AQA A-Level Chemistry Aromatic Chemistry Questions and Answer

AQA A-Level Chemistry Aromatic Chemistry Questions and Answer State the three problems with Kekule's structure 1. C=C bonds are actually all the same length rather than the predicted 3 long single bonds and 3 short double bonds. 2. Benzene doesn't undergo addition reactions which you would expect trienes to readily undergo 3. Enthalpy change of hydrogenation is 152 kJ mol-1 less than expected. What are arenes? Hydrocarbons based on benzene Why do arenes have high melting points? Due to high stability of delocalised ring Why do arenes have low boiling points? They are non-polar and so often cannot be dissolved in water State the two important factors for the reactivity of aromatic compound 1. Ring is an area of high electron density and so is attacked by electrophiles 2. Aromatic ring is very stable and so requires lots of delocalisation energy to break it What is the most common reaction of aromatic compounds? Electrophilic substitution Give the initial equation for production of nitrobenzene H2SO4 + HNO3 - H2NO3+ + HSO4- H2NO3+ - H2O + NO2+ What is Friedel-Crafts acylation? When the delocalised electron ring acts as a nucleophile, leading to the attack on acyl chlorides. What does Friedel-Crafts acylation require? 1. A reactive intermediate to be produced from acyl chloride 2. An aluminium chloride catalyst Give a general equation for the production of a reactive intermediate for Friedel-Crafts acylation? R-COCl + AlCl3 - R-CO+ + AlCl4- Give the sequence of enthalpy of hydrogenation -120, -240, cyclohexane -360, benzene -208 What type of molecule is benzene? Cyclic planar What is the bond length of the C-C bonds in a benzene ring? 139pm How do you measure stability of benzene? Comparing enthalpy change of hydrogenation in benzene and cylohexa-1,3,5-triene. By what amount is benzene more stable than cyclohexa-1,3,5-triene? 152kj/mol What does the value of enthalpy of hydrogenation suggest about the bonding in benzene? More energy is required to break the bonds in benzene than in cyclohexa-1.3.5-triene. What are aromatic compounds? Molecules that contain a benzene ring What do you call benzene with an amine group? Phenylamine How is an electrophile made in Friedel-Crafts Acylation? An acyl chloride reacts with the halogen carrier AlCl3 Why is nitrating benzene useful? Allows us to make dyes for clothing and explosives How is an electrophile made in nitration? Sulphuric acid reacts with nitric acid to form NO2+ What are the conditions needed for nitration of benzene? Temperature below 55 degrees, above this will result in multiple substitutions What's the order of increasing bond lengths? 1. Double bond 2. Benzene bond 3. C-C bond IUPAC name for Kekule's structure? Cyclohexa-1,3,5-triene In recrystallisation, why would the minimum quantity of hot water be used? To ensure the hot solution would be saturated In recrystallisation, why would the flask be cooled to room temperature before the crystals were filtered off? Yield would be lower if still warm In recrystallisation, why would the crystals be compressed in the funnel? To allow better drying as air is forced through the sample rather than just around it In recrystallisation why was a little cold water poured through the crystals? To wash away soluble impurities Name the reagents used in the nitration of chlorobenzene Concentrated HNO3 Concentrated H2SO4