Measures of Dispersion and Variability: Range, QD, AD and SD
Benzene
1. BENZENE AND DERIVATIVES
MEMBERS GROUP :
NUR HIDAYAH BADARUDDIN
SYAZANA ISMAIL
NOOR AZURAH ABDUL RAZAK
IRA NUSRAT JAAFAR
NOR FADILAH ZAKARIA
2. INTRODUCTION
• Benzene is a chemical that is a colourless or
light yellow liquid at room temperature. It
has a sweet odour and is highly flammable.
• Natural sources of benzene include volcanoes
and forest fires. Benzene is also a natural part
of crude oil, gasoline, and cigarette smoke.
3. Michael Faraday
• The word "benzene" derives
historically from "gum
benzoin", sometimes called
"benjamin" an aromatic resin
known to European pharmacists
and perfumers since the 15th
century as a product of southeast
Asia.
• Michael Faraday first isolated and
identified benzene in 1825 from
the oily residue derived from the
production of illuminating
gas, giving it the name bicarburet
of hydrogen.
8. b) DISUBSTITUTION
• 1)Ortho- disubstituted benzene has two
substituent in a 1,2 positions:
• Principle functional group is the benzene
therefore root = benzene O-dichlorobenzene
• There are two chlorine
substituent therefore dichloro.
• The substituent locants are
1 and 2 therefore ortho.
9. 2) Meta -disubstituted benzene has
two substituents in a 1,3 positions:
• Principle functional group is the benzne
therefore root = benzene m-bromochlorobenzene
• There is a chlorine substituent
therefore chloro.
• There is a bromine substituent
therefore bromo.
• The substituent locants are 1 and 3
therefore meta.
12. c) POLYSUBSTITUTION
• Benzene with more than 2 substituents are
named by numbering the position of each
substituent with lowest possible numbers:
• Principle functional group is the aromatic amine
therefore = aniline
• There is a C1 substituent therefore methyl
• There is a C2 substituent therefore ethyl
• Numbering from the -NH2 (priority group at C1)
gives the substituents the locants =2 and 3
15. 1.The Kekulé Structure of Benzene
• German chemist Friedrich August Kekulé von
Stradonitz
• A structure of benzene, containing 3 cyclic
conjugated double bonds which systematically
called 1,3,5-cyclohexatriene
16. Cont.
• The true structure of benzene is a resonance
hybrid of the two Lewis structures, with the
dashed lines of the hybrid indicating the
position of the π bonds.
hybrid
18. Cont.
• Benzene does not undergo addition reactions typical of other highly
unsaturated compounds, including conjugated dienes.
• Benzene does not react with Br2 to yield an addition product. Instead, in the
presence of a Lewis acid, bromine substitutes for a hydrogen atom, yielding
a product that retains the benzene ring.
19. 3.Boiling Point
• The only attractions between neighbouring molecules are van der Waals
dispersion forces.
• Benzene boils at 80°C -
o-dichlorobenzene m-dichlorobenzene p-dichlorobenzene
b.p. 1810C b.p. 1730C b.p. 1700C
20. 4. The Criteria for Aromacity :
Hückel's Rule
• 4 structural criteria must be satisfied for a
compound to be aromatic:
a) Cyclic
b) Planar
c) Completely Conjugated
d) Contain a particular number of π electrons
21. Cont.
[4] A molecule must satisfy Hückel’s rule, which requires a particular number
of electrons.
Hückel's rule:
•Benzene is aromatic and especially stable because it contains 6 electrons.
•Cyclobutadiene is antiaromatic and especially unstable because it contains 4
electrons.
22. Cont.
Note that Hückel’s rule refers to the number of electrons, not the number of atoms in a
particular ring.
23. Electrophilic Aromatic substituent
An electrophile (E⁺) reacts with an aromatic ring and substitutes
for one of the hydrogen
Benzene does not undergo addition reactions because addition
would yield a product that is not aromatic
25. Step 1 : Formation of arenium ion
Positive ion X+ = electrophile
Example :
Step 2 : Loss of H +
26. Reactants : Benzene and halogens (Clorine or Bromine)
Conditions : Lewis acid like FeCl₃ or FeBr₃
Analogous reaction with I2 and F2 are not synthetically useful because I2 is
too unreactive and F2 is too violently
Electrophile : Cl⁺ or Br⁺
Example :
+ Br₂ no reaction
(decolorization not observed)
+
27. MECHANISM :
Step 1 : Formation of Cl⁺ or Br⁺
Step 2 : Electrophilic substitution
(The electrophile attacks the π electron system of the benzene ring
to form a arenium ion)
28. Step 3 : Loss of proton to reform the aromatic ring
bromobenzene
Function of Lewis acid :
Incerase the polarity of halogen molecules to produce positive
halogen ions (Cl⁺ or Br⁺) = electrophile
29. Reactants : Benzene and concentrated HNO₃
Conditions : Concentrated H₂SO₄
Example :
nitrobenzene
30. MECHANISM :
Step 1 : Formation of Nitronium ion (NO₂⁺)
- Sulfuric acid ionizes to produce a proton
-Nitric acid accepts a proton from a stronger acid (H₂SO₄) and
form a protonated nitric acid
- The protonated nitric acid dissociates to form a nitronium ion
(+NO2)
31. Step 2 : Electropjilic substitution
Step 3 : Loss of proton to re-form the aromatic ring
32. Reactants : Benzene and alkyl halide
Condition : catalyst (Lewis acid like AlCl₃)
+
33. Example :
1) AlCl₃
tolouene
2)
Tert-butylbenzene
34. MECHANISM :
Step 1 : Formation of carbocation
Step 2 : Electrophilic substitution
(electrophile attacks the π electron system of the benzene ring to
form an arenium ion)
35. Step 3 : loss of proton to re-form the aromatic ring
38. Mechanism:
Step 1:
Dissociation of a chloride ion
to form an acyl cation
("acylium ion")
Step 2:
The resulting acylium ion or
a related adduct is subject
to nucleophilic attack by
the arene
Step 3:
Chloride anion (or AlCl4-)
deprotonates the ring (an
"arenium ion") to form
HCl, and the AlCl3 catalyst
is regenerated
39. Aromatic Sulfonation
Organic reaction in which a hydrogen atom on
an arene is replaced by a sulfonic acid
functional group in an electrophilic aromatic
substitution
40. Mechanism:
Step 1:
The p electrons of the aromatic C=C act as a nucleophile, attacking the
electrophilic S, pushing charge out onto an electronegative O atom. This
destroys the aromaticity giving the cyclohexadienyl cation intermediate.
Step 2:
Loss of the proton from the sp3 C bearing the sulfonyl- group reforms
the C=C and the aromatic system.
Step 3:
Protonation of the conjugate base of the sulfonic acid by sulfuric acid
produces the sulfonic acid.
45. Ortho-Para Activator
Electron donating groups (EDG) with lone pairs on the atoms
adjacent to the p system activate the aromatic ring by increasing
the electron density on the ring through a resonance donating
effect.
The resonance only allows electron density to be positioned at
the ortho- and para- positions.
Hence these sites aremore nucleophilic, and the system tends to
react with electrophiles at these ortho- and para- sites.
51. Explanation of meta- deactivators
Meta directors slow the reaction by raising the energy of the
carbocation intermediate because they have (in one resonance
form, shown below) a positively charged atom attached to the
ring. Two positively charged atoms so close together is very high
in energy (especially unstable).
52. SOME EXAMPLES OF "META DIRECTORS“
the acyl group in benzaldehyde
the NO2 group in nitrobenzene
acyl , -CN , -SO3H , CF3 , and -NO2 are meta directors and
deactivate the ring toward electrophilic aromatic substitution.
53. HALOGENATION OF ALKYLBENZENE SIDE CHAIN
Side chain bromination at the benzylic position occurs when aklybenzene is treated
with N-bromosuccinimide (NBS)
54. Mechanism of NBS (Radical) Reaction
•Abstraction of a benzylic hydrogen atom generates an intermediate benzylic radical
•Reacts with Br2 to yield product
•Br· radical cycles back into reaction to carry chain
•Br2 produced from reaction of HBr with NBS
55. Free radical also occurs between alkylbenzene side chain with halogen in the
presence of heat or light (hv)
CH3 hv CH2Cl
+ Cl2
56. OXIDATION OF ALKYLBENZENE
•Alkyl Benzene ring is inert to strong oxidizing agents such as KMnO4 and chromic
reagent
•side chains react readily with oxidizing agents are converted into carbonyl group
–COOH (Benzoic acid)
CH3 KMnO4 COOH
CH2 CH3 KMnO4 COOH
H3C CH3 KMnO4
COOH COOH
