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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?
Wait! You aren’t thinking of Huckel’s rule, are you?
Sure, We can just use the 4n+2 rule here and give you the answer right now.
Don’t! Don’t do that, please. It’s one of the steps, just NOT the first one.
Chances are if a question comes in JEE to identify an aromatic compound, blindly applying the 4n+2 rule isn’t going to give you the right answer.
So what are the rules then? How can you identify if a compound is aromatic, anti-aromatic or non-aromatic?
In this guide, I will share with you the exact 3-STEP system I use to decide aromaticity. 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.
Rules for finding out aromaticity
It turns out that there isn’t one rule, there are actually three rules a compound must satisfy to be aromatic. 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.
- It must satisfy the 4n + 2 rule, also called the Huckel’s rule.
What that means is when you want to decide aromaticity, you can ask three questions.
Q#1. 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. That’s it, done!
How about these?
These compounds are cyclic. But does that mean they are aromatic as well? No, they MAYBE aromatic or not. Two of them are in the above example. Can you identify them?
To become aromatic, a cyclic compound has to pass another test.
Q#2. 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.
Only those systems that are plane-conjugated,( where resonance is possible ) qualify for the third test.
Q#3. Does it follow the 4n+2 rule?
Now you’re ready to answer the question of whether the compound is aromatic or not. Let's look at what the rule says first.
If a compound has:
- 4n+2 pi electrons, it is aromatic.
- 4n pi electrons, it is anti-aromatic.
- Otherwise, it is non-aromatic.
The key is to realize that Huckel’s rule is only applied when the compound passes the first two tests. Sadly, most students ignore this and get it wrong.
I like to say it this way.
“Cyclic Comes First, Conjugation comes Second, and 4n+2 comes Third.”
But what is this “n”?
n is not a magic number. In fact, you won’t be given the value of n. 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.P.
“2 6 10 14 18 22…..” and so on.
So if the pi-electrons in the conjugated system matches any of these values, it satisfies the rule.
For example, if you have 2 pi-electrons, n = 0. 4(0) + 2 is satisfied.
How can we write this series easily?
Simple. Write the general term. And that’s where 4n + 2 comes from. It represents a series of numbers.
For 6 pi-electrons, n = 1. 4(1) + 2 = 6.
For 10 pi-electrons, n = 2. 4(2) + 2 = 10.
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 the help of a few examples later in this post.
When counting pi-electrons for Huckel’s rule, only count those electrons that take part in resonance.
The 3-Step Method to Identify Aromatic Compounds
The 3-step method to identify aromatic and anti-aromatic compounds is basically a checklist. There are three items on the list that you need to check in the order given.
#1. FIRST CHECK IF THE COMPOUND IS CYCLIC.
If a compound is not cyclic, it is not aromatic. And if it is cyclic, it may be aromatic.
#2. CHECK IF THE CYCLIC COMPOUND HAS RESONANCE.
If it has resonance, it MAYBE aromatic. Otherwise, it is not aromatic.
#3. USE THE HUCKEL’’ S RULE.
Remember, this rule comes last. Count the total number of pi-electrons.
- If it fits in the series 4n+2, it is aromatic.
- If it is a multiple of 4, it is anti-aromatic.
- Not cases 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. And so, it’s time…
…for you to take action. Test out what you learned on these sample problems. Go hard at them, make sure you work them out.
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