
  • Forgetting absolutely dry ether during Grignard preparation.
  • Allowing water in the preparation step. Water immediately destroys RMgX to give RH.
  • Writing the C–Mg bond polarization in the wrong direction. Carbon is approximately Cδ− and magnesium is Mgδ+.
  • Forgetting acidic hydrolysis after Grignard addition to a carbonyl, nitrile, ester, acid chloride or CO₂.
  • Writing methanal + RMgX as a secondary alcohol. Methanal gives a primary alcohol.
  • Writing an ordinary aldehyde + RMgX as a tertiary alcohol. It gives a secondary alcohol.
  • Writing ketone + RMgX as a secondary alcohol. It gives a tertiary alcohol.
  • Forgetting that CO₂ adds one carbon and gives a carboxylic acid after work-up.
  • Confusing HCN with RCN. HCN mainly protonates the Grignard reagent to give RH, whereas an organic nitrile RCN undergoes addition and hydrolysis to give a ketone.
  • Stopping ester or acid-chloride reactions at the ketone under ordinary Grignard conditions; a second Grignard addition normally occurs.
  • Using wet alcohol as solvent. Alcohol also protonates and destroys RMgX.

16. Worked Examples

Worked Example 1: Prepare ethylmagnesium bromide C₂H₅Br + Mg → C₂H₅MgBr   (dry ether)

The apparatus and solvent must be completely dry.

Worked Example 2: CH₃MgBr + methanal CH₃MgBr + HCHO → CH₃CH₂OMgBr → CH₃CH₂OH

Product: ethanol, a primary alcohol.

Worked Example 3: CH₃MgBr + ethanal CH₃MgBr + CH₃CHO → (CH₃)₂CHOMgBr → (CH₃)₂CHOH

Product: propan-2-ol, a secondary alcohol.

Worked Example 4: CH₃MgBr + propanone CH₃MgBr + (CH₃)₂CO → (CH₃)₃COMgBr → (CH₃)₃COH

Product: 2-methylpropan-2-ol, a tertiary alcohol.

Worked Example 5: Grignard + CO₂ C₂H₅MgBr + CO₂ → C₂H₅COOMgBr → C₂H₅COOH

Product: propanoic acid. The chain gains one carbon.

Worked Example 6: Grignard + nitrile C₂H₅MgBr + CH₃CN → intermediate → CH₃COC₂H₅

Product after hydrolysis: butan-2-one.

17. Important Exam Questions

Short-Answer Questions

  1. Define an organometallic compound with two examples.
  2. Why is sodium ethoxide not an organometallic compound?
  3. Write the general formula and one example each of organolithium, organocopper and organocadmium compounds.
  4. Explain the polar nature of the metal–carbon bond.
  5. Define Grignard reagent and write its general formula.
  6. How is ethylmagnesium bromide prepared?
  7. How is phenylmagnesium bromide prepared?
  8. Why is dry ether essential in Grignard preparation?
  9. What happens when RMgX reacts with water?
  10. What type of alcohol is obtained from RMgX + methanal?
  11. What type of alcohol is obtained from RMgX + an aldehyde other than methanal?
  12. What type of alcohol is obtained from RMgX + ketone?
  13. What is formed when RMgX reacts with CO₂ followed by hydrolysis?
  14. What is the reaction of a Grignard reagent with HCN?
  15. What is obtained from a nitrile + RMgX followed by hydrolysis?
  16. Why do esters normally require two equivalents of Grignard reagent?
  17. What product is normally obtained from acid chloride + excess RMgX?

Long-Answer Questions

  1. Define organometallic compounds and discuss organolithium, organocopper and organocadmium compounds with examples.
  2. Explain the nature and polarity of the metal–carbon bond.
  3. Describe preparation of Grignard reagents from haloalkanes and haloarenes.
  4. Explain why Grignard reactions require strictly dry conditions.
  5. Explain preparation of primary, secondary and tertiary alcohols using Grignard reagents.
  6. Describe reactions of RMgX with water and CO₂.
  7. Differentiate the reactions of RMgX with HCN and RCN.
  8. Explain the reaction of Grignard reagent with esters.
  9. Explain the reaction of Grignard reagent with acid chlorides.

Conversion Questions

  1. Bromoethane → ethylmagnesium bromide.
  2. Bromobenzene → phenylmagnesium bromide.
  3. CH₃MgBr + HCHO → ethanol.
  4. CH₃MgBr + CH₃CHO → propan-2-ol.
  5. CH₃MgBr + CH₃COCH₃ → 2-methylpropan-2-ol.
  6. C₂H₅MgBr + CO₂ → propanoic acid.
  7. CH₃MgBr + CH₃CN → propanone.
  8. Ethyl ethanoate + CH₃MgBr → tertiary alcohol.
  9. Ethanoyl chloride + CH₃MgBr → tertiary alcohol.

Diagram Questions

  1. Draw the defining direct C–M bond in an organometallic compound.
  2. Draw examples of organolithium, organocopper, organocadmium and Grignard compounds.
  3. Draw the polarization Cδ−–Mδ+.
  4. Draw preparation of Grignard reagent from haloalkane and haloarene.
  5. Draw RMgX + water reaction.
  6. Draw the alcohol-classification map for methanal, aldehydes and ketones.
  7. Draw RMgX + CO₂ carboxylation.
  8. Draw RMgX + nitrile → ketone.
  9. Draw the two-addition sequence with an ester.
  10. Draw the two-addition sequence with an acid chloride.
  11. Draw the complete Grignard reaction map.
Exam Strategy This is only a 2-hour unit, but it is highly reaction-based. First memorize RMgX preparation + dry ether. Then learn the product map: H₂O/HCN → alkane; HCHO → 1° alcohol; aldehyde → 2°; ketone → 3°; CO₂ → acid; RCN → ketone; ester/acid chloride → usually 3° alcohol.

18. One-Minute Revision

  • Organometallic compounds contain at least one direct carbon–metal bond.
  • Typical families: RLi, R₂CuLi, R₂Cd and RMgX.
  • The C–M bond is polarized Cδ−–Mδ+ for electropositive metals.
  • Carbon therefore behaves as a strong nucleophile/base.
  • Grignard reagent general formula = RMgX.
  • RX + Mg in absolutely dry ether gives RMgX.
  • Haloarenes can similarly give ArMgX.
  • Water destroys RMgX: RMgX + H₂O → RH.
  • Methanal + RMgX → primary alcohol after hydrolysis.
  • Other aldehyde + RMgX → secondary alcohol.
  • Ketone + RMgX → tertiary alcohol.
  • CO₂ + RMgX → carboxylic acid after hydrolysis.
  • CO₂ adds one carbon atom to the organic chain.
  • HCN protonates RMgX to give RH.
  • RCN + RMgX → ketone after hydrolysis.
  • Ester + 2 RMgX → usually tertiary alcohol after hydrolysis.
  • Acid chloride + 2 RMgX → tertiary alcohol after hydrolysis.
  • Acidic work-up is required after nucleophilic addition to obtain the neutral organic product.
  • Never use water or alcohol as the solvent for Grignard preparation.

19. Diagram Practice

Students should practice these labelled diagrams for the NEB examination:

  1. Direct carbon–metal bond criterion.
  2. Organolithium, organocopper, organocadmium and Grignard examples.
  3. Polarization of the metal–carbon bond.
  4. Preparation of RMgX and ArMgX in dry ether.
  5. Reaction of Grignard reagent with water.
  6. Formation of 1°, 2° and 3° alcohols from carbonyl compounds.
  7. Carboxylation with CO₂.
  8. Nitrile → ketone using RMgX.
  9. Ester + two equivalents of RMgX.
  10. Acid chloride + two equivalents of RMgX.
  11. Complete Grignard product map.
Source handling: The original Nepal eNotes PDF remains embedded above. The typed section follows the verified NEB/CDC syllabus and is designed as a searchable, responsive study companion. Where the PDF viewer does not expose handwritten page text, the typed section is a syllabus-aligned reconstruction and is not claimed to be a word-for-word transcription.
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