You are correct when saying that reaction of alkyl bromides with magnesium to form the grignard reagent, followed by quenching in water gives the corresponding hydrocarbon.
However, in such a reaction there cannot be any "active hydrogens" present, or the grignard reagent will not form.
See page 7 and 8 in this document: http://www.chem.umd.edu/courses/chem233mazz/chapternotes/chapter10notes.pdf (http://www.chem.umd.edu/courses/chem233mazz/chapternotes/chapter10notes.pdf)