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
What are the three steps of electrophilic substitution in arenes
- generation of an electrophile
-electrophilic attack - Regeneration aromaticity
What is the equation for forming the electrophile
H2SO4 + HNO3 = NO2+ HSO4- + H2O
What is the equation for reforming a catalyst
HSO4- + H2 = H2SO4
Why do arenes predominantly undergo substitution reactions over addition reactions
He delocalisation of electrons in arenes is the main reason why
What happens during the hydrogenation of arenes
It is an addition reaction during which the aromatic stabilisation of the arene is completely lost
True or false
The cyclohexene formed during the hydrogenation of arenes is less energetically stable than benzene. Also why
True. This is because one of the delocalised electrons that gives the ring the electron cloud has been used to form a bond with the H atom added
State some examples of metal halide carriers
Iron (iii) bromide
Aluminium chloride
True or false. Can the electrophile be a halogen
True
State the equation for halogenation
C6H6 + X2 = C6H5 + HX
What is the equation for the formation of the electrophile for the chlorination of benzene
AlCl3 + Cl2 = Cl+ + AlCl4-
How is the catalyst reformed for the chlorination of benzene
AlCl4- + H+ = AlCl3 + HCl
What is an acyl group
An acyl group is an alkyl group containing a carbonyl, C=O group
In the presence of which catalyst does the benzene ring react with an acyl chloride
AlCl3 catalyst
How can you spot a phenol
When the hydroxyl group is directly attached to the benzene ring
How is a phenyl group formed
When a hydrogen is taken off to attach something else
What is the difference between phenyl and phenol
Phenyl- one hydrogen is missed
Phenol- an OH group has been attached in the place of the missing hydrogen
State properties of phenol
Phenol is soluble in water and is a solid at room temperature and pressure
Why is phenol a solid at RTP
This is because phenol consists of hydrogen bonding and hydrogen bonding is the strongest of the three types of intermolecular forces. Because of this hydrogen bonding, extra energy has to be put to separate the molecules. To do this you are overcoming the strong hydrogen bonds which increases melting point. This shows that it is a solid due to its high melting point
Phenol reacts with alkali
Since phenol is a weak acid, it reacts with alkali to produce salt and water.
how is the salt formed with phenol
The salt is formed when the H+ of an acid (from the OH group in phenol) is replaced with a metal ion or an ammonium ion
How does phenol react with metals
Acid + metals = salt + hydrogen e.g. 2C6H5OH + 2K= 2C6H5O-K+ +H2
True or false. When phenol reacts with bromine it needs a halogen carrier catalyst
False. It doesn’t need a halogen carrier catalyst
What happens to bromine when it reacts with phenol
Bromine becomes colourless and a white precipitate is formed
What type of reaction is the reaction of bromine with phenol
It is an electrophilic substitution reaction as bromine acts as an electrophile in this reaction and is replacing the hydrogens on the phenol.
Why does phenol not need a halogen carrier but benzene does
One of the lone pairs attached to the oxygen in the OH group of phenol becomes delocalised and becomes part of the pi-electron cloud. So essentially this increases the density of the pi electron cloud. This makes the electron pair in the bromine bond repel making one bromine slightly negative and the other slightly positive . Then the pair of electrons can come out from the delocalised electron cloud and then the reaction can take place.
Meanwhile in benzene, there is not enough electron density in the benzene molecule to polarise Br-Br. So thus makes the Br-Br remain non-polar. So there can’t be a reaction between benzene and Br-Br alone.
At which positions does the electron density of the benzene ring increase
2,4,6
What are the uses of phenol
- antiseptics
- detergents
- pharmaceuticals
- dyes
At which positions does the OH group direct substitution to occur at
2,4,6
Why does the OH group direct substitution to occur at only positions 2,4,6
This is because the lone pair of electrons on the O are in the p orbital and the p orbital can overlap with the pi electron cloud of the benzene ring. So the lone pair is becoming delocalised and is being shared amongst all six carbons as well as the oxygen. This increases the electron density of the pi electron cloud. The electron density in the pi electron cloud increases at position 2,4,6
How do you label the carbons in a benzene ring
2- ortho position
3- meta position
4- para position
6- ortho position
What is an activating group and state examples
An activating group directs substitution to positions 2,4,6 e.6 OH and NH2
What is a directing group
This is the NO2 (nitro) group which is positions 3,5 (meta). The NO2 group withdraws electron density in the pi cloud from positions 2,4,6. So substitutions occurs at positions 3, 5 as the electron density at 2,4 and 6 is lower here now.
