Why do the nitro groups change the relative leaving tendencies of the carboxy and 2,4-dinitrophenoxy groups in the tetrahedral intermediate in Problem 11?

Verified step by step guidance

1. Step 1: Analyze the structure of the tetrahedral intermediate. The intermediate contains two potential leaving groups: the carboxy group (-COO⁻) and the 2,4-dinitrophenoxy group (-OAr, where Ar is the 2,4-dinitrophenyl ring). Both groups are attached to the central carbon atom.
2. Step 2: Consider the electronic effects of the nitro groups on the 2,4-dinitrophenoxy group. Nitro groups are electron-withdrawing due to their strong -I (inductive) and -M (mesomeric) effects. These effects stabilize the negative charge on the phenoxide ion (ArO⁻) that forms when the 2,4-dinitrophenoxy group leaves.
3. Step 3: Compare the leaving group tendencies. The carboxy group (-COO⁻) is also a good leaving group due to resonance stabilization of the carboxylate ion. However, the presence of two nitro groups on the phenoxy group significantly enhances its ability to act as a leaving group by stabilizing the negative charge more effectively than the carboxylate ion.
4. Step 4: Evaluate the relative leaving group tendencies in the context of the reaction mechanism. The 2,4-dinitrophenoxy group is more likely to leave because the nitro groups increase its leaving group ability, making it more favorable for the reaction to proceed via the elimination of this group.
5. Step 5: Conclude that the nitro groups change the relative leaving tendencies by stabilizing the phenoxide ion through electron-withdrawing effects, thereby making the 2,4-dinitrophenoxy group a better leaving group compared to the carboxy group in the tetrahedral intermediate.

Key Concepts

Tetrahedral Intermediates

Tetrahedral intermediates are key species in nucleophilic acyl substitution reactions, formed when a nucleophile attacks a carbonyl carbon, resulting in a four-coordinate carbon atom. In this context, the stability of the tetrahedral intermediate is influenced by the substituents on the aromatic ring, such as nitro groups, which can affect the electron density and sterics around the leaving groups.

Leaving Group Ability

The ability of a leaving group to depart from a molecule is crucial in determining the rate of a reaction. Good leaving groups are typically stable after departure, while poor leaving groups are less stable. In this case, the presence of nitro groups can enhance the leaving ability of certain groups by stabilizing the negative charge that develops during the transition state.

Electron-Withdrawing Groups

Electron-withdrawing groups, such as nitro groups, can significantly influence the reactivity of organic compounds. They stabilize negative charges through resonance or inductive effects, which can enhance the leaving tendencies of groups attached to the same molecule. In the context of the question, the nitro groups affect the relative leaving tendencies of the carboxy and 2,4-dinitrophenoxy groups by altering the electronic environment of the tetrahedral intermediate.