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## Organic chemistry

# Aromatic stability I

The aromaticity of benzene. Created by Jay.

## Want to join the conversation?

- At1:57, what is
**Atomic Orbitals**and**Molecular Orbitals**? What is the difference between them? Is there a video covering this?

Thank you ^-^(8 votes)**Atomic orbitals**are the areas around a particular*atom*where electrons are most likely to be found, and these areas form common "shapes" of high electron density (or "shapes" of where around the atom you would most likely find an electron). The most common "shapes" are: s (spherical), p (dumbbell-like), d, and f (both of which are a little more complicated to describe).**Molecular orbitals**are essentially formed by the interaction of proximate atomic orbitals within a*molecule*. This will result in hybridized orbitals with different portions of the original atomic orbitals; for example, an sp^3 hybridized orbital contains three parts p-character and one part s-character, which means it will look and behave 25% like a spherically-shaped s orbital and 75% like a dumbbell-shaped p orbital. There's more to it of course, but those are the essentials.

Khan videos for reference:

- All videos in the "Quantum Numbers and Orbitals" section in*Chemistry->Electronic Structure of Atoms*will help with atomic orbitals (https://www.khanacademy.org/science/chemistry/electronic-structure-of-atoms/orbitals-and-electrons)

- All videos in the "Hybridization" section in*Organic Chemistry->Structure and Bonding*will help with molecular orbital basics (https://www.khanacademy.org/science/organic-chemistry/gen-chem-review/hybrid-orbitals-jay/)

- Molecular Orbital theory is covered in*Organic Chemistry->Conjugation, Diels-Alder, and MO Theory*(https://www.khanacademy.org/science/organic-chemistry/conjugation-diels-alder-mo-theory/molecular-orbital-theory/)

Hope this helped!(37 votes)

- How did you know to assign 1 for N?(8 votes)
- n is just a positive number... 1 is the smallest natural no...and for a benzene ring ..the huckel rule fits perfectly for ..a benzene (6 ) pie electrons.....(0 votes)

- At5:06, why is n=1 and not 6?(2 votes)
- Hückel's rule says the number of π electrons must equal 4n+2, where n = 0,1,2, etc.

4n+2 = no. of π electrons

The no. of π electrons = 6.

4n + 2 = 6

4n = 4

n = 4/4 = 1(6 votes)

- What does the number n in Huckel's rule represent?(3 votes)
- n is any positive integer (whole number).

So for example, we know for anti- aromatic compounds, 4n= number of p orbitals. The number of p orbitals in an [10] annulene (cyclodecapentane) is 10 and so we can find out that n is not an integer if we use the equation mentioned above and therefore conclude that cycldecapentane is aromatic. Assuming that the annulene is planar.

(Annulene: hydrocarbons with alternating single and double bonds)(1 vote)

- How do you know when a compound is aromatic, non aromatic or antiaromatic? If you follow the 2 rules it will be aromatic and if you have 4n rather than 4n +2 it will be antiaromatic but how do you know when the conformation changes to become more stable and the P orbitals are not in phase anymore making it nonaromatic?(2 votes)
- This really depends on a case-by-case basis. An antiaromatic compound will change its conformation to be non-aromatic (by moving its p orbitals out of the plane) anytime the energy gained in moving to this new conformation is smaller than the energy lost by no longer being antiaromatic (ie it happens whenever there is a net energy loss and thus the changed-conformation compound is more stable). Usually, unless a compound is very conformationally restrained (for example a very small ring), you can expect there to be at least some conformational distortion to get the p orbitals out of plane if the compound is antiaromatic.(3 votes)

- At2:35, what that circle is called ? And why it is used here ?(1 vote)
- The circle is called a Frost circle (Sal names it at2:38). Drawing a Frost circle is an easy way to quickly visualize the pi molecular orbitals of a conjugated ring to determine if it is aromatic or not. The polygon (in this case a hexagon) is inscribed in the circle, and each point where the hexagon hits the circle is a molecular orbital; those in the top half of the circle are antibonding pi molecular orbitals, and those in the bottom half of the circle are bonding pi molecular orbitals (had there been any molecular orbitals exactly halfway up the circle, then they would be non-bonding molecular orbitals). You can learn more about Frost circles and their use here: http://www.chemgapedia.de/vsengine/vlu/vsc/en/ch/12/oc/vlu_organik/aromaten/aromaten/aromaten_gesamt.vlu/Page/vsc/en/ch/12/oc/aromaten/aromaten/frost/frost.vscml.html(5 votes)

- how will we come to know that the given molecule is planar?

thank u 4 answering.(2 votes)- using its hybridisation ...basically from valence bond theory.(1 vote)

- What is the logic behind Huckel's rule and the frost circle?(2 votes)
- Huckel's Rule is simply a summary of many experimental observations: a planar, cyclic, conjugated system is aromatic if it contains 4n + 2 π electrons. Similarly, In 1953, A. A. Frost noted that these systems were inscribed within a circle with a point at the bottom, the heights of the vertices on the circles reflected the relative energies of their molecular orbitals. It was all a matter of recognizing patterns in the observations.(1 vote)

- Why did Hofmann start describing in terms of smell if not all aromatics have a distinctive smell?(1 vote)
- Originally it was thought that all aromatics had that smell, however upon trying to define it other molecules were added. As the unusual stability of aromatic compounds was investigated, the term aromatic came to be applied to compounds with this stability, regardless of their odor.(2 votes)

- i didn't understand why he drew the molecular orbitals of benzen as "bonding" and not as "anti"?(1 vote)
- Imagine his drawing on a graph. Energy is on a y-axis (meaning the top, anti-bonding orbital is the least stable because it has the highest potential energy).

Electrons in molecular orbitals tend to fill the low-energy spots first. Notice the order in which he drew the electrons. That is the order you must use.(2 votes)

## Video transcript

In this series of
videos, we're going to look at aromaticity or
aromatic stabilization. We've already seen
that bromine will add across a double bond of a
simple alkene like cyclohexene to give us a mixture of
enantiomers for our products. If we try the same
reaction with benzene, we're not going to get
anything for our product. So there's no reaction. And so benzene is more
stable than cyclohexene. At first, you might
think that the stability is due to the fact that
benzene is conjugated. But numerous other
experiments have shown that it is even more
stable than we would expect. And that extra stability is
called aromaticity or aromatic stabilization. So benzene is an
aromatic molecule. Let's look at the
criteria to determine if a compound is aromatic. So a compound is
aromatic If it contains a ring of continuously
overlapping p orbitals. And so if the
molecule is planar, that's what allows the
p orbitals to overlap. It also has to have 4n plus
2 pi electrons in the ring, where n is equal to 0, 1 2,
or any other positive integer. And this is called
Huckel's rule. So let's go ahead
and analyze benzene in a little bit more detail. So if I look at
the dot structure, I can see that benzene has 2
pi electrons there, two here, and two more here, for a
total of six pi electrons. If I look at the
carbons of benzene, I can see that each carbon
has a double bond to it. So each carbon is
sp2 hybridized. And if each carbon
is sp2 hybridized, that means that each carbon
has a free p orbital. So I'm going to go
ahead and sketch in the unhybridized
free p orbital on each of the six carbons of benzene. Now, since benzene
is a planar molecule, that's going to allow those
p orbitals to overlap side by side. So you get some overlap side
by side of those p orbitals. And so benzene contains a ring
of continuously overlapping p orbitals. So p orbitals are considered
to be atomic orbitals. And so there are a total of
six atomic orbitals in benzene. According to MO theory,
those six atomic orbitals are going to cease to exist. And we will get six
molecular orbitals instead. So benzene has six
molecular orbitals. Drawing out these
molecular orbitals would be a little bit too
complicated for this video. So check out your textbook
for some nice diagrams of the six molecular
orbitals of benzene. However, it is important
to understand those six molecular orbitals in terms of
their relative energy levels. And the simplest way to do
that is to draw a frost circle. And so here I have a
circle already drawn. And inside the circle we're
going to inscribe a polygon. And since benzene is
a six-membered ring, we're going to inscribe a
hexagon in our frost circle. I'm going to go ahead and draw a
center line through the circle, just to help out with
the drawing here. And when you're inscribing your
polygon in your frost circle, you always start at the bottom. So we're going to
start down here. So we're going to
inscribe a hexagon. Let's see if we can
put a hexagon in here. And so we have a
six-sided figure here in our frost circle. The key point about
a frost circle is everywhere your
polygon intersects with your circle, that
represents the energy level of a molecular orbital. And so this intersection right
here, this intersection here, and then all the way around. And so we have our six
molecular orbitals. And we have the
relative energy levels of those six molecular orbitals. So let me go ahead and
draw them over here. So we have three
molecular orbitals which are above the center line. And those are higher in energy. And we know that
those are called antibonding molecular orbitals. So these are antibonding
molecular orbitals, which are the highest in energy. If we look down here, there
are three molecular orbitals which are below the center line. And those are our bonding
molecular orbitals. So those are lower in energy. And if we had some
molecular orbitals that were on the center
line, those would be non-bonding
molecular orbitals. We're going to go ahead and
fill our molecular orbitals with our pi electrons. So go back over here. And remember that benzene
has 6 pi electrons. And so filling
molecular orbitals is analogous to
electron configurations. You're going to fill the
lowest molecular orbital first. And each orbital can
hold two electrons, like electron configurations. And so we're going to go
ahead and put two electrons into the lowest bonding
molecular orbital. So I have four more pi
electrons to worry about. And I go ahead and put those in. And I have filled the bonding
molecular orbitals of benzene. So I have represented
all 6 pi electrons. If I think about
Huckel's rule, 4n plus 2, I have 6 pi electrons. So if n is equal to 1,
Huckel's rule is satisfied. Because I would do
4 times 1, plus 2. And so I would get a
total of 6 pi electrons. And so 6 pi electrons
follows Huckel's rule. If we look at the
frost circle and we look at the molecular
orbitals, we can understand Huckel's rule
a little bit better visually. So if I think about these
two electrons down here, you could think
about that's where the two comes from
in Huckel's rule. If think about these
four electrons up here, that would be four electrons
times our positive integer of 1. So 4 times 1, plus 2
gives us six pi electrons. And we have filled the
bonding molecular orbitals of benzene, which confers the
extra stability that we call aromaticity or
aromatic stabilization. And so benzene is aromatic. It follows our
different criteria. In the next few
videos, we're going to look at several other
examples of aromatic compounds and ions.