How Do You Know It Is the Most Stable Resonance Stucture
If a molecule has more than than one Lewis structure, it tin be represented by the corresponding resonance forms. They are separated by a double-headed pointer (two arrows are used for equilibrium):
Remember, resonance structures have the same placement of atoms, meaning that they stand for the same compound and only the system of electrons is unlike .
What is important as well, is that not all the resonance structures are as stable. In fact, the nigh stable resonance form is the resonance hybrid since it delocalizes the electron density over a greater number of atoms:
Notwithstanding, cartoon the resonance hybrid is not very practical and often, certain properties and reactions of the molecule are improve explained by a single resonance form.
And for some of these explanations, nosotros need to decide the more than stable resonance form. In that location are a few principles you demand to retrieve to determine which resonance structure is more than stable/of import, i.e. information technology contributes more to the resonance hybrid:
Principle 1. Atoms, in general, "don't like" charges, then having no charge is better:
Sometimes, it is impossible to avoid charges, so if both resonance structures are charged, then the octet dominion needs to be considered.
Principle ii. The resonance structure with a complete octet is more stable:
If the resonance structures have charges and the octet is not a determining factor either, then nosotros need to look at the general trends for stabilizing negative and positive charges.
Stability of Negative Charges
The negative charge is a high density of electrons, so in guild for the charge to exist better-stabilized, these electrons need to be on a more than electronegative cantlet.
This observation works best just when the 2 atoms begetting the formal charge are in the same row of the periodic table since they have comparable atomic sizes.
However, equally nosotros go downwardly the group, the diminutive sizes increment which helps to handle the negative charge more efficiently considering the accuse density decreases with large volume/surface.
Therefore, if you are comparing elements from unlike rows in the periodic table, choose the one where the accuse is on the bigger cantlet every bit the major resonance structure:
If the negative charge is on the same cantlet in both resonance structures, then look for other factors that can stabilize it. For example, if the charge is next to an electronegative atom that helps to stabilize it too (anterior effect):
Check the stability of the conjugate base in acrid-base reaction for more than details about stabilizing the negative charge.
Stability of Positive Charges
The stability trends for a positive charge are, as expected, opposite to the ones for the negative charge. The positive accuse is a centre of electron deficiency, therefore to stabilize it, we need electron-donating groups. Alkyl groups (CHiii-, CHiiiCH2-, etc.) being electron-altruistic stabilize the positive accuse on a carbon atom (carbocation):
1o, 2o, and 3o stand for master, secondary, and tertiary respectively. This shows the number of carbons (alkyl groups) continued to the politely charged carbon. The more alkyl groups, the more stable the carbocation.
For the aforementioned reason, putting the positive charge next to an electron-withdrawing group makes it less stable:
Notice that in none of the examples, we had a structure with more than than one formal charge. So, remember that any a resonance form with an cantlet bearing a +2 or −2 charge is very unstable and cannot be a significant contributor to the resonance hybrid.
1.
Determine which resonance construction makes the greatest contribution to the resonance hybrid. I.eastward. which resonance course is more stable?
a)
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b)
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c)
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d)
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eastward)
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f)
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g)
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h)
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i)
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j)
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g)
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l)
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2.
Utilize curved arrows to describe all the meaning resonance structures for the following molecule and determine the major correspondent to the resonance hybrid:
respond
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iii.
Rank the following resonance structures from the most to the least stable:
answer
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