Grignard Reagent, Reaction, Mechanism and Shortcut

Leah4sci
18 Feb 202009:12
EducationalLearning
32 Likes 10 Comments

TLDRThis video from Leah4Sci.com dives into Grignard reagents, detailing their formation, reactivity, and mechanism. Grignard reagents, organometallic compounds with a carbon chain bound to magnesium and a halogen, are crucial for chain elongation in organic chemistry. The video explains the formation process using alkyl halides and magnesium in an aprotic solvent like ether or THF, emphasizing the avoidance of polar protic solvents to prevent premature reaction termination. It also outlines the Grignard reaction mechanism with carbonyl compounds, leading to the formation of alcohols, and provides a mini cheat sheet for common reactions on leah4sci.com/grignard.

Takeaways
  • 🧪 Grignard reagents are organometallic compounds with a carbon chain bound to magnesium and a halogen, named after Victor Grignard, who won a Nobel Prize for their discovery.
  • 🔬 The electronegativity difference between carbon and magnesium creates a partial negative charge on the carbon, making it highly reactive, almost like a carbanion.
  • đź“š Grignard reagents are used for chain elongation in organic chemistry, being the strongest among organometallics.
  • 🌟 The formation of a Grignard reagent involves the reaction of an alkyl halide with magnesium in the presence of an ether solvent, such as diethyl ether or THF.
  • ⚠️ Polar protic solvents like water or alcohol should not be used because they can react with the Grignard reagent, leading to its destruction.
  • đź›  The Grignard reaction mechanism involves the attack of the carbon ion-like carbon on a partially positive carbonyl carbon, followed by rearrangement to form a new carbon-carbon bond.
  • 🔍 The reaction with a carbonyl compound, such as acetone, results in the formation of an alcohol, with the Grignard reagent acting as a nucleophile.
  • đź“ť The final step in the Grignard reaction involves the addition of water or acid to protonate the oxygen, leading to the formation of the alcohol product.
  • đź“‹ Leah provides a mini cheat sheet for Grignard reactions on her website, leah4sci.com/grignard, to help understand the reactivity and common reactions.
  • đź“ť To deduce the reactants from a given Grignard product, identify the alcohol, break the pi bond, and add the Grignard group and a hydrogen to the oxygen.
  • 🔑 A shortcut for writing Grignard reactions involves showing the Grignard attack first, followed by the acid workup to form the final alcohol product.
Q & A

  • What is a Grignard reagent?

    -A Grignard reagent is an organometallic compound with a carbon chain (R group) bound to a magnesium metal, which is in turn bound to a halogen, typically chlorine, bromine, or iodine. It is used for chain elongation in organic synthesis.

  • Who is the Grignard reagent named after and why?

    -The Grignard reagent is named after Victor Grignard, who won a Nobel Prize for its discovery. It is a significant reagent in organic chemistry due to its reactivity and utility in chain elongation reactions.

  • Why are Grignard reagents considered to be very reactive?

    -Grignard reagents are very reactive because the carbon atom bound to magnesium is almost like a carbanion due to the significant difference in electronegativity between carbon and magnesium, making it very electron-rich and eager to form new bonds.

  • What is the role of the solvent in Grignard reagent formation?

    -The solvent is crucial in Grignard reagent formation because it must be a polar aprotic solvent, such as diethyl ether or THF, to prevent the highly reactive Grignard reagent from attacking the solvent itself, which would destroy the reagent.

  • Why should a polar protic solvent not be used in Grignard reagent formation?

    -A polar protic solvent should not be used because the Grignard reagent would react with the solvent, attacking the hydrogen atom and forming products that are not desired, thus destroying the reagent before it can be used in synthesis.

  • What is the general mechanism of a Grignard reaction with a carbonyl compound?

    -The general mechanism involves the nucleophilic attack of the electron-rich carbon in the Grignard reagent on the partially positive carbonyl carbon. This leads to the formation of an intermediate with a negative charge on oxygen, which is then protonated by an acid to yield the final alcohol product.

  • How does the Grignard reagent facilitate chain elongation in organic synthesis?

    -The Grignard reagent facilitates chain elongation by adding the R group from the Grignard to the carbonyl carbon, effectively extending the carbon chain by one unit and forming a new C-C bond.

  • What is the significance of the order of reagent addition in the Grignard reaction mechanism?

    -The order is significant because the irreversible C-C bond formation must occur first, ensuring the Grignard reagent has reacted before the acid workup is added to protonate the oxygen and form the final alcohol product.

  • How can one determine the reactants if given a product from a Grignard reaction?

    -By identifying the alcohol group in the product, which indicates the site of Grignard attack on a carbonyl, and then reconstructing the carbonyl group and adding an MGX to represent the Grignard reagent used in the reaction.

  • What is the 'Grignard effect' mentioned in the script?

    -The 'Grignard effect' refers to the initial nucleophilic attack of the Grignard reagent on the carbonyl carbon, which is the first step in the reaction mechanism before the acid workup to form the final product.

  • What resource is available for further study on Grignard reactions?

    -For further study, a mini cheat sheet on Grignard reactions and other organometallics is available on Leah4Sci's website at leah4sci.com/grignard.

Outlines
00:00
🧪 Grignard Reagents: Formation and Reactivity

The script introduces Leah from leah4sci.com discussing Grignard reagents, named after Victor Grignard, a Nobel laureate. Grignard reagents are organometallic compounds with a carbon chain bound to magnesium and a halogen, typically used for chain elongation due to their reactivity. The carbon in these reagents is almost like a carbanion, making it highly reactive. The formation of a Grignard reagent involves reacting an alkyl halide with magnesium in the presence of an ether solvent, which is crucial as polar protic solvents would destroy the reagent by reacting with it. The video explains the process and the importance of using the correct solvent to stabilize the Grignard reagent before it can participate in further reactions.

05:01
🔍 Grignard Reaction Mechanism and Practical Tips

This paragraph delves into the mechanism of the Grignard reaction, starting with the attack of the carbon ion-like electrons on a partially positive carbonyl carbon. The reaction results in an intermediate with a formal charge on the oxygen, which is then quenched by the addition of acid or water to yield an alcohol. The script emphasizes the importance of the sequence of steps in the reaction mechanism and warns against the use of polar protic solvents, which would neutralize the Grignard reagent. Leah provides a cheat sheet for Grignard reactions on her website and offers a shortcut for identifying products and reactants in Grignard reactions, highlighting the need to recognize the carbonyl carbon and the addition of the Grignard group. The summary also includes a method for reverse-engineering reactants from a given product, which involves identifying the alcohol group and reconstructing the original carbonyl compound.

Mindmap
Keywords
đź’ˇGrignard reagent
A Grignard reagent is an organometallic compound consisting of an R group (carbon chain) bonded to magnesium and a halogen, typically chlorine, bromine, or iodine. This reagent is highly reactive and useful for chain elongation in organic synthesis. In the video, it is emphasized for its formation, reactivity, and mechanism in various chemical reactions.
đź’ˇCarbanion
A carbanion is a carbon atom with a negative charge, making it very reactive. In the context of Grignard reagents, the carbon bound to magnesium behaves like a carbanion, ready to attack positively charged or partially positive species. This property is crucial for the reactivity of Grignard reagents in forming new carbon-carbon bonds.
đź’ˇEther solvent
Ether solvents, such as diethyl ether (Et2O) and tetrahydrofuran (THF), are non-polar solvents used to dissolve Grignard reagents during their formation and reactions. These solvents are preferred because they do not react with the Grignard reagent, unlike polar protic solvents like water or alcohols, which would destroy the reagent by protonating the carbanion.
đź’ˇOrganometallic
Organometallic compounds contain a carbon-metal bond. Grignard reagents are a type of organometallic compound where the carbon is bonded to magnesium. These compounds are highlighted in the video for their significant role in organic synthesis, particularly in forming carbon-carbon bonds.
đź’ˇElectronegativity
Electronegativity refers to the tendency of an atom to attract electrons towards itself. In Grignard reagents, the difference in electronegativity between carbon and magnesium makes the carbon partially negative and the magnesium partially positive, enhancing the reactivity of the carbon as a nucleophile.
đź’ˇCarbonyl group
A carbonyl group consists of a carbon double-bonded to an oxygen atom. It is a common functional group in organic chemistry and a key target for Grignard reagents. In the video, Grignard reagents are shown to attack carbonyl groups, leading to the formation of alcohols after subsequent protonation.
đź’ˇFormation
Formation in this context refers to the synthesis of Grignard reagents by reacting an alkyl halide with magnesium in an ether solvent. The video details this process, explaining the importance of the solvent choice and the reactivity of the resulting Grignard reagent.
đź’ˇMechanism
Mechanism refers to the step-by-step sequence of events in a chemical reaction. The video covers the mechanism of Grignard reactions, showing how the carbanion attacks electrophilic centers like carbonyl groups, and emphasizing the importance of order in adding reactants to avoid destroying the Grignard reagent.
đź’ˇPolar protic solvent
Polar protic solvents, such as water and alcohols, have hydrogen atoms bound to electronegative atoms (e.g., O-H bonds). These solvents can protonate and destroy Grignard reagents, as illustrated in the video where they react with the carbanion, preventing the intended reaction with carbonyl compounds.
đź’ˇAldehyde
An aldehyde is an organic compound containing a carbonyl group bonded to at least one hydrogen atom. In the video, aldehydes are shown as typical reactants for Grignard reagents, leading to the formation of secondary alcohols upon reaction and subsequent protonation.
Highlights

Introduction to Grignard reagents, their formation, reactivity, and mechanism.

Grignard reagent is named after Victor Grignard, who won a Nobel Prize for this discovery.

Grignard reagents consist of an R group (carbon chain) bound to magnesium and a halogen (chlorine, bromine, or iodine).

Grignard reagents are the strongest of the organometallics and are very useful in chain elongation.

Magnesium in Grignard reagents is partially positive, making the carbon very partially negative, almost like a carbanion.

Carbanions are very reactive due to their negative charge.

Grignard reagents are formed by reacting an alkyl halide with magnesium in the presence of ether.

The solvent used for Grignard formation must not be polar protic.

Ether solvents like diethyl ether (Et2O) or tetrahydrofuran (THF) are commonly used for Grignard formation.

Polar protic solvents like water or alcohols will destroy Grignard reagents by protonation.

Grignard reagents attack the carbonyl carbon in carbonyl compounds, leading to alcohol formation.

The Grignard mechanism involves the carbon-magnesium bond attacking the partially positive carbonyl carbon.

After the Grignard attack, a proton source (like H3O+) is added to form the final alcohol product.

Proper reaction steps should first involve the Grignard attack, followed by acid workup.

Identifying the alcohol product and breaking the bond to deduce the original carbonyl and Grignard reagents is key in exam questions.

Transcripts

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Grignard ReagentsOrganometallicsChemical ReactionsElectronegativityCarbonyl CompoundsChain ElongationVictor GrignardNobel PrizeAcetone ReactionAlcohol Synthesis