CHEMISTRY 2600

Topic #7: Benzene and Its ReactionsSpring 2008

Dr. Susan Lait

2

The Structure of Benzene

Benzene was orginally isolated from coal tar and found to:

Have a molecular formula of C6H6 (DU = 4) Undergo substitution reactions but not addition reactions Form three different dibromides (C6H4Br2)

In the 1800s, several possible structures were proposed:

The Kekulé structure was the closest; however, he thought that the molecule switched rapidly from one cyclohexatriene to the other. We now know that that’s not how resonance works and that, actually, the electrons are delocalized and all six C-C bonds have the same bond order (1½) and length (140 pm).

3

Resonance Energy of Benzene

You’ve no doubt been told that benzene is very stable because it’s aromatic, but why is that? What’s the evidence?

One quantitative measure of benzene’s stability is its resonance energy, the difference between the energy of benzene and the hypothetical “1,3,5-cyclohexatriene”. We expect that, of these two, benzene is lower energy (i.e. more stable).

Since cyclohexatriene doesn’t actually exist, we can’t measure its energy directly. We can, however, assume that if the three bonds were isolated, each would behave in essentially the same way as the bond in cyclohexene.

4

Resonance Energy of Benzene

So, we compare the amount of energy released when benzene is hydrogenated to cyclohexane with the amount of energy that would theoretically be released if 1,3,5-cyclohexatriene could be hydrogenated to cyclohexane:

This gives a resonance energy for benzene of 35.9 kcal/mol. The increased stability comes from electron delocalization…

5

According to molecular orbital (MO) theory*, all -symmetric orbitals in a -system combine to give a set of -MOs in which:

The number of -MOs is equal to the number of p orbitals which combined to make them.

In a linear -system, # energy levels = # -MOs. In a cyclic -system, use the polygon-in-a-circle trick to

predict energy levels; the polygon should always point down. Note that, while benzene has a cyclic -system, 1,3-cyclohexadiene does not!

The number of delocalized -electrons is equal to 2 electrons per double bond + 2 electrons per lone pair that is part of the -system. We are primarily interested in the HOMO and LUMO of any given -system, so make sure you count your electrons carefully to correctly identify the HOMO and LUMO!

* Sorrell implies that MOs can be made from hybridized orbitals. This is a case of mixing two different (and largely incompatible) bonding theories and drives the physical chemists nuts! The (and *) MOs of any nonlinear molecule are not trivial to predict and do not resemble what you’d predict using hybridization and VB theory. Fortunately, we rarely need to know what they look like – and, if we do, we just ask HyperChem! All we need to know is how the energies of the - and -MOs generally compare: , , nonbonding, *, *

COPIED FROM “ADDITION REACTIONS OF CONJUGATED DIENES”.As we’ve already covered this material in CHEM 2600, it has onlybeen included for context and will not be re-covered in class.

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Benzene is Aromatic

Consider the molecular orbitals of benzene.

C

CC

C

CC

H

H

H

H

H

H

E

1

3

4

2

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Benzene is Aromatic

All three bonding MOs are filled and there are no antibonding (or nonbonding) electrons. Since the 1 MO is considerably lower in energy than the bonding MO of an isolated alkene, benzene is therefore lower in energy (i.e. more stable) than the theoretical 1,3,5-cyclohexatriene.

Benzene is thus aromatic. It has 4(1) + 2 = 6 electrons. For a molecule to be aromatic, it must have a cyclic system containing 4n+2 electrons. This is Hückel’s rule.

To be aromatic, a molecule must: Be cyclic. Have an atomic p orbital perpendicular to the ring on every

atom in the ring (creating the cyclic system). Be planar enough that the p orbitals can combine to make the

cylic system. Obey Hückel’s rule. (4n+2 electrons in the cyclic system)

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Benzene is Aromatic

If a molecule were to fit the first three criteria but have 4n electrons instead of 4n+2, it would be antiaromatic. Since an an antiaromatic compound is actually less stable than a nonaromatic compound, molecules that we would predict to be antiaromatic based on electron count usually violate the “be planar” criterion for aromaticity and behave as though they have isolated double bonds.e.g. 1,3,5,7-cyclooctatetraene looks like:

NOT

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Benzene is Aromatic

Which of the following molecules/ions would you predict to be aromatic?

For any of the aromatic molecules containing heteroatoms, indicate whether or not any lone pairs on the heteroatom are part of the cyclic system.

N..pyridine

N

N

H

..imidazole

..

O..furan

..

C

H

..

cyclopentadienyl anion

-C

H

cyclopentadienyl cation

+C

H

tropylium cation

+

1,3,5-cycloheptatriene

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Reactions of Benzene

Just like alkenes, benzene has an electron-rich system and can therefore act as a nucleophile; however, it is not thermodynamically favourable to lose the stability conferred by aromaticity. As such, benzene will not undergo electrophilic addition reactions:

Br

Br

Br

BrBr

Br

Br

Br+

+ x

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Reactions of Benzene

Instead, aromatic compounds undergo substitution reactions (“electrophilic aromatic substitution”) in which an electrophile is added then H+ is eliminated. The net effect is substitution of a hydrogen atom with the electrophilic group.

To make a source of Br+, add Br2 to a strong Lewis acid (FeBr3 or AlBr3):

Similarly, a source of Cl+ can be generated by reacting Cl2 with FeCl3 or AlCl3

H

H

Br

H

Br+

H

Br

H+

H+

+ +

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Reactions of Benzene

Looking at the mechanism on the previous page, it is obvious that the rate limiting step will be the first step. Any substituents that stabilize the carbocation intermediate will therefore make the reaction proceed more quickly. Compare:

The hydroxy group is considered to be an activating group because it increases the rate of electrophilic aromatic substitution.

All electron donating groups (even alkyl groups which are only electron donating by induction) are activating groups.

vs.

H

H

Br

H

Br+

H

Br

H+

H+

+ +

H

OH

Br

OH

Br+

H+

H

Br

OH

+ ++

13

Reactions of Benzene

Activating groups are ortho/para directors, introducing the electrophile 1,2- or 1,4- to the activating group. Why are these two products so heavily favoured over the meta (1,3-) product?

14

Reactions of Benzene

Electrons withdrawing groups are deactivating groups by making the system less nucleophilic.

The nitro group is electron-withdrawing by resonance and nitrobenzene reacts with electrophiles much more slowly than benzene does. It is a meta director, giving primarily the 1,3- product. Why?

15

Reactions of Benzene

Halobenzenes (e.g. chlorobenzene) undergo electrophilic aromatic substitution more slowly than benzene BUT give primarily the ortho and para products. Explain this apparent contradiction.

16

Reactions of Benzene

Halogenation (Cl or Br – not I!) is not the only electrophilic aromatic substitution reaction. It is also possible to introduce nitro groups and sulfonic acid groups:

Br

Br2 HBrFeBr3+ +

S

O

OH

O

SO3

H2SO4

heat+

N+

O

O

H2SO4

heatHNO3+

*Nitro groups can be reduced to –NH2 by hydrogenation (H2 and Pd/C)

17

Reactions of Benzene

It is also possible to introduce alkyl groups and acyl groups using an electrophilic aromatic substitution variant called the Friedel-Crafts reaction.

Friedel-Crafts Alkylation:

Friedel-Crafts Acylation (R≠H, Cl, NH2 or OH):

RAlCl3

+ RCl + HCl

C

O

R

Cl

O

R

AlCl3+ + HCl

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Reactions of Benzene

Both reactions proceed according to the standard electrophilic aromatic substitution mechanism; however, in the alkylation reaction, the electrophile can undergo carbocation rearrangements:

Cl

AlCl3+ and

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Reactions of Benzene

This problem can be circumvented by doing the corresponding acylation reaction then reducing the ketone to CH2 (using, for example, a Wolff-Kishner reduction)

O

Cl

O

AlCl3+

NH2NH2

KOH

H2O, heat

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Reactions of Benzene

What are the major organic products for the following reactions?

OCH3CH3O

Cl

O

AlCl3+

NO2H2SO4

+ SO3heat

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Reactions of Benzene

NH2H2SO4

+ SO3heat

CNH2SO4

+ HNO3heat