Module 6: Aromatic Chem
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Structure of benzene What is benzene’s formula
C6H6
Structure of benzene What is benzene basic structure
Six C atoms in a hexagonal ring with onto H atom bonded to each carbon atom Molecule is planar
Structure of benzene What is true to length of all bonds in benzene
All same length-intermediate between single and double bonds
Structure of benzene How many sigma bonds does each C atom make with what
3 sigma bonds, 2 with C atoms, one with H atom
Structure of benzene Where is the 4th unused electron on each C atom
In a p orbital perpendicular to plane of ring
Structure of benzene What happens to the p orbital with unused electron
Overlaps with neighbouring p orbital forming a pi Bond
Structure of benzene What is the overall result of pi bonds
Ring of negative charge above and below plane of the ring
Structure of benzene What does the electrons in the pi system being delocalised mean
Do not belong to any particular C atom or bond and are free to move throughout whole pi system
Structure of benzene Why is benzene molecule more stable
Electrons are delocalised and more spread out and will repel each other less
Structure of benzene What is the electron density of the pi system
Low
Structure of benzene What are the labels on a detailed benzene diagram
P orbitals Sigma bonds Pi bonds, electrons from p orbitals delocalise above and below the carbon ring =lower electron density
Structure of benzene What are the bonding angles of benzene
120
Kekulés structure What structure did kekule suggest
Six carbon ring joined by alternate single and double bonds
Kekulés structure How many sigma bonds in keklule structure
12
Kekulés structure Labels on kekule diagram
P orbitals Sigma bonds Pi bonds, sideways overlap of p orbitals= localised electrons between 2 carbon atoms = high electron density
Disproving kekulè model 3 pieces of evidence
C-C bond lengths Addition reactions Enthalpy of hydrogenation
Disproving kekulè model Explain c-c bond length
All c-c bonds are same length in benzene between length of sing and double carbon bonds If benzene was triene would expect three longer c-c single and three shorter c=c double
Disproving kekulè model Explain addition reactions
Benzene does not readily undergo addition reactions (eg doesn’t decolourise bromine water) If benzene was triene would expect it to ready undergo addition
Disproving kekulè model Explain enthalpy of hydrogenation using numbers
Would expect triene to react with 3 H2 to form cyclohexane releasing 360 kj/mol of energy Benzene only releases 208 kj/mol of energy showing benzene 152 kj/mol more stable Extra stability due to delocalisation of electrons
Naming aromatics compounds What groups are shown as prefixes to benzene
Alkyl groups, halogens and nitro groups
Naming aromatics compounds Name with a C2H5 group monosubstutudted
Ethylbenzene
Naming aromatics compounds Name with Cl monosubstituted
Chlorobenzene
Naming aromatics compounds Name with NO2 monosubstituted
Nitrobenzene
Naming aromatics compounds When does the benzene ring become the substituent
When ring is attached to alkyl group with functional group or alkyl chain with more than 7 carbon atoms