Introducing Benzene Flashcards
Benzene structure
C6H6
Two ways benzene can be shown as
What model did Friedrich August Kekulé propose for the structure of benzene?
A six membered ring of carbon atoms joined by alternate single and double bonds
Kekulé claimed to have thought of this structure while day-dreaming about a snake seizing its own tail.
What is one piece of evidence that disproves Kekulé’s model regarding benzene’s reactivity?
Benzene does not undergo electrophilic addition reactions
If benzene contained C-C bonds, it should decolourise bromine in an electrophilic addition reaction.
What did scientists observe about benzene’s ability to decolourise bromine?
Benzene does not decolourise bromine under normal conditions
This observation led scientists to suggest that benzene cannot have any C-C bonds in its structure.
What technique was used to measure the bond lengths in benzene?
X-ray diffraction
This technique allows for the precise measurement of bond lengths in a molecule.
True or False: The bond lengths in benzene support the existence of distinct C-C bonds as proposed by Kekulé.
False
The uniform bond length suggests that benzene does not have distinct single and double bonds.
What is the expected enthalpy change for reacting three double bonds with hydrogen according to the Kekulé structure?
-360 kJ mol
What is the actual enthalpy change of hydrogenation of benzene?
-208 kJ mol
How much less energy is produced in the actual enthalpy change compared to the expected value?
152 kJ mol
What does the comparison between theoretical and actual enthalpy changes suggest about the structure of benzene?
The actual structure of benzene is more stable than the theoretical Kekulé model
What model of benzene was proposed based on the stability observations?
The delocalised model of benzene
Fill in the blank: The expected enthalpy change for the Kekulé structure is _______.
-360 kJ mol
True or False: The actual enthalpy change of hydrogenation of benzene is greater than the expected change.
False
What is the delocalised model of benzene?
A model developed to disprove the Kekulé structure of benzene.
What is the molecular structure of benzene?
Planar, cyclic, hexagonal hydrocarbon containing six carbon atoms and six hydrogen atoms.
How many electrons does each carbon atom use in bonding?
Three of its available four electrons.
What is the orientation of the p-orbital electrons in benzene?
At right angles to the plane of the bonded carbon and hydrogen atoms.
What occurs when adjacent p-orbital electrons overlap?
They overlap sideways to form a ring of electron density.
What type of bonds are created by the overlapping of p-orbitals in benzene?
π-bonds (pi-bonds).
Over how many carbon atoms do the pi-bonds in benzene spread?
Over all six carbon atoms in the ring structure.
What is the characteristic of the six electrons in the pi-bond system of benzene?
They are said to be delocalised.
Draw the p orbitals on a benzene ring
Show what happens to the p orbitals and the formation of the ring
