27.1 Introduction to arenes Flashcards
Formula of benzene
What does it look like drawn
Is it stable?
C6H6
Hexagon with a circle in the middle is the skeltal formula
It is stable despite it being unsaturated
Why did the structure of benzene puzzle scientists for a long time
. Although it is unsaturated, it does not readily undergo addition reactions
. All the carbon atoms were equivalent which implies that all the carbon-carbon bonds are the same
Structure of benzene
It is a flat regular hexagon of carbon atoms, each of which is bonded to a hydrogen atom
Why are the C-C lengths in benzene an intermediate
Are shorter or longer bonds stronger
. Because normal C-C lengths are 0.154nm and C=C lengths are 0.134nm
However the benzene C-C are 0.14 so are an intermediate between the lengths of the others.
Short bonds are stronger than long bonds so the C-C in benzene is weaker than C=C
What is the reasoning for the structure of benzene
. Some electrons are delocalised so they are spread over more than two atoms, so in this case the six atoms that form the ring
. Each carbon has three covalent bonds, one to a hydrogen atom and the other two to carbon atoms
. This means the fourth electron of each carbon atom is in a p orbital.
There are six of these, one on each carbon atom.
- The p orbitals overlap and the electrons in them are delocalised
- They form a region of electron density above and below the ring
Why is this delocalised system important for benzene
It makes benzene unusually stable, called aromatic stability
Describe the thermochemical evidence for stability of benzene
. In cyclohexene, if you add H2 to it cyclohexane is made and enthalpy change is -120kjmol-1
. So the hydrogenation of a ring with three alternative double bonds would be expected to be three times this:
So enthalpy change would be -360kjmol-1
However the enthalpy change for benzene is in fact -208kjmol-1
So it is more stable than the unsaturated ring structure
