Chapter 25 Benzene Chemistry Flashcards
What was the old kekule model like
What was the however
Kekule thought
- benzene was a ring of 6 carbon atoms joined by alternate single and double bonds
- this would give the molecule LOCALISED electron density above and below the side on overlap of p orbitals
However- there were some problems as three pieces of evidence fit with this model
Lis three pieces of evidence that didn’t fit with the kekule model
1) benzene didnt decolourise bromine like ti should have based on its double bonds
2) benzene should follow the pattern of enthalpy change of HYDROGENATION of cyclohexane and cyclohex-1,3-diene as its has three double bonds, but didnt
3) benzene should follow the same length of bonds as discovered but doesn’t
1) - explain more benzene being unable to declourise bromine
What actually happened
What was new conclusion as a result
- Here due to double bonds like an alkene scientists thought it would be able to decolourise bromine in an electrophilic addition reaction
- however it doesn’t , it stays orange
As a result , though that double bonds weren’t im the structure
2) though that enthalpy change for hydrogenation is three times cyclohexene
What actaully happened
What was conclusion drawn
Cyclohene hydrogenation enthalpy value is -120 kjmol-1, and cyclones-1,3- diene has roughly twice
However benzene has less, around -208kjmol-1!
== THIS MEANT THAT BENZENE WAS THOUGHT TO BE MORE STABLE with three bonds than expected to be ! (More stable than cyclo tri version ) because more energy needed to break bonds vs energy given off when bonds remade
3) what was expected about bond lengths for evidence
What actually happened
What was conclusion
By doing X+ray diffraction the sixes of a SINGLE BOND and a DOUBLE bond in cyclohexene were calculated. Here single bigger than double (makes sense, double pulls in)
- thus thought hexagon would be shorter on double bonds but longer on single and have uneven shape
2) WHAT ACTUALLY HAPPENED IS BENZENE BONDS ARE ALL THE SAME LENGTH, 0.139nm
So thought structure was different than just three double bonds
New delocalised model propeoed
What was the new model for benzene considering evidence called
Delocalised model of benzene
What is the new delocalised model. For benzene consist of EXAM TECHNIQUE WORDING
- benzene is a planar cyclist hexagonal ring containing 6 car bonds and 6 hydrogens
- each carbon uses 3 of its electrons to bond in sigma fashion with two other carbons and a hydrogen
- the last electron of each carbon sits in a p orbital , 90° to the plane
- ADJACENT P orbitals thus overlap sideways in BOTH DIRECTIONS , above and below the carbon rings to form a RING of electron density
- this overlapping of pi bonds creates a CONTINUOUS RING OF ELECTRON DENSITY in a system of PI BONDS
How does the delocalised benzene model explain the evidence for why it can’t bromination
Delocalised model explains everything :
1) A benzene molecule cannot undergo electrophilic addition- as its electron density is delocalised and shared around the ring of carbons, on average between two carbon atoms it will have less electron density compared to localised electrons on an alkene pi bond. As a result it cannot induce dipoles on electrophiles or even attract them - thus can’t undergo bromination
How does delocalised model explain the length of bond problem
As the electron density between each carbon is continuous the length of each bind is the same too
How does delocalised model explain the benzene being more stable than cyclo hex -1,3,5- Triebe?
Aromatic rings become more stable than those without as the electron density is spread out rather than just concentrated in some areas = more stable
IN GENERAL THOSE DELOCALISED MOLECULES ARE MORE STBALE THAN WITHOUT !
Why are alkenes much less resistive to bromination compared to benzene?
SEE SUMMARY 4 POINTS
In benzene the electrons exist delocalised and ina conitnous ring network of electron density above and below the plane in a system of pi bonds. As a result between any two carbons on average the electron density being shared will be less than the electron density between two carbons on an alkene which will be localised instead. As a result the benzene attracts an electrophilic less, induces a weaker dipole or just polarises it less= eso it is harder to undergo electrophilic addition at the same conditions and there is more resistance (obv still possible at higher temp)
Summary
- benzne electroms are delocalised in ring pi system
- alkene electrons localised
- meaning between any two carbons alkenes will have more electron density then benznes as it is shared
- thus benxne induced a weaker dipole, attracts and polarised the electophile less and so resistance to bromination isnhigher
How to name benzne compounds
1) if ALKYL chain less than 7 carbons on benzene it becomes ethyl benzne propylbenzne etc
2) if more then 7 then phenyloctsne, benzne phenyl becomes the prefix
3) if it’s halogens they become prefix too AND NITRO GROUP
4) AND NOW THE ALYLBENZENE BECOMES THE WHOLE SUFFIX NOW so like bromoethylbenzneje
5) if there are functional groups then that becomes the suffix =phenyl Ethanone
Some exceptions too
Exceptions in naming?
Phenol = Oh
Benzoic acid = with a carbox group
Benzaldehyde
Phenylamine
Again why does being delocalised mske you more stable
Because electron density is spread out rather than concentrated in some areas and not in others = thus stable
What are industrial uses of aromatic compounds?
LEARN
Used to make pharmaceuticals like paracetamol as a starter material
- in explosives like tnt too
- dyes etc
Again summary why alkenes more reactive than benzne in brimination
- delocalised vs localised
- means around any two carbons the electron density in benzne is less than on alkene
- so can’t polarise or induces a little dipole onto a molecule which is insufficient for a reactions and thus more resistant
