Answer :
Final answer:
Grignard reagents react with DMF to form 4-halobenzaldehyde, and the m/z value for the molecular ion is calculated by combining the molecular weights of the Grignard reagent and DMF, adjusted for the loss of Br and gain of H.
Explanation:
The reactivity of Grignard reagents is due to the polarized C-Mg bond in these organometallic compounds. 4-halobenzaldehyde formation from the reaction of Grignard reagents with N,N-dimethylformamide (DMF) requires predicting the m/z values for the molecular ions. The reaction will generate a benzyl alcohol, which upon further processing will yield the aldehyde.
The molecular weight of chlorobenzene is 112.56 g/mol, and conversion to the Grignard reagent adds 24.31 g/mol (atomic weight of Mg) but subtracts the weight of a chloride ion (35.45 g/mol), resulting in a new molecular weight of 101.42 g/mol.
Then the Grignard reagent reacts with DMF (molecular weight 73.09 g/mol) to form an adduct. When calculating the m/z value for the molecular ion peak, you must take into account the mass change due to this reaction, subtracting the weight of a bromine atom, and adding the weight of a hydrogen atom. The expected m/z value for the ion would thus be the total molecular weight of the 4-halobenzaldehyde formed.