new way chemistry for hong kong a-level book 3a1 aromatic hydrocarbons 31.1introduction...
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New Way Chemistry for Hong Kong A-Level Book 3A
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Aromatic Hydrocarbons
31.131.1 IntroductionIntroduction
31.231.2 Nomenclature of the Derivatives of BenzeneNomenclature of the Derivatives of Benzene
31.331.3 The Stability of BenzeneThe Stability of Benzene
31.431.4 Physical Properties of Aromatic HydrocarbonsPhysical Properties of Aromatic Hydrocarbons
31.531.5 Preparation of BenzenePreparation of Benzene
31.631.6 Reactions of BenzeneReactions of Benzene
31.731.7 AlkylbenzenesAlkylbenzenes
Chapter 31Chapter 31
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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
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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
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(b)For other compounds, the substituent and the benzene ring taken together may form a new parent name
31.2 Nomenclature of the Derivatives of Benzene (SB p.148)
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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.
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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.
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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
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Example 31-1Example 31-1Draw the structural formula for each of the following compounds:
(a) 1,3,5-Trichlorobenzene
(b) 2,5-Dibromophenol
(c) 2,4-Dinitrobenzoic acidAnswer
31.2 Nomenclature of the Derivatives of Benzene (SB p.149)
Solution:
(a) (b)
(c)
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Check Point 31-1 Check Point 31-1
Give the IUPAC name for each of the following compounds:
(a) (b)
(c) (d)
31.2 Nomenclature of the Derivatives of Benzene (SB p.150)
Answer
(a) 1,2-Dimethylbenzene
(b) 1-Methyl-2-nitrobenzene or 2-nitrotoluene
(c) 3-Bromo-5-chlorobenzoic acid
(d) 4-Bromo-2,6-dinitrophenol
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31.3 The Stability of Benzene (SB p.151)
In 1865, Kekule proposed the structure of benzene:
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31.3 The Stability of Benzene (SB p.151)
According to the Kekulé structure, there should be two different 1,2-dibromobenzenes:
Only one 1,2-dibromobenzene has been found!!
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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
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31.3 The Stability of Benzene (SB p.152)
Enthalpy change of hydrogenation of cyclohexene
= –119.6 kJ mol–1
Enthalpy Changes of Hydrogenation of Benzene and Cyclohexene
Enthalpy Changes of Hydrogenation of Benzene and Cyclohexene
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31.3 The Stability of Benzene (SB p.152)
The enthalpy change of hydrogenation of 1,3-cyclohexadiene i
s expected to be twice that of cyclohexene
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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
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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
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31.3 The Stability of Benzene (SB p.153)
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 > 0.139 nm > 0.154 nm C = C carbon bond in benzene C – C
The Resonance Explanation of the Structure of Benzene
The Resonance Explanation of the Structure of Benzene
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31.3 The Stability of Benzene (SB p.154)
All carbon atoms in benzene are sp2-hybridized
The side-way overlap of unhybridized 2p orbitals on both sides gives a delocalized electron cloud above and below the plane of the ring
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31.3 The Stability of Benzene (SB p.154)
The delocalization of electrons gives benzene extra stabili
ty and determines the chemical properties of benzene
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31.3 The Stability of Benzene (SB p.154)
Structural formula of benzene:
The circle represents the six electrons that are delocalized
about the six carbon atoms of the benzene ring
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31.3 The Stability of Benzene (SB p.155)
• All C atoms in the ring is sp2-hybridized
• The C atom in the methyl group is sp3-hybridized
• The delocalized electron clouds give rise to extra stability
Structure of MethylbenzeneStructure of Methylbenzene
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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
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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
Ethylbenzene 136 –94 0.867
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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
1,4-Dimethylbenzene
138 13.3 0.861
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31.5 Preparation of Benzene (SB p.157)
Catalytic reforming converts alkanes and cycloalkanes int
o aromatic hydrocarbons
e.g.
Industrial PreparationIndustrial Preparation
Catalytic Reforming of Alkanes
PtC6H14 C6H6 + 4H2500°C, 10 – 20 atm
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31.5 Preparation of Benzene (SB p.157)
• 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
Destructive Distillation of Coal
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31.5 Preparation of Benzene (SB p.158)
When sodium benzoate is fused with sodium hydroxide, the carboxylate group is removed and benzene is formed
Decarboxylation of Sodium Salt of Benzoic Acid
Laboratory SynthesisLaboratory Synthesis
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31.5 Preparation of Benzene (SB p.158)
Phenol vapour is passed slowly over heated zinc dust to p
roduce benzene and zinc(II) oxide
Reduction of Phenol
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31.6 Reactions of Benzene (SB p.158)
Comparative Investigation of Chemical Properties of Cyclohexane, Cyclohexene and Benzene
Comparative Investigation of Chemical Properties of Cyclohexane, Cyclohexene and Benzene
ReactionCyclohexane (a saturated alicyclic hy
drocarbon)
Cyclohexene (an unsaturated alicycli
c hydrocarbon)
Methylbenzene (an aromatic
hydrocarbon)
Action of Br2 in CH3Cl3 (in dark)
No reaction Br2 decolourized and no HBr evolved
No reaction with Br2 alone
In the presence of FeBr3, Br2 decolourized and HBr fumes evolved
Action of H2 (with Ni catalyst)
No reaction 1 mole of cyclohexene reacts with 1 mole of H2 at room temperature
1 mole of methylbenzene reacts 3 moles of H2 at high temperature and pressure
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31.6 Reactions of Benzene (SB p.159)
ReactionCyclohexane (a saturated alicyclic hy
drocarbon)
Cyclohexene (an unsaturated alicycli
c hydrocarbon)
Methylbenzene (an aromatic
hydrocarbon)
Action of acidified KMnO4
No reaction KMnO4
decolourized
No reaction
Action of conc. HNO3 and conc. H2SO4
No reaction Cyclohexene oxidized and colour darkens
A yellow liquid is formed
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• Methylbenzene is highly unsaturated, but it is resistant to oxidation and addition reactions
31.6 Reactions of Benzene (SB p.159)
• The resistance of oxidation and addition reactions of aromatic compounds is used to distinguish from unsaturated alkenes
• Methylbenzene reacts with Br2 in the presence of
FeBr3. It is through substitution reaction
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31.6 Reactions of Benzene (SB p.160)
Electrophilic Aromatic Substitution ReactionsElectrophilic Aromatic Substitution Reactions
• 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 (+)
Most characteristic reaction of aromatic compounds:
Electrophilic substitution reactions
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31.6 Reactions of Benzene (SB p.160)
• Conc. H2SO4 increases the rate of reaction by increasing
the concentration of the electrophile, NO2+ (nitronium io
n)
Nitration
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31.6 Reactions of Benzene (SB p.160)
• Benzene reacts with fuming sulphuric(VI) acid at room temperature to give benzenesulphonic acid
Sulphonation
• Heating aqueous solution of benzenesulphonic acid above 100°C, benzene and sulphuric(VI) acid are formed
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31.6 Reactions of Benzene (SB p.161)
Benzene reacts with chlorine and bromine in the presence of catalysts such as AlCl3, FeCl3, FeBr3, to give chlorobenzen
e and bromobenzene respectively
Halogenation
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31.6 Reactions of Benzene (SB p.161)
• When benzene is warmed with a haloalkane in the presence
of catalysts such as AlCl3, an alkylbenzene is formed
Alkylation
• Important step in chemical industry to produce
polystyrene, phenol and detergents
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Example 31-2Example 31-2Complete each of the following by supplying the missing reactant or product as indicated by the question mark:
(a)
(b)
(c)
Answer
31.6 Reactions of Benzene (SB p.162)
Solution:
(a)
(b) conc. H2SO4, conc. HNO3
(c) fuming H2SO4
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Check Point 31-2 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).
Answer
31.6 Reactions of Benzene (SB p.162)
(a) UV radiation or diffuse sunlight must be
present for the free radical addition
reaction to take place.
(b)
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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.
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31.7 Alkylbenzenes (SB p.163)
Alkylbenzenes are oxidized to benzoic acid by strong oxidizing
agents such as hot alkaline potassium manganate(VII)
This type of oxidation is limited to those molecules with
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31.7 Alkylbenzenes (SB p.163)
Examples:
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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.
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Example 31-3Example 31-3State the conditions under which methylbenzene can be converted in the laboratory to
(a) C6H5CH2Cl
(b) C6H5COOH Answer
Solution:
(a) Reagent: Cl2
Condition: in the presence of light
(b) Reagent: (1) KMnO4–, OH–, (2) H3O+
Condition: heating under reflux
31.7 Alkylbenzenes (SB p.164)
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Check Point 31-3 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
31.7 Alkylbenzenes (SB p.164)
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Two different types of chlorination reaction of methylbenzene are:
Type I: free radical substitution reaction
Type II: electrophilic aromatic substitution reaction
31.7 Alkylbenzenes (SB p.164)
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