Benzene Flashcards
What is the formula for benzene
C6H6
What is hydrogenation
The addition of hydrogen to something
What does the negative sign show during enthalpy change
It shows that heat has evolved
What does the positive sign show about enthalpy change
Heat has been absorbed
Where does this heat energy during enthalpy change come from
When the reaction happens, bonds are broken, C=C double bonds, and this costs energy. Other bonds have to be made andthis releases energy. Because the bonds made are stronger than those broken, more energy is released than was used to break the original bonds and so there is a net evolution of heat energy
What is delocalisation of energy
This is the increase in stability of benzene
How is is a sigma bond formed
A sigma bond is formed by the end-to-end overlap between atomic orbitals
How are pi bonds formed
There is an overlap between the p orbital electrons on each carbon atom with those on both sides of it. This extensive sideways overlap produces a system of pi bonds which are spread out over the whole carbon ring.
What are the bond angles of benzene
120 degrees
Why is benzene a planar regular hexagon
This is because all the bonds are identical. The delocalisation of the electrons means that there aren’t alternating double and single bonds. It is planar because that is the only way that the p orbitals can overlap sideways to give the delocalised pi system.
How would other atoms be added to a benzene ring
Use some delocalised electrons to join the new atoms to the ring. That would disrupt the delocalisation and the system would become less stable.
Why is benzene reluctant to undergo addition reactions
Since about 150 kJ mol^-1 of benzene would have to be supplied to break up the delocalisation. This would not be easy to do.
What is an aromatic compound
A compound containing a benzene ring
What type of reaction does benzene undergo
Substitution reaction
What bonds does carbon form in benzene
Carbon uses three out of four of its bonding regions to form 3 sigma bonds
What happens when the pi bonds are formed
Once the pi bonds are formed the electrons are shared for each carbon. Once al pi bonds are formed the electrons become delocalised and move in a circular motion which gives benzene its stability
Why does benzene require a catalyst when it reacts with a halogen
This is because the benzene ring is unable to polarise halogen molecules
When is a phenyl group formed
When one H atom is removed. The substituent contains a functional group
What is an electrophilic substitution reaction
This is a reaction whee at least one of the H atoms from benzene are replaced
What are the stages of electrophilic substition
Step 1: since an electrophile has a positive nature, it is attracted to the electron density of the delocalised pi ring in benzene
Step2 :Now the electrophile is bonded to the benzene ring but so still is the H atom. The positive nature of the electrophile is transferred to the pi ring which is now partially delocalised.
Step 3: the partially delocalised pi ring has now lost the positive proton and given it of to the H atom and the electrons from the C-H bond restore the delocalised pi system
How is the electrophile added to the reaction with benzene
The electrophile can’t just be added to the reaction mixture to react with benzene. The electrophile has to be produced in situ, by adding appropriate reagents to the reaction mixture
