Alkyne and Grignard Reagent Reaction

Alkynes which have terminal hydrogen atom react with grignard reagent and give an alkane as the product. Grignard reagent is a strong nucleophile and a base. Therefore, they can take protons from acidic compounds. Finally, grignard reagent is destroyed due to reaction with alkyne which has terminal hydrogen atom.

We consider grignard reagent as CH₃MgBr.

Mechanism of alkyne and grignard reagent reaction

• Terminal hydrogen atom (acidic hydrogen atom) has a small positive charge. Because of that, those kind of alkynes shows weak acidic properties.
• Grignard reagents are strong nucleophiles and they like to attack positively charged parts like terminal hydrogen atoms.
• Now, negatively charged alkyl group of grignard reagent attacks terminal hydrogen atom and takes it to form an alkane.
• With that, the bond between carbon atom and terminal hydrogen atom is broken. Electrons of the bond are taken by carbon atom and a carbanion is formed as a result.
• ⁺MgBr part is reached towards anion of alkyl group and form an ionic bond.

Examples of reactions of alkyne which has terminal hydrogen and grignard reagent

Acetylene (ethyne) and CH₃MgBr reaction

Acetylene gas two terminal hydrogen atoms. With methylmagnesium bromide (CH₃MgBr), acetylene reacts and methane is produced by taking a terminal hydrogen atom. If, in the presence of excess CH₃MgBr, other terminal hydrogen bonds reacts with CH₃MgBr and produce more methane.

Finally, grignard reagent is destroyed. Therefore, grignard reagent cannot be produced when respective alkyl halide compound which contains a terminal hydrogen atoms connected to a alkyne group. Produced grignard reagent react with with terminal hydrogen atoms of own grignard reagents's