Question

Ninhydrin when undergoes hydration, which carbonyl carbon is first attacked by \(\mathrm{H}_{2} \mathrm{O}\) ? (A) 1 (B) 2 (C) 3 (D) None

Short answer

The carbonyl carbon that is first attacked during hydration by water (H₂O) in ninhydrin is (A) Carbonyl Carbon 1, as it has the highest electron density and is more reactive towards nucleophiles like water.

Step-by-step solution

Understand the Structure of Ninhydrin

First, let's draw out the structure of ninhydrin to identify its carbonyl carbons. Ninhydrin is a compound with the chemical formula C9H6O4. Its structure has three carbonyl carbons in a cyclic ketone arrangement.

Analyzing Carbonyl Carbon Stability and Electron Density

Now that we know the structure of ninhydrin, we need to evaluate the stability and reactivity of the three carbonyl carbons. Carbonyl carbon 1 is an alpha-hydroxy ketone. Carbonyl carbon 2 is part of a carbon dioxide (CO2) group, while carbonyl carbon 3 is bound between two oxygen atoms. Because carbonyl carbon 1 is an alpha-hydroxy ketone, it has the highest electron density, meaning it's more likely to attract nucleophiles like water (H₂O). On the other hand, carbonyl carbon 2 is likely to be more stable due to the CO2 group, while carbonyl carbon 3 exhibits lower reactivity due to its double bonding with oxygen atoms.

Identify the Most Reactive Carbonyl Carbon

As discussed, carbonyl carbon 1 in the ninhydrin molecule has the highest electron density which makes it more reactive towards a nucleophile like water. Therefore, during hydration, carbonyl carbon 1 is most likely to be attacked by the water molecule first.

Conclusion

To answer the given exercise, based on the analysis of the stability and electron density of the carbonyl carbons in the ninhydrin molecule, the carbonyl carbon that is first attacked during hydration by water (H₂O) is: (A) Carbonyl Carbon 1.

Key concepts

Carbonyl Carbon Reactivity

Carbonyl carbon reactivity refers to the tendency of carbonyl groups to engage in chemical reactions, such as with nucleophiles. Ninhydrin, a compound with multiple carbonyl groups, provides a useful example for studying this concept.

In ninhydrin, you have three distinct carbonyl carbons, each with a different reactivity level. This can influence which carbonyl carbon will be attacked initially during a chemical reaction such as hydration.

Reactivity is greatly influenced by the electron density present around the carbon. Carbonyl carbon 1, being part of an alpha-hydroxy ketone, has a higher electron density compared to the other carbonyl groups present in ninhydrin.

This elevated electron density makes it more susceptible to attack by nucleophiles like water molecules. On the other hand, factors like steric effects and the nature of substituents attached to the carbonyl group may lower the reactivity.

• Carbonyl carbon 2, associated with a CO2 group, tends to be more stable and less reactive.
• Carbonyl carbon 3 is between two oxygen atoms, which usually results in lower electron availability for interactions.

Alpha-Hydroxy Ketone

The term 'alpha-hydroxy ketone' refers to a ketone that has a hydroxyl group attached to one of its alpha carbons.

This specific functional group arrangement impacts the chemical behavior of the molecule significantly. One of the carbonyl carbons in ninhydrin is an alpha-hydroxy ketone, making it unique in terms of electronic effects.

The presence of the hydroxyl group enhances the electron density around its attached carbonyl carbon by donating electron pairs through the resonance or inductive effect.

This means that the carbon in the alpha-hydroxy ketone part of ninhydrin can pull in nucleophiles more efficiently than others.

In terms of reactivity, **alpha-hydroxy ketones** are often more reactive because:

• The hydroxyl group increases the overall electron flow towards the carbonyl carbon.
• This elevates attraction to nucleophiles, which can start the reaction.

Therefore, the unique nature of the alpha-hydroxy ketone group makes it an exciting target in hydration reactions, like the hydration of ninhydrin.

Nucleophile Attack Mechanism

Nucleophile attack mechanisms play a crucial role in understanding hydration reactions involving carbonyl groups. A nucleophile is typically an electron-rich species that seeks to donate a pair of electrons to an electron-poor site, such as a carbonyl carbon.

In the case of the ninhydrin molecule, water ( ucleophile) is the attacking species. It targets the carbonyl carbon that has the highest electron density and least steric hindrance.

Let's delve into how this mechanism occurs:

• The nucleophile, water, approaches the electron-deficient carbonyl carbon.
• The electron pair from the oxygen in water is donated to the carbon, forming a bond.
• This leads to the transformation and opens the path for further chemical reactions.

The choice of which carbonyl carbon gets attacked first depends on the relative reactivity of the groups involved.

In ninhydrin, carbonyl carbon 1 becomes the prime target, due to its alpha-hydroxy ketone configuration that makes it more electron-rich.

Understanding the underlying **nucleophile attack mechanisms** is fundamental for predicting and explaining how various organic compounds, like ninhydrin, respond to hydration and other reactions.