Question

When \(2,4,6\) -trinitroanisole is treated with methoxide in methanol, a red anion having the composition \(\left(\mathrm{C}_{8} \mathrm{H}_{8} \mathrm{O}_{8} \mathrm{~N}_{3}\right)^{-}\) is produced. Such anions are called Meisenheimer complexes after the chemist who first suggested the correct structure. What structure do you think he suggested? One of Meisenheimer's experiments compared the product of reaction of \(2,4,6\) -trinitroanisole with ethoxide ion with the product of \(2,4,6\) -trinitrophenyl ethyl ether with methoxide ion. What do you think he found?

Short answer

Meisenheimer likely suggested a structure for the \(\mathrm{(C_8H_8O_8N_3)^{-}}\) complex where the nucleophile (methoxide or ethoxide ion) adds to the ortho carbon, causing the adjacent nitro group to resonate and take a negative charge. In his experiments, he probably found that both reactions (2,4,6-trinitroanisole with ethoxide ion and 2,4,6-trinitrophenyl ethyl ether with methoxide ion) lead to the formation of stable Meisenheimer complexes at the ortho carbon, with the primary influence over stability and structure being the arrangement of the three nitro groups and their electron-withdrawing nature.

Step-by-step solution

Identify the molecules involved

The starting materials are 2,4,6-trinitroanisole and methoxide ion (or ethoxide ion in the second part of the problem). The product, as stated, is a Meisenheimer complex, specifically a red anion with the composition \(\mathrm{(C_8H_8O_8N_3)^{-}}\). We need to propose the structure of this complex. #Step 2: Analyzing the reactions#

Analyze electron-donating and electron-withdrawing groups

2,4,6-trinitroanisole has three nitro groups as electron-withdrawing groups (EWGs) and a methoxy group as an electron-donating group (EDG). Since Meisenheimer complexes are formed by the attack of a nucleophile (methoxide or ethoxide ion) on an electron-deficient carbon, we should look for a carbon that is electron-deficient due to the influence of the nitro groups. The carbon between the nitro groups and the methoxy group, i.e., the carbon at the ortho position, is expected to be the most electron-deficient. #Step 3: Proposing the structure of the Meisenheimer complex#

Construct the Meisenheimer complex

Considering that methoxide or ethoxide ion will act as a nucleophile and attack the most electron-deficient carbon (ortho position), we can propose the structure of the Meisenheimer complex as follows: The nucleophile adds to the ortho carbon, causing the adjacent nitro group to resonate and take a negative charge. The complex will now have a composition of \(\mathrm{(C_8H_8O_8N_3)^{-}}\) as given in the problem. #Step 4: Comparing the two reactions#

Analyze the reactions of 2,4,6-trinitroanisole and 2,4,6-trinitrophenyl ethyl ether with different nucleophiles

When 2,4,6-trinitroanisole reacts with ethoxide ion, and 2,4,6-trinitrophenyl ethyl ether reacts with methoxide ion, both form similar Meisenheimer complexes with respective nucleophiles, as the ortho carbon being attacked is the most electron-deficient in both cases. The difference lies in the presence of the methoxy group in 2,4,6-trinitroanisole, while there's an ethoxy group in 2,4,6-trinitrophenyl ethyl ether. #Step 5: Concluding the findings#

Conclude the results of Meisenheimer's experiments

Based on the analysis, we can conclude that Meisenheimer might have found that both reactions lead to the formation of stable Meisenheimer complexes by the addition of the nucleophile (methoxide or ethoxide ion) to the ortho carbon of the starting material. The presence of a methoxy or ethoxy group in the starting material did not significantly affect the stability or formation of the complex, because the primary influence over the stability and structure is the arrangement of the three nitro groups and the electron-withdrawing nature of these groups.

Key concepts

Nucleophilic Aromatic Substitution

Nucleophilic aromatic substitution is a fascinating type of chemical reaction commonly observed in organic chemistry. It involves the replacement of a group attached to an aromatic ring with a nucleophile. A nucleophile is a chemical species that donates an electron pair to an electrophile to form a bond. A classic example is when a halogen atom, such as chlorine, attached to an aromatic ring, is substituted by a nucleophile.

However, unlike aliphatic nucleophilic substitution reactions, aromatic rings don't readily undergo this process due to the high stability of the aromatic system. For nucleophilic aromatic substitution to occur, the ring usually must have electron-withdrawing groups that make certain positions on the ring more susceptible to attack by nucleophiles. In the problem described, the methoxide ion is the nucleophile that attacks an electron-deficient carbon on the aromatic ring of 2,4,6-trinitroanisole, leading to the formation of the Meisenheimer complex through this substitution mechanism.

Electron-Withdrawing Groups

Electron-withdrawing groups (EWGs) play a critical role in nucleophilic aromatic substitution. These groups are capable of drawing electron density away from the aromatic ring through various mechanisms, including resonance and induction. This action stabilizes the intermediate and lowers the energy required for the reaction to proceed.

Examples of strong EWGs include nitro, cyano, and carbonyl groups. In the case of 2,4,6-trinitroanisole, the nitro groups are the EWGs that make the ortho and parapositions on the ring electron-poor and more reactive towards nucleophiles. The presence of the EWGs is essential in facilitating the chemical transformation that leads to the formation of the Meisenheimer complex.

Stability of Anions

The stability of anions is directly influenced by the nature of substituents on the aromatic ring. When a nucleophile attacks the ring and an anion is formed, as in the case of the Meisenheimer complex, the substituents can either stabilize or destabilize the negative charge.

Anions are stabilized by electron-withdrawing groups that can delocalize the negative charge through resonance. The nitro groups in the Meisenheimer complex are electronegative and can spread out the negative charge over multiple atoms, making the anion less reactive and more stable. On the other hand, electron-donating groups would concentrate the negative charge and destabilize the anion. Thus, the pattern and type of substituents are pivotal in determining the overall stability of the resulting anions in these reactions.

Organic Reaction Mechanisms

Understanding organic reaction mechanisms involves studying the process through which organic substrates transform into products. A mechanism describes the steps, or stages, that involve breaking and forming of bonds, the movement of electrons, and the structure of intermediates and transition states.

In the mechanism for nucleophilic aromatic substitution, the nucleophile attacks the electron-deficient carbon of the aromatic ring to form a sigma complex. Subsequently, the bond rearranges, and a group leaves, often resulting in a new aromatic system. The stability of intermediates, such as the Meisenheimer complex, often determines the course of the reaction, which can vary with different substituents or conditions. By understanding these mechanisms, chemists can predict and explain reactions' outcomes and design strategies to synthesize complex organic molecules.