Aromatic Compounds
Click to watch a video
Introduction
Benzene is a compound with six carbon atoms and six hydrogen atoms. It contains three pi bonds. Benzene and its derivatives are often called aromatic compounds. They were originally called this because of the odor these compounds had. But, more recently, the term “aromatic” has come to mean that these compounds are unusually stable.
When three pi bonds are drawn in the benzene ring of dimethylbenzene, it would incorrectly predict that two different isomers exist. One of the isomers would have a double bond between the methyl groups and the other would have a single bond between the methyl groups. In fact, there is only one type of dimethylbenzene. Somehow, both of these forms of dimethylbenzene are the same.
One way of thinking about how these two are the same is to consider them two resonance forms of each other. Each bond of the benzene ring is halfway between a double and single bond. We say that each of these bonds has a bond order of 1.5. The resonance hybrid of a benzene compound is sometimes drawn with a circle in the benzene ring. This is sometimes called the toilet bowl structure.
The toilet bowl structure is a pretty good approximation for the bonding in a benzene ring. With three pi bonds, each carbon atom of the benzene ring is sp2 hybridized with an unhybridized p-orbital on it. These p-orbitals can all overlap causing equally sharing the pi electrons around the ring.
The unusual stability of benzene
The pi bonds in benzene do not react like normal pi bonds. Many of the alkene reactions we learned previously do not apply to benzene rings. For instance, the bromination of an alkene is simple, yet it does not occur with a benzene ring.
We see that the benzene ring is unusually stable. Its pi bonds do not behave like regular pi bonds. Why is benzene so stable? How can we explain that? At first glance, you might think benzene is so stable because we can draw resonance forms with it. But, this is incorrect. We can draw resonance forms with other similar compounds, yet they are not very stable.
Annulenes
Annulenes are cyclic hydrocarbons with alternating double and single bonds throughout its structure. Cyclobutadiene, benzene, and cyclooctatetraene are all annulenes.
Cyclobutadiene is very unstable. Benzene is very stable. Cyclooctatetraene has the stability of a normal annulene. All three of these annulenes have other good resonance forms. We clearly see that the unusual stability of benzene does not come from its ability to have another resonance form.
We need another theory beyond resonance forms to explain the unusual stability of benzene. We must use molecular orbital theory to explain it.
Molecular orbital diagrams of annulenes
Benzene is extra stable in its ring form. It is aromatic. The benzene pi system contains six sp2 hybridized carbon atoms. Each carbon atom has an unhybridized p-orbital. These six p-orbitals can overlap. When they overlap, they can overlap constructively to make bonding sites or destructively to make antibonding sites.
The lowest energy is when all of the p-orbitals are in phase with each other. This totally overlapping set of orbitals has zero nodes. This makes a pi-bonding molecular orbital, 𝜋1. There are two ways to overlap the p-orbitals to make one node, 𝜋2 and 𝜋3. There are two ways to overlap the p-orbitals to make two nodes, 𝜋4* and 𝜋5*. In these, there is more destructive overlap than constructive overlap, so these are antibonding molecular orbitals. We indicate they are antibonding by using *. Finally, there is one way to overlap p-orbitals where each p-orbital is out of phase with the next. This makes a totally anti-bonding pi orbital, 𝜋6*.
Once we know the ways the p-orbitals can overlap to form the various molecular orbitals, we can begin to draw a molecular orbital energy diagram. Benzene, with its three pi bonds, has six pi-system electrons. The molecular orbitals are filled in from the lowest energy up. This fills up 𝜋1, 𝜋2, and 𝜋3 with a pair of electrons in each. Each of these molecular orbitals is a bonding molecular orbital. By sharing the pi electrons over more than two atoms in these molecular orbitals, we get even lower energy electrons. The amount of energy we save by dropping below the nonbonding line is all saved energy giving us extra stability for benzene.
Overall, because benzene is more stable in its ring form than it is in its non-ring form, we say that benzene is aromatic.
MO diagram of cyclobutadiene
Cyclobutadiene is especially unstable. We call it antiaromatic. Its molecular orbital diagram will help us understand why it is so reactive.
For cyclobutadiene, there are four sp2 hybridized carbon atoms with an unhybridized p-orbital on each carbon atom. The lowest energy form for the overlapping p-orbitals is if they all overlap and make a totally bonding molecular orbital with zero nodes in it, 𝜋1. There are two ways to overlap to make one node, 𝜋2 and 𝜋3. There is one way where the p-orbitals are all out of phase with each other to make two nodes and a totally antibonding molecular orbital, 𝜋4*.
Notice that the molecular orbitals2 and 3 are the same as the two resonance forms of the Lewis structures we draw for cyclobutadiene.
Once we know the ways the p-orbitals can overlap to form the various molecular orbitals, we can begin to draw a molecular orbital energy diagram. Cyclobutadiene, with its two pi bonds, has four pi-system electrons. The molecular orbitals are filled in from the lowest energy up. The molecular orbital 𝜋1 is filled with a pair of electrons. Then, following Hund’s rule where each orbital of the same energy is filled with one electron before the electrons are paired together, we get two electrons that are in separate orbitals, 𝜋2 and 𝜋3. This diradical (two unpaired electrons) is very reactive.
The molecule is unstable. Therefore, we say that cyclobutadiene is antiaromatic. Cyclobutadiene is less stable than its open ring counterpart.
Counting Pi-System electrons
Let’s practice counting pi electrons that are in the pi-system of these conjugated cyclic compounds. Each pi bond of the double bonds is obviously part of the pi system. There are two electrons for each of these pi bonds. If there is a carbocation in the system, there is an empty p-orbital there. This extends the conjugation of the system because this empty p-orbital can overlap with the other p-orbitals, but it donates zero electrons to the pi-system. Each carbanion is sp2 hybridized with two electrons in the p-orbital adding to the electron count of the pi-system.
Polygon rule
The MO diagram of the pi system of a planar, totally conjugated ring system has the same polygon shape as the molecule does with the vertex (point) at the bottom of the diagram. We can remember the vertex is at the bottom because there is always only one lowest energy molecular orbital, the one where all of the p-orbitals overlap. There is always only one way to do this and have zero nodes.
1. For the following MO diagrams of the compounds above, circle the word “aromatic” or “antiaromatic” to indicate whether the MO diagram represents an aromatic or an antiaromatic compound. Hint: Look at whether all of the electrons are paired up or if there are two unpaired electrons.
2. Shade in the atomic p-orbitals to represent what would overlap to make the following molecular orbitals.
Aromatic, antiaromatic, nonaromatic
4N+2, Hückel’s rule
In these flat, completely conjugated ring systems, we find that there is a pattern for which types of molecules are aromatic and which are antiaromatic.
2 e- = aromatic
4 e- = antiaromatic
6 e- = aromatic
8 e- = antiaromatic
We find that is the number of electrons in the pi system = 4N+2, where N is an integer, then the system is aromatic. If the number of electrons in the pi system = 4N, then the system is antiaromatic. This is Hückel’s rule.
2 e- = 4N+2 = 4(0)+2 = aromatic
4 e- = 4N = 4(1) = antiaromatic
6 e- = 4N+2 = 4(1)+2 = aromatic
8 e- = 4N = 4(2) = antiaromatic
Identifying if aromatic, antiaromatic, or nonaromatic
4N+2, Hückel’s rule
Aromatic systems are cyclic, completely conjugated systems that are more stable than their open-ring counterparts. Antiaromatic systems are cyclic, completely conjugated systems that are less stable than their open-ring counterparts. In order for a system to be aromatic or antiaromatic, it MUST have the following characteristics.
1. It must be a cyclic compound. If it is not a ring, it is neither aromatic nor antiaromatic.
2. It must be completely conjugated at every atom in the ring. Each atom of the ring must have an unhybridized p-orbital that can overlap.
3. The ring must be a flat, planar structure. All of the p-orbitals must be able to overlap with each other so the pi-electrons can be shared.
If any of the three above considerations is not true for a compound, then the compound is a nonaromatic system.
3. Each of the following compounds is nonaromatic. Which of the previous rules, 1-3, does each compound break?
a)
b)
c)
4. For the following compounds, determine whether the molecule is aromatic, antiaromatic, or nonaromatic. Assume the molecules are planer.
a)
b)
c)
d)
e)
f)
g)
h)
i)
5. Predict whether the following compounds would be flat (planar) or not.
a)
b)
c)
d)
e)
Aromatic or Antiaromatic Reaction Intermediates
Consider cyclopentadiene. Would we predict the protons on cyclopentadiene to be very acidic or not?
Like any time we consider the acidity of a proton, we look at the conjugate base once the proton is removed and see how stable that anion is. In this case, the cyclopentadienyl anion is a completely conjugated, flat ring system that contains six pi electrons in the pi system. This is an aromatic compound and is very stable. This is a species that would like to be made, so we know that the protons on cyclopentadiene are quite acidic. These protons have a pKa of 16.
Let’s predict the acidity of the protons of cyclopropene.
Once again, let’s consider the stability of its conjugate base. In this case, the cyclopropenyl anion is a completely conjugated, flat ring system that contains four pi electrons in the pi system. This is an antiaromatic compound and is very unstable. This is a species that would not like to be made, so we know that the protons on cyclopropene are not very acidic. They have a pKa of 61. This is 1045 times less acidic than cyclopentadiene.
Two other reactions that give stable, aromatic species are the formation of a tropylium ion and the formation of a cyclooctatetraene dianion. A tropylium ion is a cycloheptatrienyl cation. It is a completely conjugated, flat ring system with six pi electrons in the pi system. It is aromatic. It can be formed by the acid dehydration of cycloheptatrienyl alcohol.
When cyclooctatetraene reacts with potassium metal, it can form a dianion. The cyclooctatetraenyl dianion is a fully conjugated, flat ring with ten pi electrons in the pi-system. This is a stable, aromatic compound.
The general rule is if we make a stable, aromatic compound as a product, this is good and the reaction will proceed easily. If we make an unstable, antiaromatic compound as a product, this is bad and the reaction will not proceed very well.
6. Predict whether or not the following reactions would proceed well.
a)
b)
c)
d)
Answers
1. a) aromatic b) antiaromatic c) antiaromatic d) aromatic e) antiaromatic
2.
3. Each of the following compounds is nonaromatic. Which of the previous rules, 1-3, does each compound break?
4.a) aromatic b) antiaromatic c) antiaromatic d) nonaromatic e) aromatic f) aromatic g) antiaromatic
h) nonaromatic i) antiaromatic
5. a) no b) no (a and b would be antiaromatic if flat) c) yes d) yes e) yes (c, d, and e would be aromatic if flat.)
6. a) yes b) no c) no d) no