In benzene, each carbon is bonded to two other carbons and one hydrogen atom and the final lone pair is in the p-orbital which sticks out above and below the planar ring
The lone electrons in the p-orbitals combine to form a delocalisedring of electrons which means that all bonds have the same length
This is kekules structure:
Kekules structure shows benzene with double bonds alternating around the benzene
This is the more common structure:
This structure has a circle to show the ring of delocalisedelectron system
Benzene is morestable than kekules structure (cyclohexa-1,3,5-triene)
We measure the stability of benzene by comparing the enthalpyofhydrogenation in benzene and cyclohexa-1,3,5-triene
If you hydrogenate cyclohexene has enthalpy change of -120 so cyclohexa-1,3,5-triene has enthalpy change of -120x3=-360
The hydrogenation of benzene is far lower than that of cyclohexa-1,3,5-triene at -208
More energy is required to break bonds in benzene than cyclohexa-1,3,5-triene because it is less exothermic and suggests that it is more stable because of its delocalisedelectron structure
Arenes are an aromatic compounds that contain a benzene ring and are named in 2 ways
The priority group ranking is
Carboxylics (Can)
Esters (Elephants)
Amides (Ask)
Nitriles (Narwahls)
Aldehyde (About)
Ketone (Keeping)
Alcohol (Anklets)
Arenes (As)
Alkenes (An)
Alkanes (Accessory)
Halogens (Huh?)
Nitros (No!)
Anything above arenes in priority is a phenylprefix, where as anything below arenes in priority have the benzenesuffix
Arenes undergo electrophilicsubstitution
Arenes undergo electrophilicsubstitution because it has a highelectrondensity due to its delocalised ring of electrons which attracts electrophiles
Benzene is verystable so do not undergo electrophilic additiobn
Electrophilicsubstitution with arenes is when a hydrogen group is substituted for the electrophile
Two mechanisms for Arenes is:
Friedel-craftsAcylation
Nitration
Benzene is a very important chemical in pharmaceuticals and dyes
Benzenes stability is an issue because it makes it difficult to react, Friedel-crafts acylation solved this problem
Friedel-Crafts acylation is when an Acylgroup (RCO-) is added onto a benzenemolecule to weaken the benzene structure and make it easier to modify
In Friedel-Crafts acylation, we react an AcylChloride with AlCl3to create a stronglypositiveelectrophile to attack the benzene
The electrophile in Friedel-Crafts Acylation is made by this reaction where the AlCl3accepts a pair of electrons from the acylgroup forming a carbonation which then goes on to react with benzene
The electrophile now reacts with benzene to make a phenylketone under reflux and a dry ether solvent
The electrophile is attracted by the delocalisedelectrons and forms a bond which breaks the ring leaving a positive charge. The hydrogen then breaks off feeding electrons back into the benzene and reforming its delocalised ring.
Nitration of benzene is useful because it allows for the production of dyes and explosives
To form a positive electrophile for the nitration of benzene there is a 2 step process:
React sulphuric acid with nitric acid
H2NO3+ion decomposes in nitronium ion
The reaction for the first step of forming an electrophile for nitration of benzene is:
HNO3+H2SO4→H2NO3++HSO4−
The second step of forming an electrophile for nitration of benzene is:
H2NO3+→NO2++H2O
The Nitronium ion (electrophile made) is attracted by the delocalisedelectrons and forms a bond leaving a positive charge, the hydrogen breaks off and feeds electrons back into benzene and reforms the delocalisedring
When doing the nitration of benzene, the temperature must be below 55degrees because it can undergo multiple substitutions above this temperature and become explosive
Nitrobenzene compounds can be used to make:
Dyes and pharmaceuticals by reduction to aromatic amines