Ch2 Hydrocarbon Aromatic

Chapter 2_4:

Aromatic Hydrocarbon

Prepared by:Arbanah Muhammad

arbanah@micet.unikl.edu.my

TOPICS

• Benzene• Naming benzene derivatives

– Mono Substituted Benzene– Di substituted Benzene– Three Or More Substituents

• Phenyl• Preparation of Benzene• Reaction of Benzene

P/S: IF more than one Lewis structure can be drawn, the molecule or ion is said to have resonance

H

HH

H H

H

OR

Kekule Structure

However, Kekule structure can’t explain why benzene molecules are not undergoes addition reaction.

Kekule structure

Kekule suggested that the carbon atoms of benzene ring are in the ring, bonded to each other by alternating single and double bonds.

Resonance Structure

In 1930s , Linus Pauling introduced the concept of hybrid orbital and resonance.

The real structure of benzene is the resonance hybrid from two Kekulé structure as shown below:

Hybrid resonance structure

AROMATIC COMPOUNDS

Nomenclature of benzene and its derivatives

Arenes = aromatic compounds:the word aromatic has nothing to do with odour.

NAMING BENZENE DERIVATIVES

Many organic molecules contain a benzene ring with one or more substituents.

Many common name are recognized by the IUPAC system

CH3

Common: tolueneIUPAC: methylbenzene

EXAMPLE:

Benzene is the parent name and the substituent is indicated by a prefix.

F

fluorobenzene

Cl

chlorobenzene

Br

bromobenzene

NO2

nitrobenzene

CH2CH3

ethylbenzene

MONO SUBSTITUTED BENZENE

IUPAC rules allow some common names to be retained.

CH3

Toluene

OH

phenol

NH2

aniline

COOH

benzoic acid

CHO

benzaldehyde

Relative position of subsituents are indicated by prefixes ortho, meta, and para ( o–, m–, and p–) or by the use of number.

Br

Br12

1,2–dibromobenzene

or

o–dibromobenzene

Br

Br

12

3

1,3–dibromobenzene

or

m–dibromobenzene

Br

Br

1,4–dibromobenzene

orp–dibromobenzene

12

34

DISUBSTITUTED BENZENE

Two Same Substituents

NO2

NO21

2

1,2–dinitrobenzene

or

o–dinitrobenzene

12

3

1,3–dinitrobenzene

orm–dinitrobenzene

NO2

NO2

NO2

NO2

1,4–dinitrobenzene

orp–dinitrobenzene

12

34

COOH

NO21

2

2–nitrobenzoic acid

or

o–nitrobenzoic acid

12

3

3–nitrobenzoic acidor

m–nitrobenzoic acid

COOH

NO2

4–nitrobenzoic acid

or

p–nitrobenzoic acid

12

34

COOH

NO2

Select one of the substituent that give new parent name and numbered as C1.

DISUBSTITUTED BENZENE

Two Different Substituents

Position of substituents must be indicated by numbers.

Br

Br

Br

1 2

34

1,2,4–tribromobenzene

The substituents are listed alphabetically when writing the name.

Cl

Br

I

2–bromo–1–chloro–3–iodobenzene

THREE OR MORE SUBSTITUENTS

C atom bearing the subtituent that define the new parent name is numbered as C1.

OH

NO2

NO2

2,4–dinitrophenol

HO OH

COOH1

2

34

12

3

4

5

6

3,5–dihydroxybenzoic acid

Br

CH3

CH3

1

23

4

4–bromo–1,2–dimethylbenzene

Br

CH3

CH3

1

23

4

4–bromo–1,2–dimethylbenzene

o–, m– and p– naming system is used for arenes with 2 substituents only!

4–bromo–o–dimethylbenzene

correct

If alkyl substituent is larger than the ring (more than 6 C), the compound is named as phenyl-substituted alkane.

CH–CH2–CH2–CH2–CH2–CH3

CH21

23 4 5 6 7

2–phenylheptane

PHENYL GROUP

Benzene ring as substituent.

Phenyl = C6H5– = Ph

If the chain is unsaturated (have C═C or C≡C) or contains important functional group, the benzene ring is considered as phenyl substituent.

CH2–C C–CH3

1 2 3 4

1–phenyl–2–butene

CH2–CH2–OH

2–phenylethanol

12

phenyl group

CH2—

benzyl group

CH2Br

benzyl bromide

CH2OH

benzyl alcohol

BENZYL GROUP

PROPERTIES OF BENZENE

Physical Properties

Molecular weight

78.11

Melting point

5C

Boiling point 80.1CSolubility Slightly soluble in water

Very soluble in organic solvents and oil

Benzene is a colourless, volatile, highly flammable andnon-polar liquid. Its physical properties include:

Preparation of Benzene

Reaction of Benzene1.HALOGENATION, X22.NITRATION, NO33.FRIEDAL CRAFT AKYLATION,

RCl4.FRIEDAL CRAFT ACYLATION,

RC=0Cl

XX2, FeX3

(X = Cl Br) + HXhalogenatio

n

HNO3

H2SO4

NO2

+ H2Onitratio

n

RCl , AlCl3

(R can rearrange)

R+ HCl

Friedel-CraftsAlkylation

RCCl , AlCl3

O

C–R

O

+ HCl

Friedel-CraftsAcylation

1. Halogenation

The reaction occurs in the presence of Lewis acid catalyst such as Fe, FeX3,AlX3 ( X is Cl or Br)

The catalyst polarized the halogen molecule & acts as an electrophile

+ X2Fe or FeX3

Br

+ Br2

EXAMPLE: Bromination rxn

X

+ HBrFeBr3

+ FeBr3

2. Nitration

Reagents used for nitration of benzene are conc. nitric acid and conc. sulphuric acid as catalyst. The rxn mixture is heated to yield nitrobenzene.

+ HNO3

H2SO4 (conc.)

Heat (50-55C)

+ H2ONO2

3. Friedel-Crafts Alkylation

This rxn used Lewis acid as a catalyst…such as AlCl3.

R

+ RXAlX3

+ HX

Benzene Alkyl halides Alkylbenzene

EXAMPLE:

Benzene

+ CH3CH2Cl

Chloroetane

AlCl3

CH2CH3

+ HCl

Ethylbenzene

4. Friedel-Crafts Acylation

Introducing acyl group R C

O

CH3C

O

acetyl group(ethanoyl group)

C

O

Benzoyl group

Two common acyl groups are the acetyl group and the benzoyl group

In the presence of anhydrous aluminium chloride, benzene react with acyl group to form ketone.

+ CH3C Cl

acetyl chloride

AlCl380C C

O

CH3 + HCl

O

For example:

ketone

Effect of substituents group on the benzene ring

1. A substituted group that is already present will havethe following effects on the subsequent substitutions on the ring.

Change the reactivity of benzene ring @ rate of rxn

The orientation of the incoming substituents.

Group that effect the reactivity of benzene ring

1. Activating group

G

2. Deactivating group

G

is the electron donating group that cause the ring to be more reactive than benzene.

is the electron withdrawing group that cause the ring to be less reactive than benzene.

The group that directs an electrophile to the ortho-para position.

The group that directs an electrophile to the meta position

Group that effect the orientation

1. Ortho-para director

2. Meta director

Common substituent groups and their effect on

Electrophilic Aromatic Subtituent:

-NH2, -NHR, -NR2

-OH-OR-NHCOCH3

-C6H5

-R-H-X-CHO, -COR-SO3H-COOH, -COOR-CN-NR3

+

-NO2

incr

easi

ng r

eact

ivity ortho/para directors

meta directors

Groups Ortho-paradirector Meta director

Activating

-NH2, -NHR, -NR2,-OH, -OR,C6H5-, R-

Deactivating Halogen-NO2, -COOH, -CN, -COOR, -SO3H,-CHO, -COR

EXAMPLE:What is the products of the rxn if these givenCompound goes nitration process?

CH3

NO2

NHO3 / H2SO4

heatmethylbenzeneOrtho-para director

CH3

NO2

CH3

NO2

and

NHO3 / H2SO4

heatnitrobenzene

meta director

NO2

NO2

ADDITIONAL PROBLEMS

1. Write structural formulas for each of the following;

CH3

Br

Br

Br

OH

NO2O2N

CH2BrC

CH3CH3

CH3

p-bromotulueneo-dibromobenzene 3,5-dinitrophenol

Benzyl bromidetert-buthylbenzene