17.4 1 Benzene Flashcards
Compound benzene
Makes an aromatic compound - described as arene
Physical properties of benzene
Colourless liquid
Insoluble of water
Present in crude oil
Toxic : carcinogen
Molecular formula
C6H6
Kekulè structure
See book
Problem with Kekulè structure - bromine water
Does not decolourise bromine water suggest no c=c bonds
Instead a substitution reaction occurs
Problem with Kekulè structure - isomers
Dibromobenzene should be four isomers (see book) however only three known to excist
Isomers with bromine on adjacent atoms are the same
Suggest bonds between carbon atoms in benezene ring are the same
Problems with Kekulè structure - bond length
Data about length of covalent bonds became available
Data shows bonds all the same and maybe a intermediate character between single and double bonds
Problems with Kekulè structure - enthalpy changes
Data about enthalpy change of hydration
Should be much higher than cyclohexane but is lower
Using theoretical structure proposed by kekule is actually much lower
See book
New model
After all sigma bonds formed each carbon remains with one electron in p orbital
Six electrons formed delocalise pi bond
Pi electron cloud above and below plane of carbon atoms by p orbital overlapping sideways
See book
How are problems solved with new model
No individual C=C
Are three isomers as no difference between adjacent
All carbon- carbon bonds are the same
Charge is spread around the species increase stability which explains lower enthalpy (greater stability ) or benzene
Why C6H5Br substitution instead of C6H4Br2 addition
Substiution preserves stability of delocalised electrons in pi bond
Addition would produce compound with two c=c lack stability
Aromatic
Delocalised electrons forming pi bonding in a hydrocarbon ring
