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Aromatic Chemistry (Benzene) - Coggle Diagram
Aromatic Chemistry
(Benzene)
Benzene
physical
properties
colourless liquid
not coloured as delocalisation
not extended enough
low MP + BP
non-polar, weak VDW forces
distinctive aroma
carcinogenic
immiscible
in water
issues
cause
leukaemia
carcinogenic
molecular formula
C6H6
empirical formula
CH
industry
petroleum
= industrial
source of benzene
obtained when crude oil
is fractionally distilled
catalytic reforming of hexane
obtained
from naphtha fraction (of crude oil)
CH3CH2CH2CH2CH2CH3 --------> C6H6 + 4H2
REACTIVITY
Delocalisation of
π electrons in Benzene
gives benzene
greater
thermodynamic stability
lowers energy of benzene
making it
less reactive
than alkenes
differences
to alkenes
benzene do not undergo
addition reactions
e.g. Br cannot be added to benzene
in same way as alkenes
obs:
Br water remain orange
(not decolourise)
similarities
to alkenes
both
react w/ electrophiles
have regions of HIGH ELECTRON DENSITY
alkenes - electron rich double bond
benzenes - delocalised ring of pi electrons
Bonding
total of
42 electrons
6
involved in
pi delocalised bonding system
24
involved in
covalent bonding
6 identical C---C bonds (INTERMEDIATE)
(C-C σ bond + partial C-C π bond)
formed through delocalisation of electrons in C-C π bond
forms
partial C-C π bonds
OVERLAP of 6 atomic p orbitals
6 spare electrons delocalised
not in fixed position
regions where found form 2 donuts
above + below ring
strength
of bond
intermediate between
alkane + alkene
STORNGER than C-C
WEAKER than C=C
length
of bond
intermediate between
alkane + alkene
SHORTER than C-C
LONGER than C=C
each C 3 covalent σ bond
uses 3 outer-shell electrons
between H + 2C atoms
1 electron per C is in p orbital
sideways overlap of p orbitals
delocalisation of electrons form pi ring
Naming
Phenyl group
C6H5
how formed:
removing H group
from benzene ring
examples
alkyl
- radical obtained
from alkane
aryl
- general term for aromatic radical obtained by removing H from nucleus
(eg phenyl)
acyl
- obtained by removing -OH group from carboxylic acids
(eg acyl chlorides)
Shape + Structure
planar hexagonal
ring molecule as the
sigma bonding pairs arranged
in
trigonal planar arrangement
about each carbon
electrons involved in π-bonding delocalised around ring
bond angle = 120°
carbon atoms form 6-membered ring
each C uses 3 outer-shell e-
σ bond w/
hydrogen atom
& 2 neighbouring C atoms
use 4th outer-shell e-
to form C-C π bond
w/ neighbouring C atom
Evidence for Stability
of Benzene
Method 1
Enthalpy Change for Hydrogenation
enthalpy change for hydrogenation
of cyclohexane = -120kJ mol-1
suggested enthalpy change for benzene
(if D+S bonds) = -360 kJ mol-1
should be 3 times greater than for cyclohexane
results
:
actual enthalpy change for hyd of benzene =
MUCH LESS
reflects additional stability
associated w/ delocalisation of π electrons
method
:
compare enthalpy change of hyd for C=C bond in alkene
with enthalpy of hyd for benzene
Method 2
Comparing Bond Enthalpy
total bond enthalpy for process
C6H6 (g) --> 6C (g) + 6H (g)
Route 1
convert gaseous benzene into atoms directly
C6H6 (g) ---> C6H6 (l)
C6H6 (l) ---> 6C (s) + 3H2 (g)
6C (s) + 3H2 (g) ---> 6C (g) + 6H (g)
Route 2
incorrectly assuming 3 double bonds + 3 single bonds
bond energies
C=C
C-C
C-H
method:
compare bond energies from
ROUTE 1 + ROUTE 2
results:
route 2 much lower than value route 1
shows stabilising effect of delocalisation
more energy needed to break ring
if it has π delocalised electron system
Preparation of
methyl 3-nitrobenzoate
properties
cream coloured solid
(MP = 78°C)
prepared by nitrating
methyl benzoate
why temp kept below
10°C during reaction?
prevents multiple nitrations
eg formation of methyl 3,5-dinitrobenozote
method:
place methyl benzoate (MB)
in
conical flask
add
conc H2SO4
to dissolve MB
cool
mix in ice
prepare
nitrating mix
(conc H2SO4 + conc HNO3)
cool
nitrating mix
add
nitrating mix
dropwise
to solution of MB
keep temp below 10°C
allow sit @
room temp
for 10 mins
pour mix over
crushed ice
filter + wash
crystals
recrystallise
to purify
using ethanol
filter + dry
crystals
determine
melting point
+
compare in data booklet
functional group in
methyl 3-nitrobenzoate which can
be hydrolysed =
ESTER
equation