3.3.10 - aromatic chemistry Flashcards
benzene is a _____, ________ molecule with the formula __________
benzene is a cyclic, planar molecule with the formula C6h6
carbon has __ valent electrons
carbon has 4 valent electrons
each carbon is bonded to __ other carbons and __ hydrogen atom.
each carbon is bonded to 2 other carbons and one hydrogen atom
where is the final lone electron
in a p orbital which sticks out above and below the planar ring
the lone electrons in the p orbital combine to form a ….
delocalised ring of electrons
What’s important about bond lengths of c-c bonds and why does this occur
All C-C bonds are of the same bond length
Due to the delocalised electron structure
Kekules structure of benzene
Benzene has double bonds
He thought there was alternating single and double bonds
Why is kekules structure wrong?
Electrons are delocalised
Used structure benzene?
Hexagon with circle inside, the circle contains the delocalised electrons
Kekules theoretical structure with alternating single and double bonds can also be called
Cyclohexa - 1,3,5 - triene
3 ways of looking at benzenes stability
Enthalpy change of hydrogenation benzene vs its theoretical
Enthalpy of hydrogenation of cyclohexene is 120kjmol-1- also has one double bond
The theoretical version with 3 double bonds has an enthalpy change of hydrogenation of 3x-120 = -360kjmol-1
However,the actual /experimental value of enthalpy of hydrogenation of benzene is less exothermic than the predicted -360kjmol-1
The actual enthalpy hydrogenation at benzene is far less exothermic than the predicted -360kjmol
What does this tell us
Energy is required to break bonds and energy is released to make bonds
This suggests mare energy is required to break the bonds in benzene verses cyclohexa - 1,3,5 - triene thus benzene is more stable than the theoretical cyclo… with 3 double bonds
This stability is due to the delocalised electron structure
What are arenes
Aromatic compounds are molecules that contain a benzene ring
2 ways of naming arenes
benzene at the end eg bromobenzene, nitrobenzen, 1,2 - dimethyl benzene
or
phenol (benzene w hyroxyl), phenyl amine
arenes undergo what type of reaction and why
electrophillic sub
benzene ring has high electron density so electrophiles are attracted to it
is benzene stable or unstable and what does this mean for the kind of reaction it wont undergo
benzene is stable so unlike traditional alkenes they dont undergo electrophillic addition as it would distrupt stable ring of electrons
2 mechanisms of electrophilic sub that benzene undergoes
friedel-crafts acylation
nitration reaction
what must the electrophile added on to the benzene ring have that acyl groups dont have
a very strong positive charger
in order to make acyl groups have more of a positive charge we…
use a halogen carrier to act as a catalyst
(AlCl3)
producing a much stronger electrophile
equation to form stronger electrophile
RCOCl + AlCl3 —> RC(+)O + AlCl4-
Conditions for electrophilic sub
Under reflux
Dry ether solvent
how to form nitrobenzene (2 STEPS)
1) we make the electrophile by reacting sulfuric acid with nitric acid
HNO3 + H2so4 –> H2no3+ + HSO4-
H2no+ —-> NO2+ + H2O (forms nitronium ion)
what else is formed when nitrobenzene is formed in an electrophilic sub reaction and what does it do
a H+ ion is formed which reacts with HSO4- formed in the first step - making h2so4 again.
hence h2so4 is a catalyst
a temp below ____ will ensure ____ no2 sub
above this will result in _____
a temp below 55 will ensure one no2 sub
above this will result in multiple sub
