AQA Chemistry A-level 3.3.10: Aromatic Chemistry Detailed Notes PDF

AQA Chemistry A-level 3.3.10: Aromatic Chemistry Detailed Notes This work by PMT Education is licensed under https://bit.ly/pmt-cc https://bit.ly/pmt-edu-cc CC BY-NC-ND 4.0...

AQA Chemistry A-level 3.3.10: Aromatic Chemistry Detailed Notes This work by PMT Education is licensed under https://bit.ly/pmt-cc https://bit.ly/pmt-edu-cc CC BY-NC-ND 4.0 https://bit.ly/pmt-cc https://bit.ly/pmt-edu https://bit.ly/pmt-cc 3.3.10.1 - Bonding Benzene is an ​aromatic compound​ consisting of a ring of ​six carbon atoms​ with ​six hydrogen atoms ​and a ring of ​delocalised electrons​: Example: Each bond in the benzene ring has an ​intermediate length​ in between that of a double and single bond. The outer electron from the ​p-orbital​ of each carbon atom is ​delocalised​ to form the central ring. This ring structure makes benzene ​very stable​ compared to other molecules of a similar size. Cyclohexatriene vs. Benzene When benzene was first discovered its structure was not known. It was predicted from empirical measurements that it had a structure similar to that of ​cyclohexatriene​, with three double bonds and three single bonds. Example: https://bit.ly/pmt-cc https://bit.ly/pmt-edu https://bit.ly/pmt-cc Therefore the enthalpy change of hydrogenation for benzene was ​predicted to be -360kJmol​-1​, three times the enthalpy change of cyclohexene. Example: It was later discovered that the enthalpy change of hydrogenation of benzene was ​actually -208kJmol​-1​ leading to the conclusion of its ​different, unusual structure​. Arenes Compounds that ​contain benzene as part of their structure ​are called arenes or aromatic compounds. They have​ high melting points​ due to the high stability of the delocalised ring, but low boiling points​ as they are ​non-polar​ molecules and ​often cannot be dissolved​ in water. https://bit.ly/pmt-cc https://bit.ly/pmt-edu https://bit.ly/pmt-cc 3.3.10.2 - Electrophilic Substitution The delocalised ring in benzene is an ​area of high electron density​ making it susceptible to attack from ​electrophiles​. When these species attack the electron ring, it is ​partially destroyed then ​restored​ in the process of ​electrophilic substitution​. This mechanism means aromatic amines and nitrobenzene can be produced from benzene. Mechanism The electrophile is shown as A​ +​. Nitrobenzene In this form of electrophilic addition, the electrophile is the ​NO​2​+​ ion​. This is a ​reactive intermediate​, produced in the reaction of concentrated sulfuric acid (H​2​SO​4​) with concentrated nitric acid (HNO​3​). Example: When heated with benzene these reagents lead to the ​substitution of the NO​2​+​ electrophile onto the benzene ring, ​removing a hydrogen​ ion. Mechanism https://bit.ly/pmt-cc https://bit.ly/pmt-edu https://bit.ly/pmt-cc This reaction shows a ​mono-substitution​ of a single NO​2​+​ electrophile which takes place when the reaction temperature is​ 55​o​C​. At temperatures greater than this, multiple substitutions can occur. It is vital that only one substitution occurs for the production of ​aromatic amines​. Friedel-Crafts Acylation The delocalised electron ring in benzene can also act as a ​nucleophile​, leading to the ​attack on acyl chlorides​. This reaction is known as ​Friedel-Crafts acylation​. In order for it to take place, a ​reactive intermediate ​must be produced from the acyl chloride and an ​aluminium chloride catalyst​. Example: This reactive intermediate is then attacked by the benzene ring. Mechanism At the end of the reaction, the ​H+​​ ion​ removed from the ring reacts with the ​AlCl​4​-​ ion ​to reform the aluminium chloride, showing it to be a ​catalyst​. The product of this reaction is a ​phenylketone​. In this case, the benzene group is called a phenyl group​. These molecules are commonly used in the industrial production of dyes, pharmaceuticals and even explosives. https://bit.ly/pmt-cc https://bit.ly/pmt-edu https://bit.ly/pmt-cc