Question

Benzophenone does not react with (a) \(\mathrm{NaHSO}_{3}\) (b) \(\mathrm{H}_{2} \mathrm{NOH}\) (c) \(\mathrm{HCN}\) (d) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{MgBr}\)

Short answer

The correct answer is (a) Sodium bisulfite (\(\mathrm{NaHSO}_{3}\)).

Step-by-step solution

Understand the Structure of Benzophenone

Benzophenone is an organic compound with the molecular formula \(\mathrm{C}_{13} \mathrm{H}_{10} \mathrm{O}\). Its structure consists of two phenyl rings connected by a carbonyl group. The carbonyl group contains a carbon atom double-bonded to an oxygen atom, making it an electrophilic center. This electrophilic center can react with nucleophiles, converting Benzophenone into various other compounds.

Analyzing the Reagents and Their Reactions

(a) \(\mathrm{NaHSO}_{3}\): Sodium bisulfite is a mild reducing agent and reacts with aldehydes to form bisulfite adducts. However, Benzophenone is a ketone, not an aldehyde, and hence does not react with sodium bisulfite. (b) \(\mathrm{H}_{2} \mathrm{NOH}\): This reagent, hydroxylamine, reacts with both aldehydes and ketones to form oximes. Since Benzophenone is a ketone, it will react with hydroxylamine. (c) \(\mathrm{HCN}\): Hydrogen cyanide reacts with both aldehydes and ketones, by adding a nitrile group to the carbonyl group via nucleophilic addition to form cyanohydrins. So, Benzophenone will react with hydrogen cyanide. (d) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{MgBr}\): This reagent is phenylmagnesium bromide, a Grignard reagent. Grignard reagents are strong nucleophiles and react with carbonyl groups, which are present in Benzophenone. As a result, Benzophenone will react with phenylmagnesium bromide.

Identify the Reagent that Does Not React

After analyzing all four reagents, we can conclude that Benzophenone does not react with Sodium bisulfite (\(\mathrm{NaHSO}_{3}\)). Therefore, the correct answer is (a) \(\mathrm{NaHSO}_{3}\).

Key concepts

Benzophenone Reactivity

Benzophenone is quite an intriguing compound in organic chemistry. It has a molecular structure

• comprised of two phenyl groups (benzene rings)
• attached to a carbonyl group (C=O).

The carbonyl group is significant because it presents an electrophilic carbon that interacts readily with nucleophiles, groups that donate electrons. You can think of Benzophenone as a compound eager to embrace electron-rich partners to form stable new molecules. This reactivity is central to understanding how Benzophenone engages in various chemical reactions. Whether it transforms depends on the nature of the reagent it encounters, based on its carbonyl functionality. Thus, the structure inherently favors interactions with nucleophiles due to the oxygen's partial negative charge, drawing in electron pairs.

Nucleophilic Addition

In chemistry, a nucleophilic addition reaction is when a nucleophile—a molecule rich in electrons—adds to an electrophilic center of another molecule. For Benzophenone, the electrophilic center is the carbonyl carbon due to its partial positive charge. When a nucleophile approaches this center, it donates its electrons, forming a new bond. This is a key mechanism in organic chemistry.
Benzophenone typically undergoes nucleophilic addition when reacting with reagents like:

• Hydrogen cyanide ( HCN), forming cyanohydrins.
• Hydroxylamine ( H_2NOH), forming oximes.
• Phenylmagnesium bromide ( C_6H_5MgBr), a Grignard reagent, leading to secondary alcohols.

These reactions significantly expand the structural diversity of organic compounds. They allow for the synthesis of substances with varied chemical properties and potential utilities.

Grignard Reagent

Grignard reagents are essential tools in organic synthesis. Named after the French chemist Victor Grignard, these reagents are organomagnesium compounds with a generic formula R-Mg-X, where R represents an organic group and X a halogen. Grignard reagents are superb nucleophiles. They excel at attacking the electrophilic carbon in carbonyls, such as that in Benzophenone, leading to alcohols once the reaction is complete. For example, when Benzophenone meets phenylmagnesium bromide ( C_6H_5MgBr), the carbon bonded to the Mg becomes the nucleophile. This process effectively extends the carbon chain and transforms them into alcohols via formation involving new carbon-carbon bonds. It's a reliable process that highlights the versatility and utility of Grignard reagents in synthetic chemistry.

Reducing Agents

In chemistry, reducing agents are substances that donate electrons to another compound, reducing it in the process. Sodium bisulfite ( NaHSO_3) is a mild reducing agent, commonly used for its ability to form bisulfite adducts with aldehydes. However, Benzophenone, being a ketone, cannot form these adducts. This lack of reactivity with sodium bisulfite is due to structural differences between aldehydes and ketones; aldehydes have a hydrogen atom attached to the carbonyl carbon, whereas ketones have two carbon groups.
Essentially, the inherent stability of ketones, compared to aldehydes, means they're less reactive toward these certain reducing agents. Understanding this distinction clears up why sodium bisulfite and Benzophenone do not react, an important insight when predicting organic reactions.

Sodium Bisulfite Reaction

The Sodium bisulfite ( NaHSO_3) reaction is particularly effective with aldehydes due to the formation of bisulfite adducts, which are water-soluble. This is extremely useful for isolating and purifying aldehydes. Sodium bisulfite interacts with the carbonyl group in aldehydes to form a sulfonate product, stabilized via hydrogen bonding and other intermolecular forces.
For Benzophenone, which is a ketone, this reaction does not take place. The reason lies in the ketonic structure, which does not support the hydrogen required for this type of reaction. Understanding this specificity is valuable when determining the reactivity and compatibility of different compounds in organic synthesis routes.