Organometallic Compounds

(c) Tertiary alcohol. A Grignard reagent adds to the carbonyl carbon of a ketone, and hydrolysis of the adduct gives a tertiary alcohol.

R2C=O + R'MgX → R2R'C-OMgX --H2O/H+--> R2R'C-OH (3° alcohol)

(d) i), ii), iii) and v) only. An organometallic compound has at least one metal-carbon bond.

• $CH_3MgI$: Mg-C bond, yes.
• $Ni(CO)_4$: metal carbonyl with Ni-C bonds, yes.
• $C_4H_9Li$: Li-C bond, yes.
• $(C_4H_9)_2SnCl_2$: Sn-C bonds, yes.
• $C_2H_5ONa$: sodium ethoxide, Na is bonded to oxygen, not carbon, so it is not organometallic.

(a) Primary alcohol. Grignard reagent adds to formaldehyde; hydrolysis of the adduct gives a primary alcohol.

HCHO + RMgX → R-CH2-OMgX --H2O/H+--> R-CH2OH (1° alcohol)

Grignard's reagent is an alkyl (or aryl) magnesium halide, $RMgX$, prepared by reacting an alkyl halide with magnesium turnings in dry ether.

R-X + Mg --dry ether--> R-MgX

Its carbon–magnesium bond is highly polar ($R^{\delta-}-Mg^{\delta+}X$), so R acts as a nucleophile towards the carbonyl carbon. The adduct is hydrolysed with dilute acid to the alcohol.

1° alcohol: from formaldehyde

HCHO + RMgX → R-CH2-OMgX --H2O/H+--> R-CH2OH + Mg(OH)X

2° alcohol: from any other aldehyde

R'CHO + RMgX → R'-CH(R)-OMgX --H2O/H+--> R'-CH(OH)-R
(e.g. CH3CHO + CH3MgI → CH3CH(OH)CH3, propan-2-ol)

3° alcohol: from a ketone

R'2C=O + RMgX → R'2C(R)-OMgX --H2O/H+--> R'2C(OH)R
(e.g. CH3COCH3 + CH3MgI → (CH3)3C-OH, 2-methylpropan-2-ol)

(a) Zeise's salt, $K[PtCl_3(\eta^2\text{-}C_2H_4)]$. Ethene is bonded to platinum through its $\pi$-electrons, and it was the first organometallic compound ever synthesised (Zeise, 1827). Ferrocene and dibenzenechromium are $\pi$-bonded but contain cyclopentadienyl/benzene rings, not ethene, and tetraethyltin is a $\sigma$-bonded compound.

(c) ketone

The Grignard reagent adds across the $C\equiv N$ bond to give an imine salt, which is hydrolysed to a ketone: