Chapter 8 Aromaticity Reactions of Benzene

Aromatic compounds undergo distinctive reactions which set them apart from other functional groups. They are highly unsaturated compounds, but unlike alkenes and alkynes, they are relatively unreactive and will tend to undergo reactions which involve a retention of their Unsaturation. six π electrons involved are not localized between any two carbon atoms. Instead, they are delocalized around the ring which results in an increased stability.

Hückel rule

An aromatic molecule must be cyclic and planar with sp2 hybridized atoms (i.e.conjugated), but it must also obey what is known as the Hückel rule. This rule states that the ring system must have 4n + 2 π electrons where n 1, 2, 3, etc.

√

1. cyclic and planar with sp2 hybridized atoms2. 4n + 2 π

sp3sp3

cycloheptatriene cyclopentadiene

interrupted cloudnot aromatic

2 pairs of electronsnot aromatic

x

x

x

18-membered cyclic system√

√ √

1. cyclic and planar with sp2 hybridized atoms2. 4n + 2 π

Pyridine Is Aromatic

N

吡啶

Halogenation of Benzene

Electrophilic Substitution

Nitration of Benzene

Electrophilic Substitution

General Mechanism for Electrophilic Aromatic Substitution of Benzene

Y+

+

H

Y

H

Y

H

Y

Y

B:

slow

fast

Sulfonation of Benzene

Electrophilic Substitution

Electrophilic Substitution

mechanism

Electrophilic Substitution

mechanism

Friedel–Crafts alkylation and acylation is electrophilic substitution requiring the presence of a Lewis acid, and are particularly important because they allow the construction of larger organic molecules by adding alkyl (R) or acyl (RCO) side chains to an aromatic ring.

Friedel–Crafts alkylation and acylation

Friedel–Crafts alkylation and acylation

The carbocation will rearrange to a more stable species

How is it possible to make structures like 1-butylbenzene in good yield?

a mineral acid

a mineral acid

alkylation of benzene by an alkene

It is not possible to obtain a good yield of an alkylbenzene containing a straight-chain group via Friedel–Crafts alkylation

However, a Friedel–Crafts acylation–reduction works well

This method avoids using a large excess of benzene in the reaction

Friedel–Crafts acylation must be carried out with more than one equivalent of AlCl3

Electrophilic Substitution

Methodologies Used for the Reduction Step

Functional group transformations

Some substituents cannot be introduced directly onto an aromatic ring by electrophilic substitution. These include the following groups: –NH2, –NHR, NR2, NHCOCH3, CO2H, CN, OH. Although these groups cannot be added directly onto the aromatic ring they can be obtained by transforming a functional group which can be applied directly by electrophilic substitution. -NO2 -NH2

Synthetic planning

A knowledge of the electrophilic substitutions and functional group transformtions which are possible is essential in planning the synthesis of an aromatic compound

difficult to control the alkylation to only one alkyl group

gives better yields

ortho, meta and para substitution

constitutional isomers

Substituent effect

The bromination of toluene goes at a faster rate than the bromination of benzene.Why? The answer lies in the fact that the methyl substituent can affect the rate and the position of further substitution. A substituent can either activate or deactivate the aromatic ring towards electrophilic substitution and does so through inductive or resonance effects

We can classify substituents into four groups depending on the effect they have on the rate and the position of substitution:● activating groups which direct ortho/para by inductive effects 诱导致活的邻、对位定位基● deactivating groups which direct meta by inductive effects 诱导致钝的间位定位基● activating groups which direct ortho/para by resonance effects 共振致活的邻、对位定位基● deactivating groups which direct ortho/para by inductive effects. 诱导致钝的邻、对位定位基There are no substituents which activate the ring and direct meta.

Reaction profileThe first stage in the mechanism is the rate determining step and is the formation of the carbocation. This is endothermic and proceeds through a transition state which requires an activation energy (ΔG#). The magnitude of ΔG# determines the rate at which the reaction will occur and this in turn is determined by the stability of the transition state.

Activating groups-inductive o/p directing诱导致活的邻、对位定位基

1 2

Electron-donatingInductive effect

Alkyl groups are activating groups and are ortho, para directing

Alkyl groups are activating groups and are ortho, para directing

Toluene will be more reactive than benzene. The electrondonating effect of the methyl group into the aromatic ring makes the ring inherently more nucleophilic and more reactive to electrophiles, as well as providing extra stabilization of the reaction intermediate.

Deactivating groups –inductive m directing诱导致钝的间位定位基

withdrawing substituents have the opposite effect. inductive m They deactivate the ring, make the ring less nucleophilic and less likely to react directing with an electrophile. The electron-withdrawing effect also destabilizes the reaction intermediate and makes the reaction more difficult.

The positive charge is placed directly next to the electron-withdrawing nitro group. As a result, these resonance structures are greatly destabilized.

Activating groups –resonance o/p directing共振致活的邻、对位定位基

In phenol, an electronegative oxygen atom is next to the aromatic ring. Since oxygen is electronegative, it should have an electron-withdrawing inductive effect and so might be expected to deactivate the ring. The fact that thephenolic group is a powerful activating group is due to the fact that oxygen is electron rich and can also act as a nucleophile, feeding electrons into the ring through a resonance process.

√ √

Delocalizing the chargefurther stabilizes it

3 =p-π

Activating groups –resonance o/p directing共振致活的邻、对位定位基

–OH ，– OR, –OCOR, –N2, –NHR, –NR2, –NHCOR

Nitrogen is more nucleophilic than oxygen since it is better able to cope with the resulting positive charge. Therefore amine substituents are strongeractivating groups than ethers. An amide group is a weaker activating group since the nitrogen atom is less nucleophilic。

Deactivating groups –inductive o/p directing诱导致钝的邻、对位定位基

the halogen substituents

√√

Inductive effects

This resonance effect is weak since the halogen atom is a much weaker nucleophile than oxygen or nitrogen

Inductive effects> resonance effect

For halogen substituents, the inductive effect is more important than the resonance effect in deactivating the ring. However, once electrophilic substitution does take place, resonance effects are more important than inductive effects in directing substitution.

Di substituted benzenes

A knowledge of how substituents affect electrophilic substitution allows us to choose the most suitable route

Removable substituents

Oxidation

Reduction

Aromatic Compounds