Transcript Reaction
Chapter 31 Aromatic Hydrocarbons
1 31.1
31.2
31.3
31.4
31.5
31.6
31.7
Introduction Nomenclature of the Derivatives of Benzene The Stability of Benzene Physical Properties of Aromatic Hydrocarbons Preparation of Benzene Reactions of Benzene Alkylbenzenes New Way Chemistry for Hong Kong A-Level Book 3A
31.1
Introduction (SB p.147)
Benzene
•
Highly unsaturated
• Six-membered ring compound with
alternative single and double bonds
between adjacent carbon atoms • Chemically
unreactive
compared to alkenes 2 New Way Chemistry for Hong Kong A-Level Book 3A
31.2
Nomenclature of the Derivatives of Benzene (SB p.148)
1. Monosubstituted benzenes (a)
For certain compounds, benzene is the parent name and the substituent is simply indicated by a prefix 3 New Way Chemistry for Hong Kong A-Level Book 3A
31.2
Nomenclature of the Derivatives of Benzene (SB p.148)
(b)
For other compounds, the substituent and the benzene ring taken together may form a
new parent name
4 New Way Chemistry for Hong Kong A-Level Book 3A
31.2
Nomenclature of the Derivatives of Benzene (SB p.148)
2. Polysubstituted benzenes
(a) If more than one substituent are present and the substituents are identical, their relative positions are indicated by the use of numbers assigned on the ring. The prefixes ‘di-’, ‘tri-’, ‘tetra-’, and so on are used.
5 New Way Chemistry for Hong Kong A-Level Book 3A
31.2
Nomenclature of the Derivatives of Benzene (SB p.149)
(b)
When more than one substituent are present and the substituents are different, they are listed in alphabetical order.
6 New Way Chemistry for Hong Kong A-Level Book 3A
31.2
Nomenclature of the Derivatives of Benzene (SB p.149)
(c)
When a substituent is one that when taken together with the benzene ring gives a new parent name, that substituent is assumed to be in position 1 and the new parent name is used 7 New Way Chemistry for Hong Kong A-Level Book 3A
31.2
Nomenclature of the Derivatives of Benzene (SB p.149)
Example 31-1
Draw the structural formula for each of the following compounds:
Solution:
(a) 1,3,5-Trichlorobenzene
(a)
(b) 2,5-Dibromophenol
(b)
(c) 2,4-Dinitrobenzoic acid
Answer (c) 8 New Way Chemistry for Hong Kong A-Level Book 3A
31.2
Nomenclature of the Derivatives of Benzene (SB p.150)
Check Point 31-1
Give the IUPAC name for each of the following compounds: (a) (b)
9
(c)
(a) 1,2-Dimethylbenzene (c) 3-Bromo-5-chlorobenzoic acid (d) 4-Bromo-2,6-dinitrophenol New Way Chemistry for Hong Kong A-Level Book 3A Answer
31.3
The Stability of Benzene (SB p.151) In 1865, Kekule proposed the structure of benzene: 10 New Way Chemistry for Hong Kong A-Level Book 3A
31.3
The Stability of Benzene (SB p.151) According to the Kekulé structure, there should be two different 1,2-dibromobenzenes: 11
Only one 1,2-dibromobenzene has been found!!
New Way Chemistry for Hong Kong A-Level Book 3A
31.3
The Stability of Benzene (SB p.151) According to the Kekulé structure, benzene should • undergo
addition reactions
readily • it gave
substitution reaction products
rather than addition reaction products
Kekulé structure cannot explain the behaviour of benzene
12 New Way Chemistry for Hong Kong A-Level Book 3A
31.3
The Stability of Benzene (SB p.152)
Enthalpy Changes of Hydrogenation of Benzene and Cyclohexene
Enthalpy change of hydrogenation of cyclohexene = –119.6 kJ mol –1 13 New Way Chemistry for Hong Kong A-Level Book 3A
31.3
The Stability of Benzene (SB p.152) The enthalpy change of hydrogenation of 1,3-cyclohexadiene is expected to be
twice
that of cyclohexene 14 New Way Chemistry for Hong Kong A-Level Book 3A
31.3
The Stability of Benzene (SB p.152) If benzene has the structure of 1,3,5-cyclohexatriene, The enthalpy change of hydrogenation is expected to be
three times
as much as that of cyclohexene 15 New Way Chemistry for Hong Kong A-Level Book 3A
31.3
The Stability of Benzene (SB p.153) •
Benzene is more stable than Kekulé structure
• The energy difference for the stabilization of benzene is called
resonance energy of benzene
16 New Way Chemistry for Hong Kong A-Level Book 3A
31.3
The Stability of Benzene (SB p.153)
The Resonance Explanation of the Structure of Benzene
From X-ray crystallography, The length of carbon-carbon bond in benzene is
intermediate between C – C bond and C = C bond
0.134 nm C = C > 0.139 nm > 0.154 nm carbon bond in benzene C – C 17 New Way Chemistry for Hong Kong A-Level Book 3A
31.3
The Stability of Benzene (SB p.154) All carbon atoms in benzene are
sp
2 -hybridized
The
side-way overlap of unhybridized 2p orbitals
both sides gives a
delocalized
electron cloud
on above and below the plane of the ring 18 New Way Chemistry for Hong Kong A-Level Book 3A
31.3
The Stability of Benzene (SB p.154) The delocalization of electrons gives benzene
extra stability
and determines the
chemical properties of benzene
19 New Way Chemistry for Hong Kong A-Level Book 3A
31.3
The Stability of Benzene (SB p.154)
Structural formula of benzene:
20 The circle represents the
six electrons that are delocalized about the six carbon atoms of the benzene ring
New Way Chemistry for Hong Kong A-Level Book 3A
31.3
The Stability of Benzene (SB p.155)
Structure of Methylbenzene
21 • All C atoms in the ring is
sp
2 -hybridized
• The C atom in the methyl group is
sp
3 -hybridized
• The delocalized electron clouds give rise to
extra stability
New Way Chemistry for Hong Kong A-Level Book 3A
31.4
Physical Properties of Aromatic Hydrocarbons (SB p.155)
Physical properties of aromatic hydrocarbons:
• have a
fragrant
smell • generally
less dense than water
at 20 ° C • • usually
immiscible with water soluble in organic solvents
22 New Way Chemistry for Hong Kong A-Level Book 3A
31.4
Physical Properties of Aromatic Hydrocarbons (SB p.156)
Name Formula Boiling point (
°
C) Melting point (
°
C) Density at 20
°
C (g cm –3 )
Benzene 80.1
5.5
0.878
Methylbenzene 111 –95 0.867
23 Ethylbenzene 136 –94 0.867
New Way Chemistry for Hong Kong A-Level Book 3A
31.4
Physical Properties of Aromatic Hydrocarbons (SB p.156)
Name Formula Boiling point (
°
C) Melting point (
°
C) Density at 20
°
C (g cm –3 )
1,2 Dimethylbenzene 144 –25.2
0.880
1,3 Dimethylbenzene 139 –47.4
0.864
24 1,4 Dimethylbenzene 138 New Way Chemistry for Hong Kong A-Level Book 3A 13.3
0.861
31.5
Preparation of Benzene (SB p.157)
Industrial Preparation
Catalytic Reforming of Alkanes
Catalytic reforming converts alkanes and cycloalkanes into aromatic hydrocarbons e.g.
C 6 H 14 500 ° C, 10 – 20 atm C 6 H 6 + 4H 2 25 New Way Chemistry for Hong Kong A-Level Book 3A
31.5
Preparation of Benzene (SB p.157)
Destructive Distillation of Coal
• • Heating coal in the
absence of air
gives out coal gas, ammoniacal liquor,
coal tar
and coke • Coal tar is
a mixture of many organic compounds
,
mainly aromatic ones Benzene
and
methylbenzene
can be obtained 26 New Way Chemistry for Hong Kong A-Level Book 3A
31.5
Preparation of Benzene (SB p.158)
Laboratory Synthesis
Decarboxylation of Sodium Salt of Benzoic Acid
When sodium benzoate is fused with sodium hydroxide, the
carboxylate group is removed
and benzene is formed 27 New Way Chemistry for Hong Kong A-Level Book 3A
31.5
Preparation of Benzene (SB p.158)
Reduction of Phenol
Phenol vapour is passed slowly over heated zinc dust to produce benzene and zinc(II) oxide 28 New Way Chemistry for Hong Kong A-Level Book 3A
31.6
Reactions of Benzene (SB p.158)
Comparative Investigation of Chemical Properties of Cyclohexane, Cyclohexene and Benzene
29
Reaction
Action of Br 2 in CH 3 Cl 3 (in dark) Action of H (with Ni catalyst) 2
Cyclohexane (a saturated alicyclic hydrocarbon)
No reaction
Cyclohexene (an unsaturated alicyclic hydrocarbon)
Br 2 decolourized and no HBr evolved No reaction 1 mole of cyclohexene reacts with 1 mole of H 2 at room temperature New Way Chemistry for Hong Kong A-Level Book 3A
Methylbenzene (an aromatic hydrocarbon)
No reaction with Br 2 alone In the presence of FeBr 3 , Br 2 decolourized and HBr fumes evolved 1 mole of methylbenzene reacts 3 moles of H 2 at high temperature and
31.6
Reactions of Benzene (SB p.159)
Reaction
Action of acidified KMnO 4
Cyclohexane (a saturated alicyclic hydrocarbon)
No reaction Action of conc. HNO 3 and conc. H 2 SO 4 No reaction
Cyclohexene (an unsaturated alicyclic hydrocarbon)
KMnO 4 decolourized Cyclohexene oxidized and colour darkens
Methylbenzene (an aromatic hydrocarbon)
No reaction A yellow liquid is formed 30 New Way Chemistry for Hong Kong A-Level Book 3A
31.6
Reactions of Benzene (SB p.159) • Methylbenzene is
highly unsaturated
, but it is
resistant to oxidation and addition reactions
• The resistance of oxidation and addition reactions of aromatic compounds is used to distinguish from unsaturated alkenes • Methylbenzene reacts with Br 2 in the presence of FeBr 3 . It is through
substitution reaction
31 New Way Chemistry for Hong Kong A-Level Book 3A
31.6
Reactions of Benzene (SB p.160)
Electrophilic Aromatic Substitution Reactions
Most characteristic reaction of aromatic compounds:
Electrophilic substitution reactions
32 • The
electrophiles attack the benzene ring
, replacing one of the hydrogen atoms in the reaction • Electrophiles are either a
positive ion
(E + ) or some other
electron-deficient species with a partial positive charge
( +) New Way Chemistry for Hong Kong A-Level Book 3A
31.6
Reactions of Benzene (SB p.160)
Nitration
33 • Conc. H 2 SO 4
increases the rate of reaction
by
increasing the concentration of the electrophile, NO 2 +
(nitronium ion) New Way Chemistry for Hong Kong A-Level Book 3A
31.6
Reactions of Benzene (SB p.160)
Sulphonation
• Benzene reacts with fuming sulphuric(VI) acid at room temperature to give
benzenesulphonic acid
34 • Heating aqueous solution of benzenesulphonic acid above 100 ° C, benzene and sulphuric(VI) acid are formed New Way Chemistry for Hong Kong A-Level Book 3A
31.6
Reactions of Benzene (SB p.161)
Halogenation
Benzene reacts with chlorine and bromine in the presence of
catalysts such as AlCl 3 , FeCl 3 , FeBr 3 ,
to give chlorobenzene and bromobenzene respectively 35 New Way Chemistry for Hong Kong A-Level Book 3A
31.6
Reactions of Benzene (SB p.161)
Alkylation
• When benzene is warmed with a haloalkane in the presence of
catalysts such as AlCl 3
, an alkylbenzene is formed 36 • Important step in chemical industry to produce
polystyrene
,
phenol
and
detergents
New Way Chemistry for Hong Kong A-Level Book 3A
37
31.6
Reactions of Benzene (SB p.162)
Example 31-2
Complete each of the following by supplying the missing reactant or product as indicated by the question mark:
Solution:
(a) (a) (b) (b)
conc. H 2 SO 4 , conc. HNO 3
(c)
fuming H 2 SO 4
(c)
Answer New Way Chemistry for Hong Kong A-Level Book 3A
31.6
Reactions of Benzene (SB p.162)
Check Point 31-2
(a) One mole of benzene reacts with three moles of chlorine under special conditions. What is the reaction condition required for the reaction?
(b) Draw the structure of the reaction product in (a).
present for the free radical addition reaction to take place.
Answer (b) 38 New Way Chemistry for Hong Kong A-Level Book 3A
31.7
Alkylbenzenes (SB p.162) •
Alkylbenzenes
are a group of aromatic hydrocarbons in which
an alkyl group is bonded directly to a benzene ring
• also known as
arenes
e.g.
39 New Way Chemistry for Hong Kong A-Level Book 3A
31.7
Alkylbenzenes (SB p.163) Alkylbenzenes are oxidized to
benzoic acid
by strong oxidizing agents such as
hot alkaline potassium manganate(VII)
40 This type of oxidation is limited to those molecules with New Way Chemistry for Hong Kong A-Level Book 3A
31.7
Alkylbenzenes (SB p.163)
Examples:
41 New Way Chemistry for Hong Kong A-Level Book 3A
31.7
Alkylbenzenes (SB p.164) The
C = C double bond and acyl groups in the side chain
are oxidized by hot alkaline potassium manganate(VII) e.g.
42 New Way Chemistry for Hong Kong A-Level Book 3A
43
31.7
Alkylbenzenes (SB p.164)
Example 31-3
State the conditions under which methylbenzene can be converted in the laboratory to (a) C 6 H 5 CH 2 Cl (b) C 6 H 5 COOH
Answer
Solution:
(a)
Reagent: Cl 2 Condition: in the presence of light
(b)
Reagent: (1) KMnO 4 – , OH – , (2) H 3 O + Condition: heating under reflux New Way Chemistry for Hong Kong A-Level Book 3A
31.7
Alkylbenzenes (SB p.164)
Check Point 31-3
Methylbenzene undergoes two different types of chlorination reaction by different mechanisms. Compare the two different types of chlorination reaction in terms of reaction conditions as well as the products formed.
Answer 44 New Way Chemistry for Hong Kong A-Level Book 3A
31.7
Alkylbenzenes (SB p.164) Two different types of chlorination reaction of methylbenzene are: Type I: free radical substitution reaction Type II: electrophilic aromatic substitution reaction 45 New Way Chemistry for Hong Kong A-Level Book 3A
The END
46 New Way Chemistry for Hong Kong A-Level Book 3A