aromatic (benzenes) Flashcards
aromaticity refers to
planar
stable due to delocalisation of p electrons
intermediate bond angles
describe the structure of benzenes
bond lengths are longer than normal cc and shorter than c=c
each c forms 3 bonds so 1 lone electron which gets delocalised as the p orbitals overlap to form a pi bond
e cloud above and below the ring plane
molecule is stable due to electron delocalisation
3 reasons why it’s benzene and not the hypothetical cyclo135triene
- bond lengths
- C-C and C=C are different lengths. so this hypothetical molecule would be?? Impossible, less stable? - Doesn’t undergo addition reactions
- doesn’t decolourise bromine water, doesnt readily undergo addition reactions because it disrupts the stability of the e ring by ruining it basc. - Enthalpies of hydrogenation
- refers to the enthalpy change measures when different cyclic alkanes are hydrovenated to get to cyclohexene. All db get hydrogen added across them
- they increase for 1ene to diene from triene, indicating that it’s more exothermic to become cyclohexane
- but the enthalpy of hydrogenation for benzene is less than triene, indicating it is more stable
What mechanism does benzene undergo and why
Electrophilic substitution
Electron density makes it attractive to electrophiles
Addition would disrupt the delocalisation and therefore reduce stability but substitution only causes temporary disruption and preserves the delocalised system
What are the two electrophilic substitution reactions benzene undergoes
Nitration with nitronium ions to form nitro benzenes
Friedek crafts acylarion with acid anhydrides or acyl chlorides to form phenyl ketones (depends on r group)
Conditions of nitration
Formation of electrophile
Conc nitric acid
Conc sulfuric acid
50 degrees
Hno3 + h2so4 - > h2no3+ + hso4-
H2no3+ - > no2+ + h2o
Mechanism of nitration
Nitronium ion attacks the ring
Ring is disrupted and a C forms bond with N ion
Horseshoe
H on the C that is in that C-NO2 donates bond to ring
forms nitrobenzene and proton
could have successive reactions
Why are nitrobenzenes useful
Used as explosives
Important step in organic synthesis of other molecules eg amines. Produces primary amines that are used in the manufacture of dyes
Conditions of Fc acylation
Anhydrous
AlCl3
Both reagents have same group that is attached (molecule left after leaving group (different) goes):
O
II
R-C+
Formation of electrophile in Fc acylation
Regeneration of catalyst after the mechanism
ACYL CHLORIDES
AlCl3 + RCClO -> RCO+ + AlCl4-
ACID ANHYDRIDES
RCOOCOR + AlCl3 - > Cl3Al-OOCR] -
((the leaving group divide is RCO(|) OCOR))
[Cl3Al-OOCR] - + H+ - > AlCl3 + RCOOH
What two ways can nitrobenzenes - - > phenylamines
REDUCTION REACTIONS
- Sn & HCl, heating, followed by NaOH
- nitrobenzene + 6[H] - > phenylamine + 2water
this phenylamine becomes - NH3+ Anion- because it acts as a base, then forms a salt. follow by NaOH to react to give the insoluble phenylamine
CATALYTIC HYDROGENATION
- nickel catalyst
- nitrobenzene + 3H2 - > phenylamine + 2water
Why is Fc acylation good
products used as chemical feedstocks and synthetic intermediates. so they are necessary to make other useful chemicals
what’s interesting about AlCl3
Is more covalent than ionic
Al shows covalent character because small radius, high charge so high positive charge density
So high electronegativity, or polarising effect. It attracts electrons towards itself, from the anion for ex
Which distorts the E cloud introducing cov character to the bonds