ORGANIC CHEMISTRY CHM 207

CHAPTER 4: AROMATIC COMPOUNDS (BENZENE AND TOLUENE)

NOR AKMALAZURA JANI

Aromatic compounds

• • • •

Organic compound that contains a benzene ring in its molecule is known as an aromatic compounds.

Sometimes called arenes.

Molecular formula: C 6 H 6 Represented as a regular hexagon containing an inscribed circle.

Structure of Benzene

•

Can be represented in two abbreviated ways.

•

The corner of each hexagon represents a carbon and a hydrogen atom.

Kekulé Structure of Benzene

Molecular formula is C 6 H 6 All the hydrogen atoms are equivalent Each carbon atom must have four covalent bonds.

Resonance Structure

•

Resonance theory: the structure of benzene is a resonance hybrid structure of two Kekulé cononical forms.

•

The hybrid structure is often represented by a hexagon containing an inscribed circle.

represents a resonance hybrid between the two

• •

Hexagonal ring – 6 carbon-carbon bonds are equal.

Circle – delocalised electrons of the benzene ring

CRITERIA OF AROMATIC COMPOUNDS • • • •

Structure must be cyclic, containing some number of conjugated pi bonds.

Each atom in the ring must have an unhybridized p orbital. (The ring atoms are usually sp 2 hybridized or occasionally sp hybridized).

The unhybridized p orbitals must overlap to form a continuous ring of parallel orbitals. The structure must be planar (or nearly planar) for effective overlap to occur.

Delocalization of the pi electrons over the ring must lower the electronic energy. *

Antiaromatic compound

: fulfills the first three criteria, but delocalization of the pi electrons over the ring electronic energy.

increase the

Huckel’s rule

• • • •

Used to determine aromaticity for planar, cyclic organic compounds with a continous ring of overlapping p orbitals.

If the number of pi ( π) electrons in the monocyclic system is (4N+2), the system is aromatic. N is 0, 1, 2, 3…..

Systems that have 2, 6 and 10 pi electrons for N = 0, 1, 2 is a aromatic.

Systems that have 4, 8, and 12 pi electrons for N = 1, 2, 3 are antiaromatic.

Naming Aromatic Compounds

• •

A substituted benzene is derived by replacing one or more of benzene’s hydrogen atoms with an atom or group of atoms.

A monosubstituted benzene has the formula C 6 H 5 G where G is the group that replaces a hydrogen atom.

•

All hydrogens in benzene are equivalent.

•

It does not matter which hydrogen is replaced by G.

Monosubstituted Benzenes

• Some monosubstituted benzenes are named by adding the name of the substituent group as a prefix to the word benzene.

• The name is written as one word. nitro group

ethyl group nitrobenzene ethylbenzene

•

Certain monosubstituted benzenes have special names.

•

These are parent names for further substituted compounds.

methyl group hydroxy group toluene phenol

benzoic acid

carboxyl group amino group

aniline

Disubstituted Benzenes

•

Three isomers are possible when two substituents replace hydrogen in a benzene molecule.

• The prefixes ortho-, meta- and para- (o-, m- and p-)

are used to name these disubstituted benzenes.

ortho disubstituted benzene substituents on adjacent carbons ortho-dichlorobenzene (1,2-dichlorobenzene) mp –17.2

o C, bp 180.4

o C

meta-dichlorobenzene (1,3-dichlorobenzene) mp –24.82

o C, bp 172 o C meta disubstituted benzene substituents on adjacent carbons

para disubstituted benzene substituents are on opposite sides of the benzene ring para-dichlorobenzene (1,4-dichlorobenzene) mp 53.1, bp 174.4

o C

When one substituent corresponds to a monosubstituted benzene with a special name, the monosubstituted compound becomes the parent name for the disubstituted compound.

phenol 3-nitrophenol

When one substituent corresponds to a monosubstituted benzene with a special name, the monosubstituted compound becomes the parent name for the disubstituted compound.

toluene 3-nitrotoluene

Tri- and Polysubstituted Benzenes

• • • •

When a numbered.

benzene ring has three or more substituents, the carbon atoms in the ring are Numbering starts at one of the substituent groups.

The numbering direction can be clockwise or counterclockwise.

Numbering must be in the direction that gives the substituent groups the lowest numbers.

clockwise numbering 6-chloro 1-chloro

6 5 1

4-chloro

4 2 3

1,4,6-trichlorobenzene

counterclockwise numbering chlorine substituents have lower numbers

3

2-chloro

2 1

1-chloro 4-chloro

4 6 5

1,2,4-trichlorobenzene

• When a compound is named as a derivative of

the special parent compound, the substituent of the parent compound is considered to be C-1 of the ring.

1 6 5 4

toluene

3 2 6 5 1 2 3 4

2,4,6 trinitrotoluene (TNT)

• •

When the hydrocarbon chain attached to the benzene ring is small, the compound is named as benzene derivative.

Example: CH 2 CH 3 ethylbenzene

Naming compounds that cannot be easily named as benzene derivatives



Benzene named as a substituent on a molecule with another functional group as its root by the prefix

phenyl

.

diphenylmethane 4-phenyl-2-pentene

The phenyl group, C

6

H

5

-

CH 2 phenyl benzyl CH=CH 2 NH 2 CH 2 Cl common name phenylethene phenylamine benzyl chloride

• •

If the hydrocarbon chain contains more than three carbon atoms, phenyl is used as part of the name.

Examples: CH 2 (CH 2 ) 5 CH 3 1-phenylheptane CH 3 C CH 2 CH 3 Br 2-bromo-2-phenylbutane

PHYSICAL PROPERTIES OF BENZENE AND ITS DERIVATIVES

• • • •

Benzene derivatives tend to be more symmetrical than similar aliphatic compounds, and pack better into crystals and have higher melting points.

Density: - Slightly dense than non-aromatic analogues, but still less dense than water.

- halogenated benzenes are denser than water.

Insoluble in water Boiling points depends on the dipole moments of compounds.

REACTION OF BENZENE

ELECTROPHILIC SUBSTITUTION REACTIONS OF BENZENE



stability of π-electron system is lost when benzene undergoes addition reactions.



benzene and its derivatives undergo substitution reaction rather than addition reactions.



product of substitution reactions : aromatic compounds and not saturated compounds.

Mechanism of electrophilic substitution of benzene

Step 1: Electrophilic addition of the benzene ring H E E + slow arenium ion (a carbocation) Step 2: Deprotonation of the arenium ion H E Nu fast nucleophile E H Nu

ELECTROPHILIC SUBSTITUTION REACTIONS

a) Halogenation H X X 2 H 2 SO 4 or FeX 3 HX halobenzene b) Nitration H NO 2 HNO 3 H 2 SO 4 2H 2 O nitrobenzene c) Sulphonation H SO 3 H 2 SO 4 SO 3 H benzenesulphonic acid

ELECTROPHILIC SUBSTITUTION REACTIONS

d) Friedel-Crafts alkylation H CH 3 Cl AlCl 3 e) Friedel-Crafts acylation H O AlCl 3 CH 3 CCl CH 3 HCl toluene O C CH 3 HCl acetophenone

Reagents, electrophiles and catalysts in electrophilic substitution reactions

Reactions Halogenation Reagents Cl 2 or Br 2 Nitration Alkylation HNO 3 RCl Acylation RCH=CH 2 RCOCl Sulphonation SO 3 Catalysts AlCl 3 , AlBr 3 , FeCl 3 or FeBr 3 H 2 SO 4 AlCl 3 Electrophiles Cl , Br NO 2 R H 2 SO 4 AlCl 3 RCH-CH 3 H 2 SO 4 RCO SO 3 H

HALOGENATION OF BENZENE

a)Chlorination Cl Cl 2 AlCl 3 HCl chlorobenzene b)Bromination Br FeBr 3 Br 2 HBr bromobenzene c) Iodination I 1/2I 2 HNO 3 NO 2 iodobenzene H 2 O

MECHANISM: BROMINATION OF BENZENE

Step 1: Formation of a stronger electrophile Br Br FeBr 3 Br Br FeBr 3 Br 2 .FeBr

3 intermediate (a stronger electrophile than Br 2 ) Step 2: Electrophilic attack and formation of the sigma complex H H H H H Br H H Br Br FeBr 3 H H H H H H H H Br H H H

sigma complex

FeBr 4 Step 3: Loss of a proton gives the products H H Br H FeBr 4 H H H H H H H Br H HBr FeBr 3 H H H Br H H H

MECHANISM: NITRATION OF BENZENE

Step 1: Formation of the nitronium ion, NO 2 + HO SO 3 H HO NO 2 H 2 O + NO 2 + + HSO 4 Step 2: Formation of an arenium ion as a result of electrophilic addition H NO 2 NO 2 + slow nironium ion arenium ion Step 3: Loss of a proton gives the products H NO 2 NO 2 HSO 4 fast H 2 SO 4

MECHANISM: FRIEDEL-CRAFTS ALKYLATION

Step 1: Formation of electrophile CH 3 H C Cl CH 3 AlCl 3 H CH 3 C CH 3 carbocation (electrophile) AlCl 4 Step 2: Formation of an arenium ion H C CH 3 CH 3 H CH(CH 3 ) 2 arenium ion Step 3: Loss of a proton H CH(CH 3 ) 2 AlCl 4 CH(CH 3 ) 2 HCl + AlCl 3

MECHANISM: FRIEDEL-CRAFTS ACYLATION

Step 1: Formation of electrophile O CH 3 C Cl AlCl 3 O CH 3 C AlCl 4 Step 2: Formation of an arenium ion O CH 3 C O H C CH 3 Step 3: Loss of a proton O H C CH 3 AlCl 4 O C CH 3 HCl + AlCl 3

Ortho-Para

and

Meta

Substituents Directing

•

When substituted benzenes undergo further substituents, the substituent group present in the benzene derivative will influence electrophilic substitution in 2 ways which are: i) Reactivity ii)Orientation

EFFECTS OF SUBSTITUENTS ON THE REACTIVITY OF ELECTROPHILIC AROMATIC SUBSTITUTION

• • •

Substituent group present in the benzene ring can influence the rate of reaction of further substitutions.

Electron-donating groups make the ring more reactive (called activating groups) thus influence the reaction become faster.

Electron-withdrawing groups make the ring less reactive (called deactivating groups) thus influence the reaction become slower.

EFFECTS OF SUBSTITUENTS ON THE ORIENTATION OF ELECTROPHILIC AROMATIC SUBSTITUTION •

A substituents group already in the ring influences the position of further electrophilic substitution whether at ortho, meta or para position.

•

Ortho-para directors : the groups that tend to direct electrophilic substitution to the C2 and C4 positions.

•

Meta directors : the groups that tend to direct electrophilic substitution to the C3 position.

Effetcs of substituent groups on the benzene ring Activating groups (electron donating) -NH -OH 2 -OR -NHCOCH 3 -R

ortho-para

directors -F -Cl -Br -I

ortho-para

directors Deactivating groups (electron-withdrawing)

O C O C O C R OH OR

meta

directors

SO 3 H C NO 2 N NR 3

Example: CH 2 CH 3 Br 2 FeBr 3 -CH 2 CH 3 = ortho and para directors CH 2 CH Br ortho position 3 CH 2 CH Br para position 3 major products CH 2 CH 3 Br meta position minor product

Example: NO 2 Br 2 FeBr 3 -NO 2 = meta director NO 2 NO 2 Br NO 2 Br meta position major product ortho position Br para position minor products

• • • •

REACTIONS OF BENZENE DERIVATIVES

Alkylbenzene such as toluene (methylbenzene) resembles benzene in many of its chemical properties.

It is preferable to use toluene because it is less toxic.

The methyl group activates the benzene nucleus.

Toluene reacts faster than benzene in all electrophilic substitutions.

Reactions of toluene Reactions of the methyl group

Substitution

-halogenation Reactions of the benzene ring

Oxidation Electrophilic substitutions

- Halogenation - Nitration

-

Friedel-Crafts reactions

-

Sulfonation

Addition reaction

-hydrogenation

SIDE-CHAIN REACTIONS

OXIDATION REACTION OF ALKYLBENZENE

CH 2 R hot, conc., KMnO 4 /H + reflux examples: CH 3 hot, conc., KMnO 4 /H + reflux CH 2 CH 3 hot, conc., KMnO 4 /H + reflux CH CH 3 3 hot, conc., KMnO 4 /H + reflux O C OH O C OH O C OH COOH COOH

HALOGENATION OF TOLUENE

Side chain substitution CH 3 CH 2 Cl uv light Cl 2 HCl CH 2 Cl (chloromethyl)benzene CHCl 2 uv light Cl 2 HCl CHCl 2 (dichloromethyl)benzene CCl 3 uv light Cl 2 HCl (trichloromethyl)benzene * Bromination of toluene takes place under similar conditions to yield corresponding bromine derivatives.

SYNTHESIZING A SUBSTITUTED AROMATIC COMPOUNDS Synthesis m-chloronitrobenzene starting from benzene NO 2

?

Cl

•

Two substituents: -NO 2 (meta-directing) and –Cl (ortho-

•

and para-directing) Cannot nitrate chlorobenzene because the wrong isomer (o- and p-chloronitrobenzenes) would formed.

Cl chlorobenzene

HNO 3 , H 2 SO 4

NO 2 NO 2 nitrobenzene

Cl 2 , FeCl 3

Cl m-chloronitrobenzene TWO STEPS: HNO 3 H 2 SO 4 benzene NO 2 nitrobenzene NO 2 Cl 2 FeCl 3 Cl m-chloronitrobenzene

SYNTHESIZING A SUBSTITUTED AROMATIC COMPOUNDS Synthesis p-bromobenzoic acid starting from benzene

?

COOH Br

•

Two substituents: -COOH (meta-directing) and –Br (ortho- and para-

•

directing) Cannot brominated benzioc acid because the wrong isomer

• •

(m-bromobenzoic acid) would formed.

Oxidation of alkylbenzene side chains yields benzoic acids.

Intermediate precursor is p-bromotoluene CH 3 KMnO 4 COOH Br Br

Immediate precursor of p-bromotoluene: i) Bromination of toluene or ii) Methylation of bromobenzene or CH 3 Br 2 FeCl 3 Br CH 3 separate the isomer CH 3 Br Br CH 3 Cl AlCl 3 Br CH 3 CH 3 Br separate the isomer

Immediate precursor of toluene: i) Benzene was methylated in a Friedel-Crafts reaction CH 3 Cl AlCl 3 benzene toluene Immediate precursor of bromobenzene: i) Bromination of benzene CH 3 benzene Br 2 FeBr 3 Br bromobenzene

benzene TWO WORKABLE ROUTES FROM BENZENE TO p-BROMOBENZOIC ACID Br 2 FeBr 3 Br CH 3 Cl AlCl 3 CH 3 Cl AlCl 3 CH 3 Br 2 FeBr 3 Br CH 3 KMnO 4 Br COOH

USES OF BENZENE AND TOLUENE •

Benzene:

- as solvent for oils and fats - starting material for making other chemicals. For example, benzene is used in the cumene process to produce phenol.

- making organic compounds such as phenylethene (styrene) and nitrobenzene. These organic compounds are then used to make plastics (polystyrene), dyes and nylon.

USES OF BENZENE AND TOLUENE •

Toluene:

- A common solvent, able to dissolve paints, paint thinners, silicone sealants, many chemical reactants, rubber, printing ink, adhesives (glues), lacquers, leather tanners and disinfectants.

- As a solvent to create a solution of carbon nanotubes.

- Dealkylation to benzene (industrial uses).

- As an octane booster in gasoline fuels used in internal combustion engines.

-As a coolant in nuclear reactor system loops.