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organic chemistry
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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
