Why can't you use acidic conditions (such as aqueous hydrochloric acid) for the addition of a grignard reagent to a ketone?

Grignard Reagents are very reactive class of Organo-metallic compounds. Grignard reagent is composed of Alkyl group attached through polar covalent bond to Magnesium and a Halogen atom attached via ionic bond to magnesium. Hence, being more reactive in nature it can readily react with any moisture (water) or any other Acid capable of donating H⁺ to produce hydrocarbons.

Examples:

                      CH₃MgBr  +  H₂O   →    CH₄  +  HOMgBr

                      CH₃MgCl  +  HCl    →    CH₄  +  MgCl₂

Result:
          When Grignard reagent is treated with aqueous HCl it will produce Methane (or any other Hydrocarbon) gas instead of reacting with Ketone to produce tertiary Alcohol.