Outline Aromaticity

Huckel’s rule

The Reactions(Electrophilic Substitution)

Halogenation

Friedel-Craft’s Reaction

Alkylation and acylation

Nitration and sulphonation

Oxidation and reduction of

benzene derivates

Disubstitution (Ortho, meta,

para directing groups)

Phenol and aniline

The relative acidity of phenol

The relative basicity of aniline

Diazoniums compounds

L-29,30

The Main Features The bond length is between C – C and C=C (1.38 A)

Due to delocalised electron (resonance structure)

The Main Features The structure is planar

Each carbon has p orbital that forms π bonding

Maximum bonding benzene should planar

p Cloud Formation in Benzene

Aromaticity (Hückel’s Rules) Huckel’s rules define the classification of aromatic and non-aromatic

molecule.

The criteria of aromatic molecule:

All the atoms are sp2 hybridised and in planar cyclicarrangement.

All atoms are sp2 but not acyclic.

Hence, non-aromatic

There is non-sp2 atom.

Hence, non-aromatic

All atoms are sp2 and a cyclic.

Hence, could be aromatic

Huckel’s rules

Huckel’s rule

Number of π-electrons is (4n+2),

How to calculate π-electrons?

based on the structure, p-orbitals in sp2 arrangement has 1electron

Has 6 π-electrons (4n+2, n=1)

Hence, aromatic

Has 4 π-electrons (4n, n=1)

Hence, anti-aromatic

Huckel’s Rule (summary)

Is the molecule

has no non-sp2

atoms?

YES NO

How many π-

electrons in the

molecule?

4n+2 Not 4n+2

aromatic Anti-

aromatic

non-aromatic

Huckel’s rules

Porphyrin ring in the haem

group

Huckel’s rule

Which molecules are aromatic?

6 π-electrons

Is this molecule aromatic?

2 π-electrons

The reactions

Benzene undergoes electrophilic substitution

Doesn’t undergo electrophilic addition

The consequence of aromatic properties

The reactions - Halogenation Halogenation. E.g. chlorination

Via:

The presence of Lewis acid (e.g. AlCl3) helps benzene to react with Cl2

The Reactions – Friedel-Crafts Reaction

Friedel-Crafts Reaction (Alkylation)

To substitute with hydrocarbon chain

Via:

Electrophilic

generation

The reactions Friedel-Crafts Reaction

There is a problem for this reaction when longer alkyl halide is used

Rearrangement of the electrophile (carbocation)

Trying to find the most stable carbocation

The Reactions Friedel-Crafts Reaction (Acylation)

To substitute with R-CO –

Via:

Electrophilic generation acylium ion

stabilised by resonance. Both structures are valid.

The reactions

Acylation can be used to get around the ‘messy’ long chain alkylation.

The Reactions The nitration (concentrated sulphuric acid as catalyst)

Via:

The Reactions

Sulphonation

Via:

The reactions Sulphonation

Producing strong sulphonic acid

The Reactions The Oxidation of toluene

ROH

O

1) KMnO4, OH-, Heat

2) H3O+

Where R is alkyl group

The Reduction of Aniline

NO2 NH2Fe

HCl

aniline

The Reactions• H2O

• 0 - 15 oC

Diazonium salts is a good precursor compound for:

Halogenation

formation of phenol

deamination

coupling reaction of arenediazonium salts

Formation of Diazonium saltsNH2 N

+ N

Cl-

NaNO2, HCl

The Reactions

The Reactions Coupling reaction of arenediazonium salts

Where Q is activating group ( –OH, –NR3).

E.g.:

N+ N

Cl- +

Q

N

N Q

N+ N

Cl- +

OH

N

O+ H

N

H

Cl- OH

NN

N+ N

Cl- +

OH

OH

NN

Disubstitution of Benzene

ortho (1,2)

CH3O OHO

The benzene ring can be substituted with another FG more than once.

The second position is determined by the first FG

O NH2

CH3 NH2 OH Cl HN CH3

O

Three possible positions:R

R1

R

R1

meta (1,3)

R

R1

para (1,4)

Disubstitution of Benzene The determining factor

The nature of FG electron withdrawing (EW) or electron

donating (ED) group

EW: the FG generally has partial positive charge

It deactivate the benzene ring, so it is less reactive

ED: the FG generally has partial negative charge

It activate the benzene ring, so it is more reactive

Disubstitution of Benzene

Disubstitution of Benzene E.g. Application for synthesis route

Phenol

The relative acidity

Acidity The easiness to release H+ (proton)

The stability of the acid conjugate determine the relative acidity.

The comparison with water and alcohol (e.g. ethanol)

The structureOH

Phenol

Let’s put water as the standard and the conjugate.

More stable the conjugate, more acid the substance.

In ethoxide ions the alkyl group push the electrons increasing the charge

In phenoxide ions, it forms a bigger resonance structure due to unbonding

p-orbital

OH

H H3C

O H OH

O-

O-

HH3C

O-

Phenol

The effect of substituent

The principle: The reduction of the charge

The deactivating benzene substituent will make phenol more acidic

The activating benzene substituent will make phenol less acidic.

Phenol 3-methylphenol 3-nitrophenol 3-chlorophenol

pKa = 9.89 pKa = 10.01 pKa = 8.28 pKa = 8.80

OH

CH3

OH

Cl

OH OH

NO2

Phenol

Predict the pKa of 2,4 dinitrophenol.

(a) 10.17

(b) 9.31

(c) 8.11

(d) 3.96

Phenol

Esterification of Phenol

No reaction with carboxylic acid

Only react with acyl chloride or acetic anhydride

OH

+ H3C

OH

O H3O+

No reaction

Phenol

Suggest the products from the reactions below

Phenol

How to distinguish with alcohol?

Since the phenol is more acidic

than alcohol, so it can reacts

with weaker base (e.g.

NaHCO3)

Both of them can react with Na

Aniline

The Basicity of amines

Basicity >< Acidity

Basicity How easy a compound can accept H+

The case: The relative Basicity of ethylamine, amine, and aniline

NH2

aniline

The easiness of compound to accept H+

The availability of lone pair electrons on N atom

NH3

ammonia

H3C NH2

methanamine

Aniline

The reactions

Phenylamine cannot react in the similar way like amine.

Phenylamine is not a better nucleophile than amine

the availability of the electrons on N atom to do the reaction

HNO3

concd H SO2 4

N -

O

+Fe/Sn

O

HCl

NH2 2NaNO , HCl

0 - 15 oC

N+ N

Cl-

This reaction can produce the

other amines. Could you

draw the other products?

Aniline

Phenylamine could form an amide with acyl chloride.

Important synthetic pathway for aniline-based compound