Aromatic Chemistry Flashcards
Why is benzene less reactive than expected if it had structure of cyclohexa-1,3,5-triene?
The enthalpy of hydrogenation of cyclohexene is 119 kJ per mole. Therefore benzene should be 357 kJ per mole (119×3). However the enthalpy of hydrogenation of benzene is 208 kJ per mole. This indicates that the building is more stable and the cyclohexa-1,3,5-triene structure would suggest. This extra stability is called the delocalisation energy of benzene
What are the three pieces of evidence that benzene does not have the same structure as cyclohexa–1,3,5– triene?
– The enthalpy of hydrogenation of benzene is lower than expected – Benzene does not react like an alkene e.g. it does not react with an aqueous solution of bromine, it requires vigourous conditions with a catalyst
– The bond lengths in benzene are all 0.14 nm, despite it containing single and double bonds, which are usually different in length
Why is benzene more stable than the theoretical compound cyclohexa-1,3,5-triene?
The electrons in the pi bonds of the six carbon atoms overlap and the six electrons are shared in a doughnut shaped electron cloud above and below the plane of the carbon atoms. The electrons are delocalised
What is an electrophile?
A positively charged ion or the positive end of a polar molecule that can accept electrons to form new covalent bonds
What is the electrophilic substitution mechanism of benzene?
A reaction where one or more of the six hydrogen atoms are replaced by halogen atoms or a functional group. The H atom is ‘substituted’
What are the 3 steps of the electrophilic substitution mechanism with benzene?
1) The electrophile is attracted to the delocalised electrons of cloud of the benzene ring structure
2) The electrophile bonds to the benzene ring by two of the six delocalised electrons, leaving a positively charged partially delocalised system containing four delocalised electrons spread over five carbon atoms. This temporarily causes a loss of the natural stability of the benzene system
3) In order to regain the delocalisation energy of the benzene ring, an H+ ions is lost by breaking the C-H bond. The electrons from this bond re-form the stable delocalised six–electron system
What happens when benzene undergoes nitration? How is the electrophile in this reaction generated?
A nitro group (NO2) replaces one of the H atoms. The electrophile is the nitronium ion (NO2+). This is generated by mixing concentrated nitric acid and concentrated sulphuric acid and heating to 50° C. The conc. sulphuric acid donates a proton to the nitric acid. This produces an intermediate (H2NO3)+ which decomposes to yield the electrophilic nitronium ion.
During the nitration of benzene, how does concentrated sulphuric act as a catalyst?
It is regenerated when the HSO4-reacts with the H+ released in the last step of the reaction mechanism
What does the reduction of nitrobenzene lead to? What are they used for? How is nitrobenzene reduced?
1) The nitro group can be reduced to an amine group
2) Aromatic amines are used widely in the dye and other industries
3) reduction can be achieved using iron or tin with concentrated hydrochloric acid or, alternatively, catalytic hydrogenation can be carried out with a nickel and hydrogen gas
What is a Friedel-Crafts reaction?
An electrophilic substitution reaction of benzene or some other aromatic compound with an acyl chloride, using a catalyst such as aluminium chloride or iron(III)chloride
What are Lewis acids?
Compounds which can except a lone pair of electrons and form a dative covalent bond
What are the conditions required for a Friedel-Crafts reaction?
A catalyst (AlCl3 or FeCl3) and heating to a temperature of about 80° C. Also the reaction mixture must be completely free of water because AlCl3 and FeCl3 react vigourously with water
How can the ketone functional group be reduced to an alkane? An alcohol?
1) Using nickel and hydrogen gas or zinc and concentrated HCl
2) Using sodium borohydride
