In short, the only way aromatic and antiaromatic compounds differ is the number of electrons they have in the conjugated system. All the other criteria-being cyclic, planar and fully conjugated are a must for both categories: Nonaromatic Compounds There is also the third class of compounds we need to discuss: These are the nonaromatic or not aromatic compounds. As the name suggests, nonaromatic compounds have really nothing to do with the aromaticity well, almost. For example, is this compound aromatic? And that is the idea, nonaromatic compounds are the ones not related to the aromatic and antiaromatic compounds: In fact, if any of these factors; cyclic, planar, fully conjugated does not match — the compound is said to be nonaromatic.
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May 17, By Abhijit Look at these compounds below. Some of them are aromatic. Some are anti-aromatic and some are non-aromatic. The question is — Can you identify which ones are aromatic and which are not? Sure, Abhijit! So what are the rules then? How can you identify if a compound is aromatic, anti-aromatic or non-aromatic? We will first take a look at the rules a compound must follow to be aromatic, and then, we will get to the questions that we must ask — to decide aromaticity.
They are: — It must be a cyclic compound. In simple words, you should see a ring. It must have a plane conjugated system. Big words. What it simply means is that resonance should be possible. What that means is when you want to decide aromaticity, you can ask three questions. Is the Compound Cyclic? See if you can spot a ring in any of these compounds.
No, right? They are not cyclic. And so they are non-aromatic. How about these? These compounds are cyclic. But does that mean they are aromatic as well? Two of them are in the above example. Can you identify them? To become aromatic, a cyclic compound has to pass another test. Is there a conjugated system? Now you should ask whether resonance is possible or not. Since we are not talking about resonance here, I will assume you know how to do that.
If not, check out this link where I talk about resonance. Only those systems that are plane-conjugated, where resonance is possible qualify for the third test.
Lets look at what the rule says first. Otherwise, it is non-aromatic. Sadly, most students ignore this and get it wrong. I like to say it this way. What the rule said is that aromatic compounds have pi-electrons that follow a series of magic numbers. You will see that it is an A. So if the pi-electrons in the conjugated system matches any of these values, it satisfies the rule.
How can we write this series easily? Write the general term. It represents a series of numbers. And so on… What you need to check is then that the pi-electrons that take part in resonance in the compound, match any of the above numbers in the series. That statement is simple, but you really need to understand it carefully.
I will explain them with a few examples later in this post. The 3-Step Method to Identify Aromatic Compounds The 3-step method to identify aromatic and anti-aromatic compounds is basically a check list. There are three items on the list that you need to check in the order given. First Check if the compound is cyclic.
If a compound is not cyclic, it is not aromatic. And if it is cyclic, it maybe aromatic. Check if the cyclic compound has resonance. Otherwise it is not aromatic. Remember, this rule comes last. Count the total number of pi-electrons. If it is a multiple of 4, it is anti-aromatic. Not case 1 or 2? The compound is non-aromatic. Watch me use the exact same system to solve a few problems on deciding the aromaticity of organic compounds — easy or difficult, you can solve it in a snap once you understand the process.
Practice Problems No technique does you any good unless you try it. Only then you can see the benefits. Test out what you learnt on these sample problems. Go hard at them, make sure you work them out. Need more practice problems and reference material? Did it help?
Non Aromatic Compounds
There is, however, one more criterion that compounds must match in order to be aromatic. Not all the compounds that are cyclic, planar and fully conjugated are aromatic. This indicates that the conjugate base of cyclopentadiene cyclopentadienyl anion must be more stable than regular carbanions: And indeed, the pKa of cyclopentadiene is 15 while allylic protons are generally at the range. It is clearly cyclic, but is it planar and fully conjugated? In this case, the carbon changes the hybridization from sp3 to sp2 and the lone pair is now in the p orbital, fully delocalized over the ring. Why is it slower than the reaction of other secondary alkyl halides in SN1 reaction?
Difference Between Aromatic Antiaromatic and Nonaromatic
May 17, By Abhijit Look at these compounds below. Some of them are aromatic. Some are anti-aromatic and some are non-aromatic. The question is — Can you identify which ones are aromatic and which are not? Sure, Abhijit! So what are the rules then?
This explains why being a planar, cyclic molecule is a key characteristic of both aromatic and antiaromatic molecules. However, in reality, it is difficult to determine whether or not a molecule is completely conjugated simply by looking at its structure: sometimes molecules can distort in order to relieve strain and this distortion has the potential to disrupt the conjugation. Thus, additional efforts must be taken in order to determine whether or not a certain molecule is genuinely antiaromatic. In an antiaromatic compound, the amount of conjugation energy in the molecule will be significantly higher than in an appropriate reference compound. If an experimentally determined structure of the molecule in question does not exist, a computational analysis must be performed. The potential energy of the molecule should be probed for various geometries in order to assess any distortion from a symmetric planar conformation.