ALLOW reverse arguments for Kekule
structure Any three points from
Type of reaction
Benzene reacts by (mostly electrophilic)
substitution OR does not react by
(electrophilic) addition
OR
Benzene does not react like alkenes /
does not decolourise bromine water
ALLOW
Other suitable reactions /
benzene needs a catalyst /halogen carrier
to react with bromine (1)
Di-substitution
There are only 3 isomers of di-substituted
compounds (not 4)
OR
Some di-substituted compounds are the
same,
e.g. 1,2 and 1,6 (1)
Thermochemical
Benzene’s (standard) enthalpy (change) of
hydrogenation is less exothermic than if
it had (three localised C=C) double bonds /
is not three times the value for three
(localised C=C) double bonds
ALLOW
Benzene is more stable by ~150 kJ mol-1
OR
stated enthalpies (of hydrogenation)
−205 to −210 kJ mol-1 for benzene and
−360 kJ mol-1 for 3 (localised C=C)
double bonds
OR
(Standard) enthalpy (change) of
combustion is less exothermic than if it
had three (localised C=C) double bonds
ALLOW
(Standard) enthalpy (change) of formation of
benzene is less endothermic than that of
“cyclohexa-1,3,5-triene”
(1)
IGNORE
Just ‘thermodynamically more stable’
X-ray diffraction – does not need to be
mentioned
The C-C bond lengths in benzene are midway
between that of a single bond and a
double bond / are all the same length
OR
Benzene is a regular hexagon (and Kekule
structure is not) (1)
IGNORE
Bond angles are the same
Infrared
The infrared spectrum for benzene has a
peak for an aromatic C=C at a different
wavenumber
/ absorption / frequency to an alkene C=C
OR
Benzene has peaks at 1600, 1580, 1500,
1450 (cm−1) rather than 1669 – 1645
(cm−1)
ALLOW
Benzene has no peak for alkene C=C / 1669
– 1645 (cm−1) (1)
IGNORE
different C-H absorptions / just ‘different
peaks to alkenes’
IGNORE
References to NMR
Electron density map
Benzene shows an even spread of electrons
