UPSC Exam  >  UPSC Notes  >  Chemistry Optional Notes for UPSC  >  Grignard and Organolithium Reagents

Grignard and Organolithium Reagents | Chemistry Optional Notes for UPSC PDF Download

Introduction

The alkali metals (Li, Na, K etc.) and the alkaline earth metals (Mg and Ca, together with Zn) are good reducing agents, the former being stronger than the latter. These same metals reduce the carbon-halogen bonds of alkyl halides. The halogen is converted to a halide anion, and the carbon bonds to the metal which has characteristics similar to a carbanion (R:-).

Formation of Organometallic Reagents

  • Many organometallic reagents are commercially available, however, it is often necessary to make then. The following equations illustrate these reactions for the commonly used metals lithium and magnesium (R may be hydrogen or alkyl groups in any combination).
    • An Alkyl Lithium Reagent:
      Grignard and Organolithium Reagents | Chemistry Optional Notes for UPSC
    • A Grignard Regent:
      Grignard and Organolithium Reagents | Chemistry Optional Notes for UPSC
  • Halide reactivity in these reactions increases in the order: Cl < Br < I and Fluorides are usually not used. The alkyl magnesium halides described in the second reaction are called Grignard Reagents after the French chemist, Victor Grignard, who discovered them and received the Nobel prize in 1912 for this work. The other metals mentioned above react in a similar manner, but Grignard and Alky Lithium Reagents most widely used. Although the formulas drawn here for the alkyl lithium and Grignard reagents reflect the stoichiometry of the reactions and are widely used in the chemical literature, they do not accurately depict the structural nature of these remarkable substances. Mixtures of polymeric and other associated and complexed species are in equilibrium under the conditions normally used for their preparation.
  • A suitable solvent must be used. For alkyl lithium formation pentane or hexane are usually used. Diethyl ether can also be used but the subsequent alkyl lithium reagent must be used immediately after preparation due to an interaction with the solvent. Ethyl ether or THF are essential for Grignard reagent formation. Lone pair electrons from two ether molecules form a complex with the magnesium in the Grignard reagent (As pictured below). This complex helps stabilize the organometallic and increases its ability to react.
    Grignard and Organolithium Reagents | Chemistry Optional Notes for UPSC
  • These reactions are obviously substitution reactions, but they cannot be classified as nucleophilic substitutions, as were the earlier reactions of alkyl halides. Because the functional carbon atom has been reduced, the polarity of the resulting functional group is inverted (an originally electrophilic carbon becomes nucleophilic). This change, shown below, makes alkyl lithium and Grignard reagents excellent nucleophiles and useful reactants in synthesis.

Grignard and Organolithium Reagents | Chemistry Optional Notes for UPSC

Example

Grignard and Organolithium Reagents | Chemistry Optional Notes for UPSC

Question for Grignard and Organolithium Reagents
Try yourself:
What is the product formed when an organolithium reagent reacts with water?
View Solution
 

Common Organometallic Reagents

Grignard and Organolithium Reagents | Chemistry Optional Notes for UPSC

Reaction of Organometallic Reagents with Various Carbonyls

Because organometallic reagents react as their corresponding carbanion, they are excellent nucleophiles. The basic reaction involves the nucleophilic attack of the carbanionic carbon in the organometallic reagent with the electrophilic carbon in the carbonyl to form alcohols.
Grignard and Organolithium Reagents | Chemistry Optional Notes for UPSC

Both Grignard and Organolithium Reagents will perform these reactions.

  • Addition to formaldehyde gives 1° alcohols
    Grignard and Organolithium Reagents | Chemistry Optional Notes for UPSC
  • Addition to aldehydes gives 2° alcohols

    Grignard and Organolithium Reagents | Chemistry Optional Notes for UPSC

  • Addition to ketones gives 3° alcohols
    Grignard and Organolithium Reagents | Chemistry Optional Notes for UPSC

  • Addition to carbon dioxide (CO2) forms a carboxylic acid
    Grignard and Organolithium Reagents | Chemistry Optional Notes for UPSC

Example

Grignard and Organolithium Reagents | Chemistry Optional Notes for UPSC

Going from Reactants to Products Simplified

Grignard and Organolithium Reagents | Chemistry Optional Notes for UPSC

Mechanism for the Addition to Carbonyls

The mechanism for a Grignard agent is shown; the mechanism for an organometallic reagent is the same.

  • Nucleophilic attack
    Grignard and Organolithium Reagents | Chemistry Optional Notes for UPSC
  • Protonation
    Grignard and Organolithium Reagents | Chemistry Optional Notes for UPSC

Organometallic Reagents as Bases

  • These reagents are very strong bases (pKa's of saturated hydrocarbons range from 42 to 50). Although not usually done with Grignard reagents, organolithium reagents can be used as strong bases. Both Grignard reagents and organolithium reagents react with water to form the corresponding hydrocarbon. This is why so much care is needed to insure dry glassware and solvents when working with organometallic reagents.
    Grignard and Organolithium Reagents | Chemistry Optional Notes for UPSC
  • In fact, the reactivity of Grignard reagents and organolithium reagents can be exploited to create a new method for the conversion of halogens to the corresponding hydrocarbon (illustrated below). The halogen is converted to an organometallic reagent and then subsequently reacted with water to from an alkane.

Grignard and Organolithium Reagents | Chemistry Optional Notes for UPSC

Question for Grignard and Organolithium Reagents
Try yourself:
Which of the following functional groups cannot be used with Grignard or organolithium reagents?
View Solution
 

Limitation of Organometallic Reagents

As discussed above, Grignard and organolithium reagents are powerful bases. Because of this they cannot be used as nucleophiles on compounds which contain acidic hydrogens. If they are used they will act as a base and deprotonate the acidic hydrogen rather than act as a nucleophile and attack the carbonyl. A partial list of functional groups which cannot be used are: alcohols, amides, 1o amines, 2o amines, carboxylic acids, and terminal alkynes.

Grignard and Organolithium Reagents | Chemistry Optional Notes for UPSC

Solved Examples

Example 1: Please write the product of the following reactions.
Grignard and Organolithium Reagents | Chemistry Optional Notes for UPSCAns:

Grignard and Organolithium Reagents | Chemistry Optional Notes for UPSC

Example 2: Please indicate the starting material required to produce the product.
Grignard and Organolithium Reagents | Chemistry Optional Notes for UPSC

Ans:
Grignard and Organolithium Reagents | Chemistry Optional Notes for UPSC

Example 3: Please give a detailed mechanism and the final product of this reaction.
Grignard and Organolithium Reagents | Chemistry Optional Notes for UPSC

Ans: 

  • Nucleophilic attack:
    Grignard and Organolithium Reagents | Chemistry Optional Notes for UPSC
  • Protonation
    Grignard and Organolithium Reagents | Chemistry Optional Notes for UPSC

Example 4: Please show two sets of reactants which could be used to synthesize the following molecule using a Grignard reaction.
Ans: 
Grignard and Organolithium Reagents | Chemistry Optional Notes for UPSC

The document Grignard and Organolithium Reagents | Chemistry Optional Notes for UPSC is a part of the UPSC Course Chemistry Optional Notes for UPSC.
All you need of UPSC at this link: UPSC
308 docs

Top Courses for UPSC

FAQs on Grignard and Organolithium Reagents - Chemistry Optional Notes for UPSC

1. What are organometallic reagents?
Ans. Organometallic reagents are compounds that contain a metal-carbon bond. These reagents are widely used in organic synthesis as they can undergo various reactions to form new carbon-carbon or carbon-heteroatom bonds.
2. How are organometallic reagents formed?
Ans. Organometallic reagents can be formed by reacting a metal with an organic halide or by transmetallation reactions. For example, Grignard reagents are formed by reacting alkyl or aryl halides with magnesium metal.
3. What are some common organometallic reagents?
Ans. Some common organometallic reagents include Grignard reagents (RMgX), organolithium reagents (RLi), organocopper reagents (RCu), and organozinc reagents (RZnX). These reagents are widely used in organic synthesis for various reactions.
4. How do organometallic reagents add to carbonyl compounds?
Ans. Organometallic reagents can add to carbonyl compounds (such as aldehydes, ketones, and esters) through a nucleophilic addition reaction. The metal-carbon bond acts as a nucleophile and attacks the electrophilic carbonyl carbon, resulting in the formation of a new carbon-carbon bond.
5. Can organometallic reagents act as bases?
Ans. Yes, organometallic reagents can also act as bases. Due to the presence of a metal-carbon bond, these reagents can abstract a proton from a molecule, making them strong bases. This property is often utilized in various reactions, such as deprotonation of acidic compounds or as a catalyst for certain transformations.
308 docs
Download as PDF
Explore Courses for UPSC exam

Top Courses for UPSC

Signup for Free!
Signup to see your scores go up within 7 days! Learn & Practice with 1000+ FREE Notes, Videos & Tests.
10M+ students study on EduRev
Related Searches

MCQs

,

Viva Questions

,

Grignard and Organolithium Reagents | Chemistry Optional Notes for UPSC

,

Grignard and Organolithium Reagents | Chemistry Optional Notes for UPSC

,

Extra Questions

,

Exam

,

video lectures

,

Previous Year Questions with Solutions

,

mock tests for examination

,

Summary

,

Free

,

pdf

,

Objective type Questions

,

Sample Paper

,

practice quizzes

,

past year papers

,

Semester Notes

,

ppt

,

study material

,

Grignard and Organolithium Reagents | Chemistry Optional Notes for UPSC

,

shortcuts and tricks

,

Important questions

;