benzene Flashcards
what is benzene?
a cyclic, planar molecule with general formula C6H6
describe the bonding in benzene
- 4 valent electrons
- each bonded to 2 carbons and 1 hydrogen
- lone electrons are in p orbitals
- p orbitals stick out and above plane
- lone electrons combine to form a delocalised electron
describe the C-C bonds in benzene
- same bond length due to delocalised electron structure
- 139pm
- smaller than normal C-C but bigger than double bonds
describe the kekule benzene structure
6 carbon cyclic ring
double bonds on every other carbon
describe the delocalised electron benzene structure
6 carbon cyclic ring with a circle
how is benzene stability measured?
comparing enthalpy change of hydrogenation in benzene and cyclohexa-1,3,5-triene
what is the theoretical enthalpy change of hydrogenation in benzene and why?
-360kjmol-1
cyclohexene = 120
what is the experimental enthalpy change of hydrogenation in benzene?
-208kjmol-1
what makes benzene more stable than the theoretical value?
- energy is required to break bonds and released to form bonds
- suggests more energy is required to break benzene bonds
- more stable due to delocalised electron structure
name the family of chemicals to which benzene belongs
arenes
what term is given to molecules that don’t contain a benzene ring?
alicyclic
name 4 compounds which contain a benzene ring and examples
- azo = dyes
- TNT = explosives
- aspirin = painkillers
- TCP = antiseptics
what is the difference in enthalpy between the kekule and true benzene structure known as?
delocalisation enthalpy
how is delocalisation enthalpy accounted for?
delocalised pi bond system makes the molecule more stable
if benzene had a kekule structure what would it look like?
irregular hexagon
if all C-C bond angles are 120, what would the shape of the benzene molecule look like?
regular hexagon
how many electrons are present in the pi orbital?
6
describe some properties of benzene
- colourless
- flammable
- found in crude oil and petrol
- a carcinogen which can cause cancer
state 3 pieces of evidence which reject the kekule model
- lack of reactivity
- lengths of C-C bonds
- hydrogenation enthalpies
summarise how the lack of benzene reactivity rejects the kekule model
- the presence of the double C bond should decolourise bromine
-but benzene does not undergo electrophilic addition
summarise how the lengths of C-C bonds rejects the kekule model
- bonds are 0.139nm
- between that of a single and double bond
summarise how the hydrogenation enthalpies reject the kekule model
- it would be expected to have a hydrogenation enthalpy 3x that of cyclohexene
- less energy is produced than expected
