Bonding and structure of benzene Flashcards
Why was th e bonding and structure of benzene puzzling to organic chemists?
- in spite of being unsatured, it does not readily undergo addition reactions
- all the carbon atoms were equivalent, which implied all the carbon-carbon bonds are the same
How is benzene different from cyclohexane?
benzene is a flat regular hexagon
each is bonded to a single hydrogen atom
cyclohexane is puckered
What is unusual about the C-C bond lengths?
they are intermediate between those expected for a carbon-carbon single bond and a C-C double bond
so each bond is a intermediate between a single and a dobule bond
What is delocalisation? And why is it significant in benzene?
the varied bond lengths can be explained by using the idea that some of the electrons are delecolised
delocalisation means that electrons are spread over more than two atoms - in this case the six carbond atoms that form over the ring
How does delocalisation occur in benzene?
each C atom has 3 covalent bond - 1 to a hyrogen atom and the other to carbon atoms
the fourth electron of each carbon atom is in a p-orbital, and there are six of these - one on each C atom
the p-orbitals overlap and the elctron in them are delocalised
they form a region of electron density above and below the ring
what effect does the delocalised electrons have on benzene?
make it stable
sometimes called aromatic stability
How does thermochemical evidence support this stability?
- the ethalpy change for the hydrogenation of cyclohexene is -120 kJ mol-1 - addition of one hydrogen molecule
- so the hydrogenation of a ring with alternate double bonds would be expected to be three times this (this would by hypothetical non-delocalised benzene) - additation of 3 hydrogen molecules
- the enthalpy change for benzene is in fact -208 kJ mol-1
- less exothermic
- because of delocalisation
