6.1.1 - aromatic compounds Flashcards
briefly describe Kekule’s Benzene
six memebered cyclic structure with alternating single and double bonds (cyclohexa - 1,3,5, - triene)
give the 3 pieces of evidence as to why scientists disagreed with Kekule’s model of benzene:
- reactivity with bromine
- bond lengths
- enthalpy of hydrogenation
explain why benzenes reactivity with bromine disproved Kekule’s model of benzene
benzene does not undergo electrophilic addition with bromine like alkenes typically do, and there is no colour change when bromine is added to benzene. Benzene can react with bromine in the presence of a halogen carrier catalyst in electrophilic substitution.
explain why bond lengths in benzene disproved Kekule’s model of benzene
benzene has a regular structure which is planar with equal carbon carbon bond lengths. C C double bonds are shorter than C-C bonds so Kekule’s model didn’t predict a regular hexagon for benzene.
explain why the enthalpy of hydrogenation of benzene disproves Kekule’s model of Benzene
the actual enthalpy of hydrogenation of benzene is less exothermic than Kekule’s suggested cyclohex - 1,3,5 - triene
describe the shape of benzene
benzene is a planar cyclic molecule with bon angles of 120 degrees between carbon atoms.
describe the electron structure/bonding of benzene
each carbon forms 3 sigma bonds. the 4th electron of each carbon is contained in a p-orbital. the p-orbitals overlap sideways, above and below the plane of the ring to form a pi bond. 6 pi electrons are delocalised over the 6 carbon atoms.
in terms of a question about the delocalised model of benzene, describe an alkene
alkene = kekules model of benzene
- sideways overlap of p-orbitals
- pi bonds above and below carbon ring
- 3 localised pi bonds
- 2 electrons in each pi bond
- overlap of orbitals in one direction.
in terms of a question about the delocalised model of benzene, describe the structure of benzene
- sideways overlap of p - orbitals
- pi bond (ring) above and below carbon ring
- one delocalised pi ring
- 6 electrons in one pi bond (ring)
- overlap in both directions.
compare the reactivity of an alkene and benzene in terms of reactions with Br2
in alkenes, the pi electrons are localised and there is a high electron density, which is able to polarise and attract the Br2 molecule for and electrophilic addition reaction.
in benzene, the pi electrons are delocalised, giving a lower electron density and benzene cannot polarise and attract a Br2 molecule, so it cant undergo electrophilic addition.
benzene can react with Br2 in electrophilic substitution reactions in the presence of a halogen carrier catalyst.
define arene
hydrocarbon containing at lease one benzene ring
define aromatic
compounds containing benzene rings
when is the word phenyl used?
when a benzene ring is already a part of an already well known and named molecule. eg. phenol or phenylethene.
explain why benzene cant undergo electrophilic addition.
in alkenes (which can undergo electrophilic addition) the localised 2 pi electrons cause a high electron density, so the alkene can polarise and attract an electrophile. benzene has 6 delocalised pi electrons, so has a lower electron density, and cant polarise or attract an electrophile, and cant undergo electrophilic addition
what is the name of the mechanism that allows benzene to react with halides? what are the reagents?
electrophilic substitution. a halogen carrier catalyst is required.
in electrophilic substitution: what produces an electrophile for benzene to react with?
reaction between catalyst and the other reactant. produces a positive ion (heterolytic fission) that is attracted to the electrons in the delocalised pi ring.
explain what happens to the delocalised pi ring during electrophilic substitution.
there are 6 pi electrons in benzene spread over the 6 ring carbon atoms.
during the reaction, 2 of these electrons go into making a bond between the electrophile and a carbon.
the intermediate now has 4 pi electrons spread over 5 carbon atoms.
a H atom is lost in the last step, which gives 2 electrons back to the benzene ring, which re-established the delocalised pi system.
what is the name of the reaction when a Br+ electrophile reacts with an aromatic compound?
halogenation
what is the name of the reaction when an “acyl” electrophile reacts with an aromatic compound?
Friedel-Crafts Acylation
what is the name of the reaction when a NO2+ electrophile reacts with an aromatic compound?
nitration
what is the name of the reaction when an alkyl electrophile reacts with an aromatic compound?
Friedel-Crafts Alkylation
give an example of the uses of Bromobenzene
preparation of pharmaceuticals
give 2 examples of what chlorobenzene is used for
as a solvent and in the preparation of pesticides
what are the conditions for the nitration of an aromatic compound?
Concentrated HNO3, contrentrated H2SO4 and 50-60 degrees celsius
