AROMATIC CHEMISTRY Flashcards
describe the structure of benzene
a cyclic planar molecule with formula C6H6
the lone electron on each carbon sits in a p orbital above and below the plane of the ring
the p orbitals of each carbon will overlap and combine to form a delocalised ring of electrons
each C-C bond length is the same length (139pm), between a single bond (154pm) and (134pm) double bond
what would we expect the enthalpy of hydrogenation to be in benzene and why
-360KJmol-1
as hydrogenation of 1 double bond is -120, so if benzene had 3 it would be 3x the enthalpy of 1
What is the actual value of enthalpy of hydrogenation of benze
-208KJmol-1
much lower than the expected value, showing benzene is more stable as more energy is required to break bonds in benzene than in cyclohexa-1,3,5-triene
this is due to its delocalised electron structure
What are arenes
molecules that contain a benzene ring, these are aromatic
two ways arenes are named (prefix and suffix)
-benzene or phenyl-
What types of reactions do arenes undergo
electrophilic substitution
What is another way of showing stability of benzene
even though benzene has a high electron density, it will not undergo electrophilic addition reactions as this would disrupt the stable ring of electrons
it will undergo electrophilic substitution where a hydrogen or functional group is substituted for an electrophile
Describe the Friedel-Crafts acylation mechanism and why is it used
The addition of an acyl group onto a benzene molecule to make the structure weaker and therefore easier to modify further to make useful products
Halogen carrier used as a catalyst to produce a much stronger electrophile with a strong positive charge
( Acyl chloride reacts with a halogen carrier (AlCl3))
AlCl3 accepts a pair of electrons from the acyl group causing the chlorine to leave the acyl group, which leaves a strongly positive carbocation on the acyl group and AlCl4-
react the electrophile with benzene under reflux with a dry ether solvent to form a less stable phenylketone
the delocalised ring of electrons are attracted to a carbocation so a pair of electrons move to form a bond which breaks the ring and leaves a positive charge
then the negatively charge AlCl4- is attracted to the positively charged ring and a chlorine atom breaks away to form a bond with the hydrogen
this reforms the benzene ring and leaves HCl and AlCl3 as well as the phenylketone products
Describe the mechanism of nitration of benzene and the effect of temperature
Heat benzene with concentrated nitric acid and sulfuric acid to form nitrobenzene (A POWERFUL ELECTROPHILE MUST BE MADE FIRST)
react sulfuric acid with nitric acid to form H2NO3+ + HSO4-
the H2NO3+ decomposes to give H2O and NO2+ which is a powerful electrophile
a pair of electrons from the delocalised ring of electrons in benzene will form a bond with the nitronium ion leaving a positively charged unstable ring
the electrons in the C-H bond in the intermediate will move to reform the delocalised electron ring to form nitrobenzene
The H+ the will react with the HSO4- to reform the sulphuric acid catalyst
TEMPS BELOW 55 DEGREES WILL ONLY ENSURE ONE SUBSTITUTION, ANYTHING ABOVE AND MULTPLE SUBSTITUTIONS CAN OCCUR
Why are phenols more reactive than benzene
the lone pair of electrons on the oxygen of the hydroxyl group will be partially delocalised into the pi system as the p orbital will overlap with the delocalised ring structure
this increases the electron density so will be more susceptible to attack from electrophiles
How do functional groups affect substitution reactions in terms of withdrawing groups and donating groups
Electronegative groups such as NO2 withdraw electron density from the ring and specifically from carbons 2,4 and 6, so electrophiles are more likely to attach carbons 3 and 5
electron donating groups such as NH2 and OH
How do functional groups affect substitution reactions in terms of withdrawing groups and donating groups
Electron withdrawing groups such as NO2 withdraw electron density from the ring and specifically from carbons 2,4, 6, so electrophiles are more likely to attach carbons 3 and 5
electron donating groups such as NH2 and OH will donate electrons and will increase density of the ring so electrophiles are more likely to attack carbons 2,4,6
How are phenols weak acids and what reactions can this cause
as they partially dissociate to form a phenoxide ion and a H+ ion
therefore they can react with alkalis to form a salt and water
(e.g. sodium phenoxide from phenol and sodium hydroxide)
How can we differentiate phenol and benzene
benzene will not react with bromine, whereas phenol will as the electron density in the ring is higher, so the solution will be decolourised as the bromine has reacted
Phenol can also react with dilute nitric acid to produce nitrophenols, either 2-nitrophenol or 4-nitrophenol
