3.6 aromatic compounds Flashcards
what do aromatic compounds all contain?
a benzene ring
what is the structure of benzene?
C6H6
planar,cyclic structure
each c forms single covalent bonds to the carbons on either side and to 1 hydrogen
unpaired electron on each c is located in a p orbital that sticks above and below the plane of the ring.
p-orbitals on each c form a ring of delocalised electrons
bond angles- 120
(6 electrons in a pi bond above and below the plane of the atoms)
what is the length of the bonds in a benzene ring?
all the same length (140pm)
between the length of a single C-C and double C=C
why is benzene more stable than cyclohexa-1,3,5-triene would be?
because of the delocalised electron ring system
in a delocalised ring, the electron density is shared over more atoms so energy of the molecule is lowered and it becomes more stable
evidence for benzene being more stable than cyclohexa-1,3,5-triene
cyclohexatriene would be expected to have a ∆H of 3x -120KJmol-1 (because it would have 3 double bonds)
benzene is more stable because the actual hydration enthalpy is -208 KJmol-1 so 152 JKmol-1 less exothermic than expected
so less energy is given out than the expected so must be used elsewhere (to break bonds) this energy is used to break the more stable electron ring system
what are the problems with kekules structure? (cyclohexa-1,3,5-triene)
- C-C bonds are all the same length (halfway between c-c and c=c)
- benzene doesnt undergo electrophilic addition readily
- you dont get isomers like those in my book from 5/9/24
- ∆H ∆H is 152KJmol-1 less than expected
how to name aromatic compounds
practice
suffix benzene or prefix phenyl
why does benzene attract electrophiles?
because it is a region of high electron density
what mechanism does benzene go through?
why?
electrophilic substitution
because it is so stable so doesnt undergo electrophilic addition as this would destroy the electron ring
describe the electrophilic substitution mechanism of benzene
electron dense region at centre of ring attracts a nucleophile
electrophile steals pair of e- from centre of ring and gives molecule a + charge
to regain stability of benzene ring, c which is now bound to electrophile loses a h
conditions of nitration?
overall equation?
when you warm benzene and conc sulfuric acid, you get nitrobenzene.
overall eq is:
HNO3 + benzene -> nitrobenzene + H2O
what is the sulfuric acid catalyst for in nitration?
equation for making NO2+
helps make nitronium ion (NO2+) which is the electrophile
formation of NO2+ is first step of reaction mechanism
eq is
HNO3 + H2SO4 -> HSO4- + NO2+ + H2O
(once formed it can react w benzene through electrophilic sub)
what are the uses of nitration reactions?
nitro compounds can be reduced to form aromatic amines which are used to manufacture dye and pharmaceuticals
nitro compounds decompose violently when heated so they are heated so are used as explosive such as TNT
why are friedel crafts acylation reactions important?
they add an acyl group to a benzene ring which can be modified using further reactions to make useful products
this is good because benzene is stable so is fairly unreactive
what is Friedel-Crafts acylation?
conditions?
Adding an acyl group to a benzene ring
reactants need to be heated under reflux in a non-aqueous solvent
what is the equation for friedel crafts acylation?
acyl chloride + benzene -> phenylketone + HCl
what is a halogen carrier?
what is the halogen carrier and electrophile in friedel crafts acylation?
electrophiles need to be very + to attack stable benzene and most arent polar enough so are made stronger by halogen carrier catalyst
acyl chloride provides electrophile, AlCl3 is halogen carrier
how does AlCl3 make electrophile stronger?
accepts lp from acyl chloride which increases the polarisation of the acyl chloride forming a carbocation
