231805
Description
Flashcards by jasmin.sahota, updated more than 1 year ago
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Created by jasmin.sahota
over 10 years ago
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| Question | Answer |
| Kekule model | Alternate double and single bonds |
| Evidence for this model | Only one isomer for the monoderivative |
| So? | All carbons must be the same |
| More evidence | 3 isomers of the di-derivative |
| Why was Kekule wrong? | All bonds were the same length |
| What length | In between a C=C and C-C |
| Second problem with Kekule's theory | Benzene not as reactive as you would expect if it had C=C |
| Third reason why Kekule was wrong | Enthalpy change of hydrogenation is less exothermic than expected for 3 C=C bonds |
| So what is the other model? | Delocalised ring of electrons |
| Formed by | Sideways overlap of p orbitals |
| To form | pi bonds which form a ring of charge |
| Where | Above and below the plane of the ring of carbons |
| What type of reactions do arenes undergo | Electrophilic substitution |
| For example nitration. This needs a | Nitrating mixture |
| What is that | Sulfuric & nitric acids (reagents) |
| Conditions | Concentrated acids and 50 C |
| Overall equation | C6H6 + HNO3 --> C6H5NO2 + H2O |
| Mechanism – step 1 makes a | Nitronium ion NO2+ |
| How | H2SO4 + HNO3 NO2+ + HSO4- + H2O |
| Draw the mechanism | check notes |
| How comes the H2SO4 is a catalyst? | It is remade in a last step |
| What step? | H+ + HSO4- --> H2SO4 |
| Halogenation needs a halogen carrier Why? | Because the electron density in benzene is lower than in alkenes |
| Why? | Because the 6 electrons are spread out between 6 carbons in benzene pi bond |
| Whereas in alkenes | There are 2 electrons concentrated between the 2 carbons |
| What are halogen carriers? | Fe, FeBr3 or AlCl3 |
| e.g for Cl2 we need to make | Cl+ |
| How | AlCl3 + Cl2 --> AlCl4- + Cl+ |
| Mechanism for reaction of Br2 with benzene. First | FeBr3 + Br2 --> FeBr4- + Br+ |
| Br+ is the | electrophile |
| Then (DIY) mechanism | check notes |
| Overall equation | C6H6 + Br2 --> C6H5Br + HBr |
| relative resistance to bromination of benzene | delocalised electron density of the π bonds in benzene is less than with the localised electron density of the C=C bond in alkenes |
| Phenol is | Phenol is C6H5OH |
| Reaction with aqueous alkalis | Makes a salt and water as it acts as an acid |
| equation | C6H5OH + NaOH --> C6H5O-Na+ + H2O |
| With Na | C6H5OH + Na --> C6H5O-Na+ + ½ H2 |
| What is the name of C6H5O-Na+ | Sodium phenoxide |
| With Br2 we get | A white ppt and the Br2 decolourises |
| Equation | C6H5OH + 3 Br2 --> C6H3OBr3 + 3HBr |
| What is C6H3OBr3 | 2,4,6-tribromophenol (white ppt) |
| HBr is a | Steamy gas |
| explain the relative ease of bromination of phenol compared with benzene, | Does not need a halogen carrier |
| Why? | The electron pair on the p orbital of oxygen delocalises onto the benzene ring |
| Which | Increases the electron density of the ring |
| So? | Electrophiles are more attracted to the ring |
| This is called | Ring activation |
| Use of phenol | plastics, antiseptics, disinfectants & resins for paints. |
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