Hydrocarbons 15.3 Arenes

i - Nomenclature

Benzene molecular formula C6H6, skeletal formula , structural/displayed formula

methylbenzene (toluene) ethylbenzene propylbenzene

The C6H5- aromatic ring grouping is called a phenyl group when quoted as a substituent prefix.

When have two substituents, use ortho (1,2), meta (1,3) and para (1,4)

1,2-diethylbenzene 1,3-dipropylbenzene

CH3 CH3

CH3

CH3

CH3

CH3

CH3

toluene o-xylene m-xylene p-xylene

i - Nomenclature

i - Nomenclature

If the OH group (hydroxy) is directly attached to a benzene ring, the molecule is classified as a 'phenol'.

If not, the molecule is classified as an

aliphatic alcohol.

2-chlorophenol (o-chlorophenol) 3-methylphenol (m-methylphenol)

If the OH is not attached to a benzene ring you get an aliphatic alcohol which is isomeric with a phenols or an ether.

phenylmethanol (old name 'benzyl alcohol') is a primary aliphatic alcohol

i - Nomenclature

benzene derivatives :

. Example : 2-aminophenol 4-aminophenol 2-nitrophenol

3-hydroxybenzoic acid 2,5-dichloro-4-methylphenol

2-hydroxybenzaldehyde 1-phenylethanone

McMurry Organic Chemistry 6th edition Chapter 16 (c) 2003

7

Substitution Reactions of Benzene and Its Derivatives

Benzene is aromatic: a cyclic conjugated compound with 6 electrons

Reactions of benzene lead to the retention of the aromatic core

Electrophilic aromatic substitution replaces a proton on benzene with another electrophile

.

.

9

Nitration of benzene

The combination of nitric acid and sulfuric acid produces NO2

+ (nitronium ion)

The reaction with benzene produces nitrobenzene

. MECHANISM FOR NITRATION OF BENZENE Step 1:

An acid / base reaction. Protonation of the hydroxy group of the nitric acid. This provides a better leaving group.....

Step 2: Loss of the leaving group, a water molecule provides the nitronium ion, the reactive electrophile.

Step 3: The electrophilic nitronium ion reacts with the nucleophilic C=C of the arene. This is the rate determining step as it destroys the aromaticity of the arene.

Step 4: Water functions as a base to remove the proton from the sp3 C bearing the nitro- group and reforms the C=C and the aromatic system.

Halogenation of Benzene Overall transformation : Ar-H to Ar-X

Reagent : normally the halogen (e.g. Br2) with a Lewis acid catalyst

The active catalyst is not Fe (0) but the FeX3 formed by reaction of Fe with X2

Electrophilic species : the halonium ion (i.e. X +) formed by the removal of a halide ion by the Lewis acid catalyst

Restricted to Cl2 and Br2. I- or F- are usually introduced using alternative methods

. Step 1: The bromine reacts with the Lewis acid to form

a complex that makes the bromine more electrophilic.

Step 2: The p electrons of the aromatic C=C act as a nucleophile, attacking the electrophilic Br, and displacing iron tetrabromide. This step destroys the aromaticity giving the cyclohexadienyl cation intermediate.

Step 3: Removal of the proton from the sp3 C bearing the bromo- group reforms the C=C and the aromatic system, generating HBr and regenerating the active catalyst.

13

Aromatic Chlorination and Iodination

Chlorine and iodine (but not fluorine, which is too reactive) can produce aromatic substitution with the addition of other reagents to promote the reaction

Chlorination requires FeCl3

Iodine must be oxidized to form a more powerful I+ species (with Cu+ or peroxide)

Sulfonation of Benzene Overall transformation : Ar-H to Ar-SO3H, a sulfonic acid.

Reagent : for benzene, H2SO4 / heat or SO3 / H2SO4 / heat (= fuming sulfuric acid)

Electrophilic species : SO3 which can be formed by the loss of water from the sulfuric acid

Unlike the other electrophilic aromatic substitution reactions, sulfonation is reversible.

Removal of water from the system favours the formation of the sulfonation product.

Heating a sulfonic acid with aqueous sulfuric acid can result be the reverse reaction, desulfonation.

Sulfonation with fuming sulfuric acid strongly favours formation of the product the sulfonic acid.

.

Step 1: The p electrons of the aromatic C=C act as a nucleophile, attacking the electrophilic S, pushing charge out onto an electronegative O atom. This destroys the aromaticity giving the cyclohexadienyl cation intermediate.

Step 2: Loss of the proton from the sp3 C bearing the sulfonyl- group reforms the C=C and the aromatic system.

Step 3: Protonation of the conjugate base of the sulfonic acid by sulfuric acid produces the sulfonic acid.

McMurry Organic Chemistry 6th edition Chapter 16 (c) 2003

16

Alkali Fusion of Aromatic Sulfonic Acids

Sulfonic acids are useful as intermediates

Heating with NaOH at 300 C followed by neutralization with acid replaces the SO3H group with an OH

Example is the synthesis of p-cresol

Friedel-Crafts Alkylation of Benzene Overall transformation : Ar-H to Ar-R

Named after Friedel and Crafts who discovered the reaction in 1877.

Reagent : normally the alkyl halide (e.g. R-Br or R-Cl) with aluminum trichloride, AlCl3, a Lewis acid catalyst

The AlCl3 enhances the electrophilicity of the alkyl halide by complexing with the halide

Electrophilic species : the carbocation (i.e. R +) formed by the "removal" of the halide by the Lewis acid catalyst

The reactive electrophile, the carbocation is prone to rearrangement to a more stable carbocation which will then undergo the alkylation reaction.

. Friedel-Crafts reactions are limited to arenes as or more reactive

than mono-halobenzenes Other Lewis acids such as BF3, FeCl3 or ZnCl2 can also be used .Other sources of carbocations can also be used:

from loss of water from alcohols treated with acid

from the protonation of alkenes by acid

.

MECHANISM FOR THE FRIEDEL-CRAFTS ALKYLATION OF BENZENE

Step 1:

The alkyl halide reacts with the Lewis acid to form a more electrophilic C, a carbocation

Step 2: The p electrons of the aromatic C=C act as a nucleophile, attacking the electrophilic C+. This step destroys the aromaticity giving the cyclohexadienyl cation intermediate.

Step 3: Removal of the proton from the sp3 C bearing the alkyl- group reforms the C=C and the aromatic system, generating HCl and regenerating the active catalyst.

McMurry Organic Chemistry 6th edition Chapter 16 (c) 2003

20

Limitations of the Friedel-Crafts Alkylation

Only alkyl halides can be used (F, Cl, I, Br)

Aryl halides and vinylic halides do not react (their carbocations are too hard to form)

Will not work with rings containing an amino group substituent or a strongly electron-withdrawing group

Friedel-Crafts Acylation of Benzene

Overall transformation : Ar-H to Ar-COR (a ketone) Reagent : normally the acyl halide (e.g. usually RCOCl)

with aluminum trichloride, AlCl3, a Lewis acid catalyst The AlCl3 enhances the electrophilicity of the acyl halide by complexing with the halide Electrophilic species : the acyl cation or acylium ion (i.e. RCO + ) formed by the "removal" of the halide by the Lewis acid catalyst

. The acylium ion is stabilized by resonance as shown below. This extra stability prevents the problems associated with the rearrangement of simple carbocations:

The reduction of acylation products can be used to give the equivalent of alkylation but avoids the problems of rearrangement (more details)

Friedel-Crafts reactions are limited to arenes more reactive than mono-halobenzenes

Other sources of acylium can also be used such as acid anhydrides with AlCl3

MECHANISM FOR THE FRIEDEL-CRAFTS ACYLATION OF BENZENE

Step 1: The acyl halide reacts with the Lewis acid to form a more electrophilic C, an acylium ion

Step 2: The p electrons of the aromatic C=C act as a nucleophile, attacking the electrophilic C+. This step destroys the aromaticity giving the cyclohexadienyl cation intermediate.

Step 3: Removal of the proton from the sp3 C bearing the acyl- group reforms the C=C and the aromatic system, generating HCl and regenerating the active catalyst.

. Thus, there are two types of Friedel-Crafts reactions, alkylation and acylation:

Example :

Stability of benzene towards oxidation

Double bonds of benzene are more stable than aliphatic alkenes due to the :

delocalisation of 6 electrons among the 6 carbons in the ring.

The formation of resonance structures.

So that, benzene does not undergo oxidation.

Benzene also does not undergo reduction process since it does not react with oxidizing agents such as acidified KMnO4 and acidified KCr2O7. However, the exception occur when benzene can be reduced to cyclohexane in presence of H2 gas with Nickel as catalyst at 180 oC.

The test to differentiate alkene and benzene : Put chemical in cold KMnO4. If the chemical decolourise the

dark puple colour, then the chemical does not contain ring structure.

Self-assessment

.

.

.

Chemicals Reaction of Benzene

15.3 Arenes

From previous section (benzene) here is the summary:

2

3

Substituent Effects in Aromatic Rings

Substituents can cause a compound to be (much) more or (much) less reactive than benzene

Substituents affect the orientation of the reaction the positional relationship is controlled

ortho- and para-directing activators, ortho- and para-directing deactivators, and meta-directing deactivators.

4

Origins of Substituent Effects

An interplay of inductive effects and resonance effects

Inductive effect - withdrawal or donation of electrons through a s bond = Polar Covalent Bonds

Resonance effect - withdrawal or donation of electrons through a bond due to the overlap of a p orbital on the substituent with a p orbital on the aromatic ring

5

Inductive Effects

Controlled by electronegativity and the polarity of bonds in functional groups

Halogens, C=O, CN, and NO2 withdraw electrons through s bond connected to ring

Alkyl groups donate electrons

6

Resonance Effects Electron Withdrawal

C=O, CN, NO2 substituents withdraw electrons from the aromatic ring by resonance

electrons flow from the rings to the substituents

Look for a double (or triple) bond connected to the ring by a single bond

7

Resonance Effects Electron Donation Halogen, OH, alkoxyl (OR), and amino substituents donate

electrons

electrons flow from the substituents to the ring

Effect is greatest at ortho and para positions

Look for a lone pair on an atom attached to the ring

8

Substituent Effects

Substituent that donate electrons make ring more nucleophilic Electron donating groups (EDG) activate the ring toward EAS

Substituent that withdraw electrons make less nucleophilic Electron withdrawing groups (EWG) deactivate the ring toward EAS

E++E

Nucleophile Electrophile

9

Substituent Effects

Substituents influence by induction

EDG activate the ring toward EAS EWG deactivate the ring toward EAS

Halogens, C=O, CN, and NO2

withdraw electrons through s

bond connected to ring

Alkyl groups donate electrons

10

Substituent Effects

Substituents influence by resonance

EDG activate the ring toward EAS

Halogen, OH, alkoxyl (OR), and amino substituents donate electrons

11

Substituent Effects Substituent influence by resonance

ewg deactivate the ring toward EAS

C=O, CN, NO2 substituents withdraw electrons from the aromatic ring by resonance

12

Ortho- & Para-Directing Activators: Alkyl

Groups Alkyl groups activate: direct further substitution to positions ortho and para

to themselves

13

Ortho- and Para-Directing Activators: OH and NH2

Alkoxyl, and amino groups have a strong, electron-donating resonance effect

Most pronounced at the ortho and para positions

14

Ortho- & Para-Directing Deactivators: Halogens

Electron-withdrawing inductive effect outweighs weaker electron-donating resonance effect

Resonance effect is only at the ortho and para positions, stabilizing carbocation intermediate

15

Meta-Directing Deactivators Inductive and resonance effects reinforce each other

Ortho and para intermediates destabilized by deactivation of carbocation intermediate

Resonance cannot produce stabilization

Reactivity

.

17

Orientation

1) Halogenation of alkylbenzene

There are two conditions :

If ultraviolet or sunlight present : Free radical substitution of the alkyl side chain

If Lewis acid is present : Electrophilic aromatic substitution of the benzene ring

industrial process

highly regioselective for benzylic position

1.1 Free-radical Chlorination of Toluene

CH3

Cl2

light or

heat

CH2Cl

Toluene Benzyl chloride

Similarly, dichlorination and trichlorination are selective for the benzylic carbon. Further chlorination gives:

Free-radical Chlorination of Toluene

CCl3

(Dichloromethyl) benzene

CHCl2

(Dichloromethyl) benzene

is used in the laboratory to introduce a halogen at the benzylic position

Benzylic Bromination

CH3

NO2

+ Br2

CCl4, 80C

light

p-Nitrotoluene

+ HBr

NO2

CH2Br

p-Nitrobenzyl bromide (71%)

is a convenient reagent for benzylic bromination

N-Bromosuccinimide (NBS)

NBr

O

O

CCl4

benzoyl peroxide,

heat

CH2CH3

+

CHCH3

NH

O

O

+

Br

(87%)

.

.

1.2 EAS on benzene ring

Substitution in the ring happens in the presence of aluminium chloride (or aluminium bromide if you are using bromine) or iron.

This is exactly the same as the reaction with benzene, except that you have to worry about where the halogen atom attaches to the ring relative to the position of the methyl group.

Methyl groups are 2,4-directing, which means that incoming groups will tend to go into the 2 or 4 positions on the ring - assuming the methyl group is in the 1 position. In other words, the new group will attach to the ring next door to the methyl group or opposite it.

. With chlorine, substitution into the ring gives

a mixture of 2-chloromethylbenzene and 4-chloromethylbenzene.

26

2)Oxidation of Alkylbenzene Methylbenzene is heated under reflux with a solution of

potassium manganate(VII) made alkaline with sodium carbonate. The purple colour of the potassium manganate(VII) is eventually replaced by a dark brown precipitate of manganese(IV) oxide.

The mixture is finally acidified with dilute sulphuric acid.

Overall, the methylbenzene is oxidised to benzoic acid.

CH3

CH2R

CHR2

or

or

COH

O Na2Cr2O7

H2SO4

H2O

heat

.

Example

Na2Cr2O7

H2SO4

H2O

heat

COH

O

CH3

NO2

p-Nitrotoluene

NO2

p-Nitrobenzoic acid (82-86%)

Example

Na2Cr2O7

H2SO4

H2O

heat

CH(CH3)2

CH3

(45%)

COH

O

COH

O

3) Friedel-Crafts acylation of alkylbenzene

The reaction is just the same with benzene except that you have to worry about where the acyl group attaches to the ring relative to the methyl group.

Normally, the methyl group in methylbenzene directs new groups into the 2- and 4- positions (assuming the methyl group is in the 1- position

4) Friedel-Crafts alkylation of alkylbenzene

Again, the reaction is just the same with benzene except that you have to worry about where the alkyl group attaches to the ring relative to the methyl group.

Methylbenzene reacts rather faster than benzene - in nitration, the reaction is about 25 times faster. That means that you would use a lower temperature to prevent more than one nitro group being substituted - in this case, 30C rather than 50C. Apart from that, the reaction is just the same - using the same nitrating mixture of concentrated sulphuric and nitric acids.

Products formed: 2-nitromethylbenzene and 4-nitromethylbenzene.

4) Nitration of alkylbenzene

15.3 Arenes part 115.3 Arenes part 2