Chapter 25 - Aromatic Chemistry Flashcards
What is the formula of benzene?
C6H6
Define aromatic compound
an organic substance that contains a benzene ring
List 3 things benzene rings can be found in
pharmaceuticals, dyes and explosives
What is a -Cl attachment called in an aromatic compound?
chloro
What is a -Br attachment called in an aromatic compound?
bromo
What is a -CH3 attachment called in an aromatic compound?
methyl
What is a -NO2 attachment called in an aromatic compound?
nitro
What are numbers used to give the lowest possible combination of numbers called?
Locant numbers
Define locant numbers
numbers used to give the lowest possible combination of numbers
What is a benzene group present in a molecule named as? What is this similar to?
Phenyl group
Methyl group for CH3
Who suggested the first structure of benzene?
Kekulé
What were the main 2 principles of his structure?
That there were 2 forms of benzene and that the position of the double bonds constantly alternated from one to the other
What is more reactive?
c=c or c-c
c=c because the pi bond is easier to break than the sigma bond
Based on the length of C bonds, explain why Kekulés structure must be incorrect
If there were both C=C and C-C the bonds would have different lengths, but in Benzene each bond is the same length, suggesting there are no C=C
Based on reaction types explain why Kekulés structure must be incorrect
Kekulés structure makes benzene look like cyclic ALKENE so we would expect it to undergo addition reactions to break the double bonds, however benzene undergoes substitution reactions which suggests there are no C=C bonds present
Based on energy profiles explain why Kekulés structure must be incorrect
Kekulés model suggests that the total enthalpy change of benzene should be the addition of the hydrogenation of each C=C bond, totalling -360kJmol-1, however experimentally it is actually around -200kJmol-1 suggesting that the suggested structure of benzene is not correct
State 3 reasons why Kekulé’s structure of benzene is incorrect
- The type of reactions benzene undergoes is not equivalent to the model
- The bond lengths of benzene is not equivalent to the model
- The enthalpy change of hydrogenation is not equivalent to the one suggested by the model.
State 10 key features of the delocalised model of benzene
- 6C and 6H atoms form a cyclic hydrocarbon (not alkene)
- These carbon atoms are arranged in a planar (which means flat) hexagonal ring
- The arrangement of atoms around each C atom is trigonal planar with bond angles of 120
- Each C atom has 4e-, 3 of which are bonded to 2C and 1H
- The 4th e- is located in a P- orbital which sticks up above and below the ring
- The P orbitals on each C atom overlap sideways with those on the 2 adjacent C atoms
- This forms a Pi cloud of electron density above and below the ring
- The pi cloud contains 6e-
- The density is spread out within the ring
- The electrons in the cloud are said to be delocalised
Which model of benzene is currently accepted to be accurate?
The delocalised model of benzene
Define delocalised
Not belonging to a specific atom
How many delocalised electrons are spread out within the pi cloud?
6e-
What shape does each C atom have in the delocalised model of benzene?
trigonal planar
What does each of the bonding electrons in a carbon atom bond to in the delocalised model of benzene?
- adjacent C atom
- adjacent C atom
- adjacent H atom
- not bonded - located in the p orbital above or below the c atom
What shape do the 6 C atoms have in the delocalised model of benzene?
Planar hexagonal ring
Is benzene more or less reactive than predicted by Kekulés model?
Less reactive -more stable
What 3 features show that benzene is less reactive than predicted by Kekulé?
Benzene does not react with bromine water, strong acids or halogens
What are the 3 types of causes for mechanisms?
Electrophilic
Nucleophilic
Free radical
What are the 3 types of effects for mechanisms?
Elimination
Addition
Substitution
What type of mechanism is benzene susceptible to and why?
Electrophilic substitution
Does not contain C=C so no addition
Has an electron cloud with a strong electron density meaning electrophiles are attracted to it
Explain the general mechanism of an electrophilic substitution reaction of benzene
- The electrophile is attracted to the benzene ring which has a high electron density
- The electrophile accepts a pair of electrons from the delocalised pi cloud
- A covalent bond is formed between the electrophile and C atom on benzene
- The delocalised pi cloud is disrupted
- Because of this the intermediate is highly reactive
- An electron pair from a covalent bond between the C atom and a H atom goes to restoring the benzene ring
- the H+ ion is rapidly lost
- The delocalised pi cloud is restored
what conditions are required for the nitration of benzene?
Conc H2SO4, 50 degrees, reflux
What group is substituted on in the nitration of benzene?
NO2+
What reagent is required in the nitration of benzene? Why?
HNO3
It supplies the NO2+ ion
Write the equation for the formation of a nitrate electrophile from its reagent
HNO3 + H2SO4 –> HSO4- + H2O
What conditions are required for the electrophilic substitution of benzene
Conc H2SO4
50 degrees
Reflux
What reagents can be used for the halogenation of benzene?
Br2 or Cl2
What catalysts can be used for the halogenation of benzene?
AlCl3 or AlBr3
Write the equation for the formation of a halogenation electrophile from its reagent
Br2 + FeBr3 –> Br+ + FeBr4-
What reagents can be used for the alkylation of benzene?
haloalkane
What catalysts can be used for the alkylation of benzene?
A halogen carrier such as AlCl3
Write the equation for the formation of a alkylation electrophile from its reagent
Haloalkane + AlCl3 –> carbocation+ + AlCl4-
What reagents can be used for the acylation of benzene?
Acyl chloride
What catalysts can be used for the acylation of benzene?
AlCl3
Write the equation for the formation of a acylation electrophile from its reagent
Acyl chloride + AlCl3 –> carbocation+ + AlCl4-
Which of cylohexene and benzene are resistant to bromination?
benzene
Explain why benzene is resistant to bromination
It has a lower pi electron density than cyclohexene which is not strong enough to polarise Br2
List the directing groups
NH2, NO2 and OH
how do electron donating groups act as a directing group?
It donates a lone pair of electrons to the benzene ring when bonded, which causes its electron density to increase and is therefore more reactive, and the benzene more readily attacks an electrophile
Does a directing group make benzene more or less reactive?
More
What does a directing group prevent the need of for benzene to react?
A cataylst
Electron donating groups direct new groups on where in the benzene?
C 2 and 4
Electron withdrawing groups direct new groups on where in the benzene?
C 3 and 5
give examples of an electron donating group
NH2 OH
give examples of an electron withdawing group
NO2
Explain how electron withdrawing groups act as directing groups
They draw electrons away from the centre of benzene, making it less reactive and the benzene less readily attacks nitrophiles
define phenols
A group of organic chemicals that contains an OH group attached directly to a benzene ring
is phenol soluble
Yes slightly because it contains an OH group
Is phenol acidic
Yes it can form weak acidic solutions due to its H+
How can you establish between a phenol and carboxylic acid?
react with sodium carbonate - CA will react, phenol will not
Phenol + NaOH –>
Phenoxide salt and water
Is phenol more reactive than benzene? why/why not?
Yes because it has an OH group which increases the electron density and hence the reactivity
Can benzene react with Br2 without a carrier?
No
Can phenol react with Br2 without a carrier?
yes
Phenol + dilute HNO3 –>
2 nitrophenol or 4 nitrophenol
