Question

Why is water used to quench a reaction involving lithium or Grignard reagents?

Answer 1

Simply because water reacts with lithium and Grignard reagents.

Explanation:

If you are preparing a lithium or Grignard reagent, the alkyl halide must be the only electrophile in the reaction.

`"R-X" + "2Li" → stackrelcolor(blue)(color(white)(l)δ^-)("R")"-"stackrelcolor(blue)(color(white)(l)δ^+)("Li") + "LiX"`

The `"C-Metal"` bond is highly polar, and the partial negative charge makes the carbon atom a strong base.

Water is a protic substance — it reacts as an acid and converts the alkyl group to an alkane.

`stackrelcolor(blue)(color(white)(l)δ^-)("R")"-"stackrelcolor(blue)(color(white)(l)δ^+)("Li") + "H-OH" → "R-H" + "LiOH"`

Moral: Always use dry ether when reacting alkyl halides with metals.