In this lesson, we will learn:
- The structure and bonding of benzene (as an example of an aromatic compound) and the evidence for this.
- To understand benzene’s reactivity in terms of its aromaticity.
- Some key organic reactions of benzene.
- Benzene is a molecule with the formula C6H6. This was known in the mid-19th century, but its exact structure was not.
The 1:1 carbon:hydrogen ratio led chemists to suggest various ring structures made from the 6 carbon atoms, with each one bonded to one hydrogen atom.
This leaves three bonds to assign to each carbon between the two carbon atoms on either side of it in the ring. How are the carbon atoms in benzene bonded?
There were quite a few suggestions. See below for some of them:
They all show a possible structure of C6H6 – three of them are real substances, but two aren’t benzene. Experiments into the isomers of benzene derivatives helped go further – they depended on benzene being a flat, symmetrical ring structure which was later confirmed – the Kekulé model is most accurate.
- The Kekulé model of benzene shows benzene as having alternating C-C and C=C bonds in a six-membered ring:
But this is not completely correct. If half the C-C bonds are single and half are double, we’d have two different bond lengths, but this evidence of benzene’s bonding is known:
- All the carbon-carbon bonds are the same length and they aren’t single or double bonds. The bond length does not match with C-C or C=C bonds found in other molecules.
- It doesn’t react like an alkene. The enthalpy change of hydrogenation of benzene does not match that of C=C bonds in other molecules (such as cyclohexene).
You could say that benzene doesn’t have ‘normal’ single or double bonds; they are somewhere in between.
You can still use the Kekulé model to show this as seen in the image below.
The above is two different resonance structures of benzene ‘contributing’ to form the real structure which is somewhere in between. Think of black and white combining together to form grey.
- Don’t be confused by the arrows; this just shows how the forms are related. Benzene is not constantly shuffling between the two resonance forms.
Grey isn’t just a flickering between black and white, it’s its own thing! Just like you wouldn’t describe grey as just black or white, another way of getting grey is to take a mix black and white.
This is an extremely important point for understanding resonance in chemistry. Benzene has a special type of bonding that can’t be fully expressed with one structure of single and double bonds, and we are trying to show it using only double and single bonds – we’ll take an average of two forms!
- The bonding in benzene can also be described by the delocalised model . This is shown by an overlap of p-orbitals that creates pi-bonds in a conjugated system.
This conjugated system makes benzene very stable compared to alkenes . The C=C double (pi) bond creates reactivity – areas of high electron density will attack electron-poor molecules (electrophiles) and form bonds.
Here is the difference between it in an alkene and benzene:
- In an ordinary alkene the C=C pi bond focuses higher electron density around and the two carbon atoms. This electron density is capable of electrophilic attack – that’s why alkenes will react easily with bromine and the double bond gets ‘opened up’ for example.
- In benzene, the conjugated pi-bond system delocalises electron density reducing it on any one atom. Conjugation ‘dilutes’ the electron density from a black/white single bond/double bond system, into a shade of grey. The electrons don’t have a focal point to attack electrophiles here. That’s why, for example, benzene will not easily react with bromine in an electrophilic addition like alkenes generally will.
When cyclic compounds display this bonding, they are called aromatic compounds. The ring that the conjugated system exists in is called an aromatic ring - benzene is the most well-known aromatic compound.
- Despite being unreactive compared to a similar alkene (e.g. cyclohexene), benzene does undergo some reactions. Many of them require a catalyst to lower the high activation energy barrier.
- Combustion of benzene produces a smoky flame :
2 C6H6 + 15 O2 → 12 CO2 + 6 H2O
This smoky flame is due to carbon particulates. The reaction above shows that 7 ½ moles of oxygen need to react for every mole of benzene – this is a lot! The result is that incomplete combustion frequently happens and so burning benzene releases soot (C particles) that makes the flame smoky.
- The bromination or chlorination of benzene can occur but unlike alkenes, benzene requires a catalyst to react in this way. When iron is added with bromine, the FeBr3 catalyst is produced:
2 Fe + 3 Br2 → 2 FeBr3
This FeBr3 catalyst interacts with and polarises Br2 molecules, creating larger partial charges on Br2 than the molecule could generate alone. This ‘activated bromine’ is a strong enough electrophile for benzene to attack. See the diagram below for the reaction mechanism:
When the bromine molecule is attacked, the FeBr3 catalyst becomes the FeBr4- intermediate, and the aromatic ring is broken. This broken aromatic ring is a very high energy intermediate itself and it will quickly restore aromaticity by breaking a C-H bond, liberating H+ which is picked up by Br- from the catalyst which gets regenerated back to FeBr3.
The final products are therefore the brominated benzene molecule, HBr and the FeBr3 catalyst regenerated.
Chlorination of benzene works the same way, using a FeCl3 catalyst made the same way as FeBr3 is made.
- The nitration of benzene is another electrophilic substitution, where a nitro (-NO2) group can be attached to the benzene ring.
Sulfuric and nitric acid are reacted together to produce the NO2+ reactive intermediate:
HNO3 + 2H2SO4 → H3O+ + 2HSO4- + NO2+
This incredibly reactive NO2+ group generated is called the nitronium ion and as a strong electrophile, benzene will attack it, breaking aromaticity to do so.
The next step is like in the bromination reaction above: a C-H bond will be broken to restore aromaticity, and H2SO4 re-forms from the HSO4- we made earlier. See the diagram below for the reaction mechanism:
The react ion produces nitrobenzene and H2SO4.
- Nitration, chlorination and bromination of benzene are all examples of electrophilic substitution of benzene. This has the following meaning.
- It is electrophilic because the reactions are driven by an electrophile. This is the very reactive, electron-poor species that benzene, being electron rich, attacks. In nitration the electrophile is the NO2+ ion and it is the delta-positive bromine/chlorine in the bromination/chlorination.
- It is a substitution because the reaction substitutes a C-H bond on benzene with a carbon bond to another atom.
- The Friedel-Crafts alkylation and acylation reactions create C-C bonds in benzene and they are also good examples of electrophilic substitution. Like with the others, an AlCl3 catalyst is needed for both these reactions.
- Friedel-Crafts acylation involves substituting a hydrogen on benzene for an acyl group – any hydrocarbon with a C=O attachment where the new bond is made. A good example of this would be ethanoyl chloride (CH3COCl), which with the AlCl3 catalyst becomes an extremely reactive acylium ion (CH3CO+).
AlCl3 + CH3COCl → AlCl4- + CH3CO+
This ion is electrophilic enough for benzene to attack. Breaking aromaticity, the ring is restored by breaking a C-H bond and forming HCl in the same way as HBr gets formed in the bromination reaction we saw above.
See below for the reaction mechanism:
This product is singly-substituted and more than one substitution generally doesn’t happen. The acyl group is electron-withdrawing – the C=O pulls electron density out of the ring, making it less prone to attack electrophiles than before. All in all, this means the substitution happens once then largely stops.
- Friedel-Crafts alkylation substitutes a hydrogen on benzene for an alkyl group, in the same way as acylation but without the carbonyl group on the reactant to be substituted onto the benzene ring. The AlCl3 catalyst produces a carbocation as the electrophile. Using chloroethane:
AlCl3 + CH3CH2Cl → AlCl4- + CH3CH2+
This carbocation is then attacked by benzene, with a C-H bond broken to restore aromaticity afterwards. Like before, the products are the regenerated AlCl3 catalyst, HCl and the new benzene derivative. See below for the mechanism:
Unlike the acylation, Friedel-Crafts alkylation can lead to many substitutions. The alkyl group now attached is has a mild inductive effect, pushing electron density into the ring. This makes it more reactive towards electrophiles than unsubstituted benzene, and the result is this reaction can have mono-, di- or trisubstituted alkylbenzene products.
- Benzene’s relatively unreactive state can be compared to phenol, which is a benzene ring with an -OH hydroxyl group attached.
- With a substituent group, the atoms on the benzene ring become named/numbered in a way that refers to the substituent group:
- Immediately next to the group is ortho or the 1/6 positions.
- Two carbons away from the group is meta or the 2/5 positions.
- The opposite side of the ring to the group is para or the 4 position.
Phenol’s hydroxyl group is electron-donating – the lone pair’s electron density gets pushed into the delocalised pi-system, making it more electron-rich (meaning reactive to electrophiles!) than before.
This means that electrophilic substitutions (like bromination and nitration) are much more feasible with phenol than with benzene alone. You can see this in the sort of conditions required – phenol reactions generally need less concentrated acids, less heat, and less catalysts or activation in general.
Electron-donating effects mean reactions are directed at some positions (atoms) on the aromatic ring. For electron donating groups, the 2, 4 and 6 positions (ortho and para) are activated.
The effect of this is that bromination is much easier for phenol than benzene which does not have these activated ortho and para positions.
These electronic effects that change the reactivity of an aromatic ring are looked at in more detail later in Electron withdrawing and donating effects.