Benzene & Aromatic Compounds

By:Dr. Shatha alaqeel

Introduction• Name= Benzene

• Chemical formula= C6H6

• Structure = Cyclic planner• Hybridization= SP2 • Bond angles= 120o

• Bond Length= 1.39 A

• 1) Benzene can be written as a six-membered ring with alternating single and doublebonds (Kekulé structure).

• 2) Benzene reacts with substitution reactions not with addition reactions

The Aromatic Character• When to say this compound is

aromatic?• 1) If it I cyclic• 2) If it has π and σ alternating bonds• 3) If it is planar

4) Reacts with substitution not addition reactions

5) Contain (4n+2) π electronsThis is called Huckel’s RuleWhere n= 0,1,2,3,…….When n=0 π=2 n=1 π=6 n= 2 π=10 n= 3 π=14

n = 0 (4x0)+2 = 2 π elect. not aromatic

n = 2 (4x2)+2 = 10 π elect. aromatic

n = 1 (4x1)+2 = 6 π electrons aromatic

N N O S....

..

......

Pyridine Pyrrole Furan Thiophene

ExcerciseApply Huckel,s rule to the following

N NH....

Naphthalene Quinoline 1H-Indole

Some Aromatic Ions

n = 0

+ -

not aromatic aromatic not aromatic

-n = 1

+

=

-

not aromatic aromatic

= +

+

n = 1

aromaticnot aromatic

etc...

Not aromatic (not planar)Not aromatic (does not

follow Huckel,s rule

Stability of BenzeneStability of Benzene

+ H2H = 28.6kcal\mol

+ 3 H2 H = 85.8 kcal\mol

+ 3 H2 H = 49.8 kcal\mol

(imaginary)

Benzene is more stable than cyclohexatriene by 36 kcal\mol : Resonance energy

Nomenclature of Organic Compounds

Cl Br I F

Chloro benzene Bromobenzene Iodobenzene Florobenzene

NO2

Ethylbenzene t-Butylbenzene Nitrobenzene

CH3

OHCHO

Tolune Phenol Benzaldehyde

CHCH2

COOH NH2

Styrene Benzoic acid Aniline

-C6H5, Ph- Φ

CH2

Benzyl group Phenyl group

OMe

Anisole 2-Phenylbutane PhenylcyclopentaneBiphenyl

Cl

Benzylchloride

Disubstituted Benzene

Br

Br

Br

Br

Br

Br

1,2-Dibromo-benzene 1,3-Dibromo-benzene1,4-Dibromo-benzene

ortho-Dibromo-benzene meta-Dibromo-benzene para-Dibromo-benzene

• 2) When the substituents are different, they are arranged alphabetically

Cl

F

I Br

NO2

1-Chloro-2-ethyl-benzene

O-Chloro ethylbenzene

1-Fluoro-4-iodo-benzene 1-Bromo-3-nitrobenzene

P-floro iodobenzene m-bromo nitrobenzene

If one of the substituents is part of a common named compound, then the disubstituted benzene is named as a derivative of the parent compound

OH

Cl

CH3

NO2

Br

COOH

2-Chlorophenolo-chlorophenol 4-nitrotolune

p-nitrotoluene

3-bromobenzoic acidm-bromobenzoic acid

Some disubstituted benzenes have common names

CH3

CH3

CH3

CH3

CH3

CH3o-xylene m-xylenep-xylene

Polysubstituted Benzene• Number so as to give the lowest possible numbering to

the substituents

Cl

Cl

Cl

Cl

Cl

Cl

Cl

Cl Cl

1,2,3-trichlorobenzene

NOT 1,4,6-trichlorobenzene

1,2,4-trichlorobenzene 1,3,5-trichlorobenzene

The three trichlorobenzene are isomers

ClF

NO2OH

Br

Br

Br

2,4,6-Tribromophenol

CH3

NO2

NO2

O2N

2,4,6-trinitrotoluene

TNT

1-chloro-3-fluoro-5-nitrobenzene

Electrophilic Aromatic Substitution ReactionsElectrophilic Aromatic Substitution Reactions

COR

RCOCl, AlCl3Acylation

Summary of the electrophilic substitution reactions

Side-Chain Reactions of Aromatic CompoundsSide-Chain Reactions of Aromatic Compounds

a.a. Halogenation of an Alkyl Side Chain Halogenation of an Alkyl Side ChainCH3

Br2

UV light

Toluene

CH2Br

+ BrH

Benzyl Bromide

b.b. Oxidation of an Alkyl Side Chain Oxidation of an Alkyl Side ChainCH3

KMnO4

Toluene

COOH

Benzoic acid

CH2CH3KMnO4

COOH

Benzoic acid

+ + OH2CO2

Orientation effects of substituents in electrophilic Orientation effects of substituents in electrophilic aromatic substitution reactions of monosubstituted aromatic substitution reactions of monosubstituted

BenzenesBenzenes

Ortho , para directors Meta directors

-OH, -OR-NH2, -NHR, -NR2-C6H5-CH3, -R (alkyl)-F, -Cl, -Br, -I

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

OH

HNO3 / H2SO4

OH OH

+

NO2

NO2o-Nitrophenol 53 % p-Nitrophenol

47 %NO2

SO3 / H2SO4

OH

m-Nitrobezenesulfonic acid

SO3H

Orientation effects of substituents in Orientation effects of substituents in electrophilic aromatic substitution electrophilic aromatic substitution

reactions of monosubstituted Benzenesreactions of monosubstituted Benzenes Alkyl groups and groups with lone pairsAlkyl groups and groups with lone pairs direct new groups to ortho-, para- direct new groups to ortho-, para- positions and positions and speed-up speed-up the reaction the reaction (activating groups).(activating groups). HalogensHalogens direct new groups to ortho-, para- positions direct new groups to ortho-, para- positions but it it slow downslow down the the reaction reaction (deactivating).(deactivating). Electron withdrawing groupsElectron withdrawing groups such as nitro, nitrile, and carbonyl direct new such as nitro, nitrile, and carbonyl direct new groups to the meta-position and groups to the meta-position and slowslow the reaction the reaction down down (deactivating).(deactivating).

Thus the order of reactivity of benzene and monosubstituted benzene derivatives as in Thus the order of reactivity of benzene and monosubstituted benzene derivatives as in the followingthe following

Substituted benzene with Substituted benzene with o,po,p directors directors >> Benzene Benzene >> Halobenzene derivatives Halobenzene derivatives >> Substituted benzene with Substituted benzene with m-m- directors directors

OH

HNO3 / H2SO4

OH OH

+

NO2

NO2

COOH

HNO3 / H2SO4

COOH

NO2

COCH3

Br2 / FeBr3

COCH3

Br