Question

While azanonatetraene is comparatively stable at room temperature, \(\mathrm{N}\) -carbethoxyazanonatetraene rapidly isomerizes under the same conditions. Account for this difference.

Short answer

The difference in stability between azanonatetraene and N-carbethoxyazanonatetraene at room temperature is due to the electron-withdrawing carbethoxy group in N-carbethoxyazanonatetraene. This group disrupts the delocalized π-electron system, making the molecule less stable and more prone to isomerization. In contrast, azanonatetraene has a stable, extended allyl structure with strong π-electron delocalization, providing good thermodynamic stability.

Step-by-step solution

Draw the structures of azanonatetraene and N-carbethoxyazanonatetraene

First, let's draw the structures of both molecules: Azanonatetraene (C9H9N) has a linear structure with alternating single and double bonds, similar to an extended allyl group.\\\[ \ce{H2C=CH-CH=CH-N-CH=CH-CH=CH2} \]\\\ N-carbethoxyazanonatetraene (C11H11NO2) has an additional carbethoxy group (COOC2H5) attached to the nitrogen atom.\\\[ \ce{H2C=CH-CH=CH-N(COOC2H5)-CH=CH-CH=CH2} \]

Analyze the electronic configuration

In azanonatetraene, there are three conjugated double bonds which result in a delocalized π-electron system. This strong delocalization of π-electrons over the entire molecule provides stability similar to allylic systems. Moreover, there are no significant steric hindrances or electronic perturbations present. For N-carbethoxyazanonatetraene, the nitrogen atom carries a positive charge due to the carbethoxy group. The presence of this electron-withdrawing group disrupts the π-electron delocalization to a certain extent and makes the molecule less stable.

Compare stabilities and isomerization tendencies

As mentioned earlier, azanonatetraene has a stable, extended allyl structure with strong π-electron delocalization. This structure provides good thermodynamic stability at room temperature. However, in N-carbethoxyazanonatetraene, the electron-withdrawing group attached to the nitrogen destabilizes the delocalized π-electron system. This destabilization makes the molecule more prone to isomerization in an attempt to achieve a more stable configuration. Consequently, N-carbethoxyazanonatetraene rapidly isomerizes under the same conditions as azanonatetraene. In conclusion, the difference in stability between azanonatetraene and N-carbethoxyazanonatetraene is due to the effect of the electron-withdrawing carbethoxy group on the delocalized π-electron system. This group destabilizes the molecule, making it more susceptible to isomerization at room temperature.

Key concepts

Isomerization

Isomerization is a process where a molecule is transformed into another molecule with the same atoms, but in a different arrangement. This often leads to a change in the physical and chemical properties of the substance. It’s like rearranging furniture in a room—you still have the same items, they've just moved around. Isomerization can occur naturally or be induced by heat, light, or chemicals.
In the context of N-carbethoxyazanonatetraene, isomerization happens due to instability brought about by the carbethoxy group. This instability encourages the molecule to rearrange into a more stable structure. Here, the rearrangement can help to minimize internal electron repulsion or achieve a more favorable distribution of electrons. Because of this isomerization tendency, some molecules can adopt different forms under varying conditions. Thus, understanding isomerization is key to predicting the behavior of molecules under specific circumstances.

Conjugated Systems

Conjugated systems are structures where alternating single and double bonds allow for electron sharing across those bonds. This creates a larger, delocalized electron cloud over the entire structure. Think of electrons moving freely like a shared blanket over the interconnected bed frames in a bunk bed.
In azanonatetraene, this extended system of conjugation involves several double bonds connected in sequence these extensive

• Electron cloud helps stabilize the molecule
• Lower energy and increased stability
• Prevent excessive internal charge buildup

This kind of electron movement is what gives conjugated systems their stability and unique chemistry. However, the introduction of the carbethoxy group in the N-carbethoxyazanonatetraene disturbs this conjugation, making its overall structure less stable than azanonatetraene.

Electron Delocalization

Electron delocalization is an essential concept that comes into play when discussing conjugated systems. It refers to the distribution of electrons across multiple atoms, rather than being localized to a single atomic orbit.
This phenomenon typically occurs in systems with alternating single and double bonds. For azanonatetraene, the delocalization of

• Electrons across multiple pi bonds
• Helps distribute the molecule's charge
• Enhances thermodynamic stability

Electron delocalization can reduce the likelihood of localized electron cloud stress, making the molecule more robust. However, when a carbethoxy group is introduced, as in N-carbethoxyazanonatetraene, electron delocalization gets disturbed.
The carbethoxy group withdraws electrons, creating a localized area of electron deficiency which complicates seamless electron flow. Such a disturbance leads to instability, encouraging isomerization as a possible path to regain stability.

Carbethoxy Group

The carbethoxy group, represented as COOC₂H₅, is an ester functional group that attaches to molecules to modify their characteristics. This group consists of a carbon (C) double bonded to oxygen (O) with a separate oxygen (O) bonded to an ethyl group (C₂H₅).
The inclusion of this group introduces an electron-withdrawing capacity, which plays a vital role in altering the molecule's electronic configuration. In N-carbethoxyazanonatetraene, the carbethoxy group

• Withdraws electrons from the nitrogen atom
• Creates a positive polarization on nitrogen
• Disrupts electron delocalization

This disruption is the primary reason for the molecule's instability compared to azanonatetraene. Understanding the influence of such electron withdrawing groups is essential in chemical stability discussions, as they may promote structural rearrangement toward more stable configurations.