Intro into Organometallic reagents Flashcards
What is the carbon-metal bond like in an organometallic?
- Carbon is more electronegative than the metal, so the bond is polarised towards carbon, making the carbon nucleophilic
The more electropositive the metal, the….
the more ionic the bond
What happens to electronegativity as we move from left to right across the periodic table?
The elements increase in electronegativity
Using what you know about the electronegativity of substituents of this compound, predict it’s properties
- Completely covalent
- Very stable, can be purifed and isolated
- Doesn’t react with water
Organometallic chemistry relies on carbon as a nucleophile
Why is this benefical?
Because it allows us to make carbon-carbon bonds
The more electropositive the metal, the….
…more ionic the bond
The potassium in MeK is very electropositive
Hence, deduct the properties
Almost completely ionic
(only stable in solution, will catch fire in air and not stable to water)
They are very reactive organometallic reagents
Why might be use MeMgBr over MeK for synthesis
Mg is less electropositive than K
Hence MeMgBr is more covalent in character, meaning it is stable is solution (won’t catch fire) but is still highly reactive
What happens to the nature of the C-M bond with changing hybridisation
- Increasing stability of C-M bond with increasing s character
- Due to the negative charge on the carbon being more stabilised in orbitals with higher s- character = closer to nucleus
What happens to the nature of the C-M bond with the addition of electron-donating substituents on the carbon
- Decreasing stability with the more electron-donating alkyl groups
- Due to inductively electron-donating alkyl groups destabilising the negative charge on the carbon
What happens to the stability and reactivity going down the list of compounds
- Increasing stability
- Decreasing reactivity
- pKₐₕ can be used as a guide to reativity/stability - lower the pKₐₕ, the more stable the organometallic
Organometallics are highly reactive (very unstable)
What maybe 3 considerations of using them?
- Organometallics are nucleophilic and basic: need to use aprotic solvents
- They react with O₂: nned to use a inert dry atmosphere, such as N₂ or Ar
- React rapidly/violently with H₂O: need to be kept absolutely free of moisture
- (keep an eye out for proton sources)
Fill in the blanks for the following reaction with Grignards
(organometallic compounds are nucleophiles)
How is Grignards formed from the following reagents?
- Grignards reagent are made by oxidative addition of Mg0 into a C-halogen bond
What is the order of reactivity for this reaction for the first 3 halogens?
- Due to bad overlap of the carbon-iodine orbitals
Why are alkyl chlorides more often used?
- Chlorides (R-Cl) are often cheap, and the by-product (MgCl₂) is insoluble and therefoe easy to remove
- Alkyl iodides sometimes undergo an unwanted side-reaction (Wurtz coupling)
