Q1(d). Why Grignard reactions cannot occur in cells

Introduction. A Grignard reagent, R–MgX, contains a strongly polarised C^(δ-)-Mg^(δ+) bond: its carbon behaves as a carbanion, one of the strongest bases in organic chemistry. It can be made and used only in dry ether, under nitrogen. Cells are the opposite environment.

Scheme: Water (pKₐ about 16) protonates the carbanion carbon (conjugate acid pKₐ about 50) at once.

1. Water. A cell is about 70 % water. Water, an acid of pKₐ≈ 16, protonates R–MgX instantly to give the alkane R–H. Every -OH, -NH, -SH and -COOH group of proteins, sugars and nucleic acids would do the same. A Grignard reagent cannot survive long enough to attack a carbonyl group.
2. Magnesium is not metallic. Grignard reagents are formed from Mg metal (oxidation state 0) and an alkyl halide. In the body magnesium is present only as hydrated Mg²⁺, mostly bound to ATP and to enzymes. There is no reducing metal to insert into a C–X bond, and alkyl halides are not normal metabolites.
3. Oxygen. R–MgX reacts with O₂ to give alkoxides and with CO₂ to give carboxylates; cells are full of both.
4. Ether solvent. The reagent needs ether oxygens to coordinate Mg (Schlenk equilibrium); the cell provides water instead.
5. How cells make C–C bonds. Biology uses stabilised carbon nucleophiles that survive in water near pH 7: enolates of thioesters (acetyl-CoA in Claisen and aldol-type steps), enamines (class I aldolase), and the thiamine (TPP) ylide. Here Mg²⁺ acts only as a Lewis acid that holds and activates phosphate groups.

Conclusion. Grignard reagents are carbanion-like bases that are destroyed at once by water, O₂ and CO₂, and they need metallic Mg and dry ether. The cell provides water at pH 7.4 and only Mg²⁺, so it makes C–C bonds through enzyme-controlled enolate and enamine chemistry instead.

Application. The same fact is used in the laboratory: D₂O quench of a Grignard reagent puts deuterium on a chosen carbon, and CH₃MgI with an -OH compound measures active hydrogen (Zerewitinoff method).