3.3.10 Aromatic chemistry Flashcards
Describe the structure of benzene and why it has these properties
- planar molecule
- 6 carbon atoms each with 4 talent electrons (2 of which bond to adjacent carbons , 1 bonded to hydrogen and 1 moving freely in the p-orbital above and below the planar ring
-lone electrons in p-orbital combine to form DELOCALISED RING OF ELECTRONS
- all length soo c-c bonds are same due to delocalised electrons
-bond length of c-c bond is between single bond (154pm) and double bond (134pm) so it is 139pm
Describe the difference in skeletal formula of kekules structure and the typical benzene structure
- kekules shows double bonds as he thought there was alternating double and single bonds
but as we know electrons are delocalised so
-typical benzene structure has a ring of delocalised electrons within the carbon ring
Describe the difference in skeletal formula of kekules structure and the typical benzene structure
- kekules shows double bonds as he thought there was alternating double and single bonds
but as we know electrons are delocalised so
-typical benzene structure has a ring of delocalised electrons within the carbon ring
explain how benzene is discovered to be more stable than kekules structure
enthalpy change of hydrogenation across a double bond is predicted to be -180KJmol-1 so overall in the kekules structure the enthalpy change of hydrogenation across all 3 double bonds should be -360KJmol-1
BUT
the experimental enthalpy of hydrogenation of benzene was found to be -208KJ mol-1 which mean that the enthalpy change of hydrogenation across the double bonds is actually greater and so the benzene is more stable than the theoretical cyclohexane-1,3,5-triene with 3 double bonds. (more energy required to break bonds)
What allows the benzene structure to be more stable than the kekules theoretical structure?
due to benezens delocalised ring of electrons
What are arenes?
aromatic compounds that contain a benzene ring
What are the 2 different ways to name arenas and when can you tell the difference
if it has anything BESIDES OH, NH2 attached to the benzene ring, then it is named with benzene at the end e.g bromobenzene
if there is OH or NH2 attached to the benzene ring then you name it phenyl (as if phenyl is a functional group) e.g Phenol, phenylamine
What type of mechanism do areas undergo?
electrophilic substitution
Why are areas unable to undergo electrophilic addition?
because the benzene ring does not actually have double bonds (c=c) it has a ring of delocalised electrons
and the benzene is stable so electrophilic addition would disrupt the stable ring of electrons
Why are arenas able to undergo electrophilic substitution”?
the benzene has a HIGH ELECTRON DENSITY from ring of delocalised electrons
this is attractive to electrophiles
what are the 2 electrophilic substitution reactions that arenes can undergo?
- friedel-crafts acylation
- nitration reaction
what are the 2 electrophilic substitution reactions that arenes can undergo?
- friedel-crafts acylation
- nitration reaction
in freidel-crafts acylation what must we react to create a strongly positive electrophile and how does this work
acyl chloride with AlCl3 (halogen carrier)
- AlCl3 accepts a pair of electrons away from the acyl group
- carbocation of acyl group is formed (this is the strong electrophile)
- AlCl4- is also formed
Describe or draw out the mechanism for the reaction of benzene with strong electrophile and AlCl4-
- delocalised electrons attracted to carbocation
- 2 electrons move to form bond and break electron ring and positive charge develops
- negative AlCl4- is attracted to positively charged ring and on of the chlorine atoms breaks away to form bond with hydrogen
-electrons in c-h bond move to neutralise positive charge and reform delocalised electron ring
- AlCl3 catalyst is reformed
What does freidel-crafts acylation produce?
Phenylketone
HCL
AlCl3(reform)
