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Aromatic Compounds
6.2.1 Benzene and Phenol…read more

Slide 2

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Bond angles in benzene are all 120 degrees
· Planar molecule
· Circle represents two rings of electrons ­
delocalised (instead of forming 3 double
Benzene bonds
· The simplest aromatic hydrocarbon/ arene ­ C6H6 (empirical
formula ­ CH)
· Colourless, sweet smelling, highly flammable liquid
· Found naturally in crude oil- component in petrol and found in
cigarette smoke
· Carcinogen ­ can cause cancer
· Derivatives of benzene:
-many pleasant smelling compounds (historically) ­ also odourless
ones too e.g. thymol and benzaldehyde
-many aromatic compounds can be synthesised from benzene
· Molecules not containing a
benzene ring- aliphatic…read more

Slide 3

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Kekulé's Structure of Benzene
· Its molecular formula C6H6 suggested a structure containing many
double bonds or double and triple bonds
· Kekulé suggested a 6-Carbon ring joined by alternate single and
double bonds
· However, this model was not able to explain many of benzene's
properties such as lack of reactivity and so many people disagreed
with the model
· Based on a dream he had!…read more

Slide 4

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Problems with Kekulé's Structure of
Benzene (1)
· Lack of Reactivity ­ If it contained the C=C bonds, it should
decolourise bromine in an electrophilic addition reaction (in the
dark at room temperature- like other alkenes)
however... it does not readily undergo electrophilic addition
reactions and it does not decolourise bromine under normal
conditions SO it is not a normal alkene…read more

Slide 5

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Problems with Kekulé's Structure of
Benzene (2)
· Thermodynamic Stability ­ enthalpy of hydrogenation- should undergo
similar reactions to cyclohexene and cyclohexa-di-ene SO benzene
should be 3 x cyclohexene's enthalpy of hydrogenation to indicate the
positioning of the C=C bonds (3 x -120 = -360 kJmol-1)
3 · Cyclohexane is
formed as expected
· However ... its hydrogenation enthalpy was found to be -208kJmol-1,
152 more than predicted (more endothermic) SO more stable than
expected if it contained 3 ordinary C=C bonds…read more

Slide 6

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Problems with Kekulé's Structure of
Benzene (3)
· Bond Lengths of Benzene ­
Cyclohexane C-C = 0.154 nm
Cyclohexene C=C = 0.134 nm
SO... if benzene had alternating double bonds its structure would be
· However... direct evidence from Scanning Tunnelling Microscopy found
benzene with an undistorted shape- all bonds lengths are the same- 0.140 nm
(between a double bond and a single bond). This suggests that the double
bonds can't be fixed in position
· Resonance suggests benzene is in such rapid equilibrium between the 2
evidence forms, we detect a blurred combination of the two. The electrons from the
for this double bonds are therefore drawn as a circle shared equally between the C
atoms. Spare electrons are delocalised around all 6-Carbon atoms…read more

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