Question

In the following compound, electron pair of Nitrogen more effectively involved in the delocalization process, for adopting aromatic character is



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Answer 1

Answer:

Now that we understand the difference between sigma and π electrons, we remember that the π bond is made up of loosely held electrons that form a diffuse cloud which can be easily distorted. This can be illustrated by comparing two types of double bonds, one polar and one nonpolar. The C=C double bond on the left below is nonpolar. Therefore the π electrons occupy a relatively symmetric molecular orbital that’s evenly distributed (shared) over the two carbon atoms. The C=O double bond, on the other hand, is polar due to the higher electronegativity of oxygen. The π cloud is distorted in a way that results in higher electron density around oxygen compared to carbon. Both atoms still share electrons, but the electrons spend more time around oxygen. The drawing on the right tries to illustrate that concept.

shown before, we can use the curved arrow formalism either to arrive from structure I to structure II, or vice versa.

resonance energy.

Since conjugation brings up electron delocalization, it follows that the more extensive the conjugated system, the more stable the molecule (i.e. the lower its potential energy). If there are positive or negative charges, they also spread out as a result of resonance.

The more resonance forms one can write for a given system, the more stable it is. That is, the greater its resonance energy.

Examine the following examples and write as many resonance structures as you can for each to further explore these points:

Let’s look for a moment at the three structures in the last row above. In the first structure, delocalization of the positive charge and the π bonds occurs over the entire ring. This becomes apparent when we look at all the possible resonance structures as shown below.

In the second structure, delocalization is only possible over three carbon atoms. This is demonstrated by writing all the possible resonance forms below, which now number only two.

Finally, the third structure has no delocalization of charge or electrons because no resonance forms are possible. Therefore, it is the least stable of the three. This brings us to the last topic. How do we recognize when delocalization is possible? Let’s look at some delocalization setups, that is to say, structural features that result in delocalization of electrons.

Delocalization Setups

There are specific structural features that bring up electron or charge delocalization. The presence of a conjugated system is one of them. Other common arrangements are:

A combination of orbital and Lewis or 3-D formulas is a popular means of representing certain features that we may want to highlight. For example, if we’re not interested in the sp2 orbitals and we just want to focus on what the p orbitals are doing we can use the following notation.

If we focus on the orbital pictures, we can immediately see the potential for electron delocalization. The two π molecular orbitals shown in red on the left below are close enough to overlap. Overlapping is a good thing because it delocalizes the electrons and spreads them over a larger area, bringing added stability to the system.

It is however time-consuming to draw orbitals all the time. The following representations are used to represent the delocalized system.

A similar process applied to the carbocation leads to a similar picture.

The resonance representation conveys the idea of delocalization of charge and electrons rather well.

Finally, the following representations are sometimes used, but again, the simpler they are, the less accurately they represent the delocalization picture.