Organometallic Reagents

Why are organometallic reagents so reactive?

They are generally made with elements much more electropositive than carbon

The M-C bond is polarised, with a negative dipole on carbon, making the carbon nucleophilic

The smaller the electronegativity difference the more covalent character the bond has → less reactive

What are some factors affecting the stability of organometallic reagents?

• The electropositivity of the metal

• The nature of the carbon (s character)

• For sp3 carbons → no. of electron donating groups

• pKaH can be used as a measure of stability

Why can pKaH be used as a guide to reactivity/stability of organometallics?

Because pKaH reflects how easily the conjugate acid donates a proton — a higher pKaH means the proton is held less tightly, so the conjugate base (the organometallic) is stronger and more reactive

Why does the carbon nature affect the stability of the corresponding organometallic

sp3 < sp2 < sp (most stable)

The higher the s character of the carbon, the closer the -ve charge is held to the nucleus, increasing its stability via proximity to the +ve nuclear charge

What conditions would you expect to have to use in the lab with a highly reactive organometallic reagent?

Use an aprotic solvent, organometallics are basic and nucleophilic

Use an inert atmosphere, e.g. N2 or AR, they react with O2

Use completely dry glassware (oven) as they react violently with water

How do organometallics react with water?

****PHOTO

How do grignard reagents react?

****PHOTO

Charge density allows nucleophilic creation of C-C bones

Intermediate must be quenched (protonated) to reach the product

How are grignard reagents synthesised?

Oxidative addition of Mg(0) to a C-X bond (+ heat)

• In the lab, the surface of the Mg can oxidise, to remove this later we can add iodine beads or dibromoethane

****PHOTO

What organohalides are best at forming grignard reagents?

Bigger halides are more reactive towards oxidative addition, however, this reactivity can become problematic. Alkyl iodides often undergo unwanted side reactions and the grignard reagent is not prodyced.

****PHOTO

Alkyl bromides can be useful but chlorides are often cheaper, and form an insoluble by-product, which is easy to remove.

****PHOTO