The source of the reactivity of the Grignard reagent is the highly polarized C-Mg bond. These active reagents are not only reactive toward carbonyl compounds but also toward water, oxygen, and protic solvents. Grignard reagents are typically prepared under rigorously anhydrous conditions, and elaborate glassware is often used to ensure that the reagent does not interact with air or water.

In this experiment, you will work with a partner to generate a Grignard reagent from chlorobenzene and react it in situ (i.e., without isolating it) with N,N-dimethylformamide (DMF). Following the work-up, the corresponding 4-halobenzaldehyde will be isolated and analyzed.

Predict the m/z value(s) for the molecular ion(s) of each of the possible products from the reaction of the Grignard reagent with DMF.

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.