Answer:
The glassware was not dried before use.
Explanation:
Phenylmagnesium bromide ( grignard reagent ) reacts with water to form benzene
C₆H₅MgBr + WATER--------→ C₆H₆ + Mg (OH) Br
This happens when glassware is not dried before use. It will decrease the effectiveness of the Grignard reagent.