Which of the following reagents is used to distinguish acetone and acetophenone? (a) \(\mathrm{NH}_{4} \mathrm{Cl}\) (b) \(\mathrm{Na}_{2} \mathrm{SO}_{4}\) (c) \(\mathrm{NaHSO}_{3}\) (d) Grignard reagent.

A Grignard reagent, named after the French chemist Victor Grignard, is a powerful chemical tool widely used in organic synthesis. It is an organomagnesium compound typically represented as R-Mg-X, where R is an organic group and X is a halogen. These reagents are highly nucleophilic, which means they readily donate electrons and form new bonds with electrophiles such as carbonyl compounds.

Upon reacting with ketones, like acetone, the Grignard reagent adds to the carbonyl carbon, forming an alcohol after subsequent protonation. The outcome is usually a tertiary or secondary alcohol, depending on the structure of the initial ketone. However, not all ketones react uniformly with Grignard reagents. Factors like steric hindrance impact the reactivity and will alter the course of the reaction, which is a crucial point in distinguishing between different ketones.

Ketones are characterized by a carbonyl group (a carbon double-bonded to an oxygen), with two alkyl or aryl groups attached to the carbonyl carbon. The presence of the double bond makes the carbonyl carbon electrophilic and susceptible to attack by nucleophiles, such as Grignard reagents.

The reactivity of ketones with nucleophiles can be influenced by several factors: the steric bulk of the substituents and the electronic effects of neighboring groups. Acetone, being a smaller molecule with less steric hindrance, is more reactive compared to acetophenone, which contains a bulky phenyl group. This concept of ketone reactivity is imperative when trying to chemically distinguish one ketone from another, as in the case of acetone and acetophenone.

Steric hindrance refers to the prevention of chemical reactions due to the spatial size of atoms or groups within a molecule. Larger groups can impede the approach of reactive species to a particular site within the molecule, thus reducing reactivity. In the context of acetophenone, the sizable phenyl group adjacent to the carbonyl carbon blocks the Grignard reagent from effectively attacking the carbonyl carbon.

In addition to steric effects, electronic effects can also play a role in reactivity. Resonance stabilization occurs when a pi bond can be delocalized over a structure, conferring extra stability on the molecule. For acetophenone, the phenyl group is capable of stabilizing the carbonyl through resonance, making it less electrophilic and less prone to reaction with nucleophiles like Grignard reagents. This combination of steric hindrance and resonance effects is central to understanding why Grignard reagents differentiate between acetone and acetophenone.