3.3.10- Aromatic compounds/chemistry (PAPER 2) Flashcards
Describe the structure of benzene
Cyclic, planar molecule with structure C6H6
Each C bonded to 2 other C and 1 H, the final lone electron is in a p-orbital which sticks out above+ below planar ring- lone electrons in p orbitals combine to form delocalised ring of electrons
Due to delocalised electron structure, all C-C bonds in molecule are the same (0.139nm): these are intermediate in length between single and double C bonds, evidence of delocalisation.
All 6C atoms have trigonal planar arrangement, internal bond angles are all 120.
What is the IUPAC name for Kekule structure of benzene?
cyclohexa-1,3,5-triene
What is the difference between Kekule structure for benzene ring and current structure?
Kekule shows a heptagon with alternating double and single carbon bonds.
Current is a heptagon with a circle in the middle representing cloud of delocalised electrons.
Why was Kekule structure deemed inaccurate?
Benzene was more thermodynamically stable than expected
Enthalpy of hydrogenation (adding a H) to cyclohexene- a cyclic alkene with one double bond was -120kjmol-1
Cyclohexa-1,3,5-triene enthalpy of hydrogenation is assumed to be -120x3 = -360kjmol-1 for 3 double bonds
However actual enthalpy of hydrogenation of benzene was -208kjmol-1
Why was benzene enthalpy of hydrogenation lower than expected?
Benzene has increased stability due to its delocalised system of electrons, more energy is required to break the bonds in benzene.
What is an arene and how are they named?
A molecule that contains a benzene ring- high melting points due to stability of delocalised ring but low boiling points due to non-polarity.
We can either name by putting benzene on the end and the functional group infront- eg 1,2-dimethylbenzene or by using phenol (benzene ring with an OH)- eg phenylamine if NH2 bonded to benzene ring
What reactions do arenes undergo + why?
Electrophilic substitution/ -nitration where nitro (NO2) group is placed into benzene ring instead of hydrogen or acylation where RCO is placed into benzene
Benzene has high electron density which is attractive to to electrophiles, does not undergo electrophilic addition like other alkenes as this would disrupt stable electron ring.
What is Friedel-Crafts acylation?
Acyl group (RCO-) is added onto benzene molecule, making benzene structure weaker. Electrophile must have very strong + charge (stronger than acyl groups) so halogen carrier can act as catalyst to produce stronger electrophile.
Acyl chloride reacts with halogen carrier to produce strong electrophile
What is the equation to make the electrophile acylium ion from AlCl3
O O
// //
R-C - Cl: + AlCl3 —> R-C+ + AlCl4 -
AlCl3 accepts pair of electrons from acyl group, polarisation increases and carbocation formed
Outline the mechanism between benzene and acylium ion to make phenylketone under reflux and dry ether solvent
Curly arrow from ring inside benzene structure to d+ C from RC=O, ring splits and C joins to one of corners where ring is split (joined to R and =O) , H shown joined to same corner. + charge drawn in horseshoe shape
Draw AlCl4-, curly arrow from one Al-Cl bond to H visible in benzene structure.
Electrons in C-H bond move to neutralise positive charge and reform ring, RC=O joined to ring, HCl formed and AlCl3 reformed.
How do we make the electrophile for the second electrophilic substitution mechanism- nitration?
React sulfuric acid with nitric acid (acts as base and accepts proton)
HNO3 + H2SO4 ⇌ HSO4- + H2NO3 +
H2NO3+ decomposes to form electrophile (nitronium ion NO2+)
H2NO3+ —-> NO2+ +H2O
Outline the mechanism between benzene and nitronium ion to form nitrobenzene.
Curly arrow from ring inside benzene structure to N of NO2-, ring splits and N joins to one of corners where it has split (with O2) , H shown on same corner, + charge added inside horseshoe.
Curly arrow from C-H bond into + charge as electrons in bond move to reform ring.
Nitrobenzene formed- NO2 added to benzene ring, H+ is formed which reacts with HSO4- produced earlier so reforms catalyst H2SO4
How to ensure a single substitution of NO2?
A temperature below 55C
What can nitrobenzene compounds be used for?
Dyes and pharmaceuticals- by reducing nitrobenzenes to aromatic amines (benzene with amine groups attached)
Explosives- made from nitrobenzene- eg 2,4,6-trinitromethylbenzene/ TriNitroToluene
