OCR A-Level Chemistry: Chapter 25 - Aromatic Compounds
This flashcard set defines benzene and related terms like arene and aromatic compounds. It also explains why benzene does not behave like a typical alkene, providing evidence against the Kekule structure through its lack of electrophilic addition reactions.
What is benzene?
A cyclic hydrocarbon compound C6H6
Key Terms
What is benzene?
A cyclic hydrocarbon compound C6H6
Define arene
An aromatic hydrocarbon containing one or more benzene rings
Define aromatic
Any compound containing one or more benzene rings
What is the electrophilic addition argument against benzene having the Kekule structure?
Three C=C bonds so it is expected
No reaction with bromine water at room temperature
No other electrophilic additi...
What is the response to the electrophilic addition argument against benzene having the Kekule structure?
Two forms in rapid equilibrium
Approaching Br-Br not attracted to bonds
Bonds change from C=C to C-C before reacti...
Outline the stability argument against benzene having the Kekule structure
One C=C bond in 6 carbon ring, enthalpy -120kJ/mol
Two C=C bonds in 6 carbon ring, enthalpy -240kJ/mol
Expect 3 C=...
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| Term | Definition |
|---|---|
What is benzene? | A cyclic hydrocarbon compound C6H6 |
Define arene | An aromatic hydrocarbon containing one or more benzene rings |
Define aromatic | Any compound containing one or more benzene rings |
What is the electrophilic addition argument against benzene having the Kekule structure? | Three C=C bonds so it is expected |
What is the response to the electrophilic addition argument against benzene having the Kekule structure? | Two forms in rapid equilibrium |
Outline the stability argument against benzene having the Kekule structure | One C=C bond in 6 carbon ring, enthalpy -120kJ/mol |
What causes benzene to have its unexpected properties? | Localised p-orbitals overlap and form delocalised π orbitals above and below the carbon structure (rings) |
Define electrophile | An atom or group of atoms that is attracted to an electron rich centre, where it accepts a pair of electrons to form a new covalent bond |
What are the conditions for nitration of benzene? | Benzene + HNO3 with H2SO4 at 50C |
As halogens cannot react on their own with benzene, what is required? | Halogen carrier |
Give two examples of chlorine carriers | AlCl3 FeCl3 |
What is formed as a byproduct of halogenation of benzene? | HX |
What are the two types of Friedel Crafts reactions of benzene? | Alkylation Acylation |
Outline alkylation of benzene | Benzene + RCl with FeCl3 Forms alkylbenzene + HCl |
Outline acylation of benzene | Benzene + RCOCl with FeCl3 Forms Phenylalkanone + HCl |
Name the mechanism for bromination of benzene | Electrophilic aromatic substitution |
Outline electrophilic aromatic substitution | Electrophile near benzene Arrow from benzene to electrophile Intermediate ion of benzene Arrow from H-C bond to ring Product and H+ |
How does a nitrating mixture produce an electrophile? | H2SO4 + HNO3 -> NO2+ + HSO4- + H2O Nitronium ion is electrophile |
How is the H2SO4 catalyst regenerated? | HSO4- + H+ -> H2SO4 H+ from benzene |
How is an electrophile formed in reaction with a halogen carrier? | X2 + AlX3 -> X+ + AlX4- |
How is the halogen carrier catalyst regenerated? | AlX4- + H+ -> AlX3 + HX |
What is the structure of phenol? | C6H6O OH attached to benzene ring |
What are the products of phenol reacting with NaOH? | Sodium phenoxide: C6H5O-Na+ Water |
| Too weak of an acid to react with weak base |
What is formed on reaction of phenol with excess Br2? | C6H3OBr3 - OH in position 1 and Br in positions 2,4 and 6 | 3HBr |
What are the reaction conditions of bromination of phenol? | Room temperature | No catalyst |
Why does phenol react so readily with bromine? | OH group acts to increase reactivity of benzene ring |
What is the term for OH increasing benzene's reactivity? | Activating group |
Define activation | An activating group increases the rate of electrophilic aromatic substitution compared to that of benzene |
Define deactivation | A deactivating group decreases the rate of electrophilic aromatic substitution compared to that of benzene |
Outline differences between phenol and nitrobenzene reacting with bromine | Phenol is rapid reaction, nitro is slower |
What do activating groups do with their electrons in benzene? | Donate electrons |
What do deactivating groups do with their electrons in benzene? | Withdraw electrons |
Where do the NH2 and OH groups direct on benzene? | 2,4 and 6 positions |
Where does the NO2 group direct on benzene? | 3 and 5 positions |