1. Name the homologous series
C4H10
C4H8
In groups discuss everything you know about these 2 homologous series.
What type of hydrocarbons do they belong to?
What about C6H12? What structures are possible with this molecular formula?
What type of hydrocarbons does this belong to?

2. Benzene
L.O. To be able to describe and explain the structure of benzene
To know specific reactions associated with benzene and other common substituted aromatic compounds
C6H6 = benzene = an aromatic hydrocarbon or an arene

3. X-ray diffraction and evidence for benzene’s structure
Two structures are used to represent benzene:
The KEKULÉ structure;
The modern DELOCALISED structure – 1930s
Cyclohexa-1,3,5-triene Bond lengths would be 0.154nm and 0.134nm alternating
X-ray diffraction data has shown that the carbon atoms in benzene are at the corners of a REGULAR HEXAGON. These data have also shown all C-C bonds to be 0.139nm; therefore an intermediate between a single and double bond

4. Thermochemical data and stability of benzene
When unsaturated hydrocarbons are reduced to the corresponding saturated compound, energy is released. The amount of heat liberated per mole (enthalpy of hydrogenation) can be measured.
When cyclohexene (one C=C bond) is reduced to cyclohexane, 120kJ of energy is released per mole.
C6H10(l) + H2(g) C6H12(l)
Theoretically, if benzene contained three separate C=C bonds it would release 360kJ per mole when reduced to cyclohexane
C6H6(l) + 3H2(g) C6H12(l)
Benzene releases only 208kJ per mole when reduced, putting it lower down the energy scale
Theoretical
- 360 kJ mol-1
(3 x -120) 2 3
Experimental
- 208 kJ mol-1
- 120 kJ mol-1

5. Thermochemical data and stability of benzene MORE STABLE THAN EXPECTED
Benzene is 152kJ per mole more stable than expected.
This value is known as the RESONANCE ENERGY.
MORE STABLE THAN EXPECTED
by 152 kJ mol-1
Theoretical
- 360 kJ mol-1
(3 x -120) 2 3
Experimental
- 208 kJ mol-1
- 120 kJ mol-1

6. Consider the electron configuration of carbon1 1s 2 2s 2p
The electronic configuration of a carbon atom is 1s22s22p2
If you provide a bit of energy you can promote (lift) one of the s electrons into a p orbital. The configuration is now 1s22s12p3 1 1s 2 2s 2p
Why would this be favourable?

7. Hybridisation of orbitals
The four orbitals (1 x s and 3 x p) combine (HYBRIDISE) to give 4 new orbitals. All four orbitals are energetically equivalent.
2s22p2
2s12p3
4 x sp3
HYBRIDISE
sp3 HYBRIDISATION

8. Hybridisation of orbitals
Alternatively, only 3 orbitals (1 x s and 2 x p) combine (HYBRIDISE) to give 3 new orbitals. All 3 orbitals are energetically equivalent. The remaining 2p orbital is unchanged.
2s22p2
2s12p3
3 x sp2 2p
HYBRIDISE
sp2 HYBRIDISATION

9. Hybridisation of orbitals
2s22p2
2s12p3
3 x sp2 2p
HYBRIDISE
sp2 HYBRIDISATION
2s22p2
2s12p3
4 x sp3
HYBRIDISE
sp3 HYBRIDISATION

10. Alkanes vs Alkenes
In ALKANES, the 4 sp3 orbitals repel each other into a tetrahedral arrangement.
In ALKENES, the 3 sp2 orbitals repel each other into a planar arrangement and the 2p orbital lies at right angles to them

11. Alkenes Covalent bonds are formed by overlap of orbitals.
An sp2 orbital from each carbon overlaps to form a single C-C bond.
The resulting bond is called a SIGMA (δ) bond.

12. Alkenes
The two 2p orbitals also overlap. This forms a second bond; it is known as a PI (π) bond.
For maximum overlap and hence the strongest bond, the 2p orbitals are in line.
This gives rise to the planar arrangement around C=C bonds and a shorter bond than in the corresponding alkane (0.134nm).

13. Ethene
2 sp2 orbitals overlap to form a sigma bond between the 2 carbon atoms
2 2p orbitals overlap to form a pi bond between the 2 carbon atoms
s orbitals in hydrogen overlap with the sp2 orbitals in carbon to form C-H bonds
the resulting shape is planar with bond angles of 120º

14. Structure of benzene – a delocalised structure
Theory: Instead of three localised (in one position) double bonds,
the six p (p) electrons making up those bonds are delocalised around the ring by overlapping the p orbitals. There would be no double bonds and all bond lengths would be equal. It would also give a planar structure.
6 single bonds
one way to overlap
adjacent p orbitals
another
possibility
delocalised pi
orbital system
This final structure fits with the reactivity of benzene:
Very stable
Resistant to electrophilic addition, typically seen in alkenes.
Undergoes electrophilic substitution, which does not affect the delocalised pi orbital system.

15. IR data to add evidence to structure suggestion p182-183
Benzene
IR data to add evidence to structure suggestion p

16. How do you name benzene derivatives with more than one substituent?
Naming arenes
Be careful, this can be confusing!
Names use either phenyl (C6H5- notation) or the benzene notation.
General rule is that when a hydroxy (OH) or amine (NH2) group is substituted into the ring, they use the PHENYL notation – Phenol and phenylamine respectively.
In majority of other cases, compounds named as substituted products (derivatives) of benzene, e.g. nitrobenzene (C6H5-NO2) or methylbenzene (C6H5-CH3)
How do you name benzene derivatives with more than one substituent?

17. Benzene – Key reactions
Benzene reacts mainly via electrophilic substitution.
The electron density of the benzene ring attracts electrophiles.
Reactions are NOT addition. Instead, electrophiles substitute into the ring, maintaining the delocalised structure and stability of the ring.
N.B. Benzene can be drawn as either:
BUT remember, benzene IS NOT made up of alternating single and double bonds!

18. Key reactions of benzene
Nitration:
Reagents conc. nitric acid and conc. sulphuric acid (catalyst)
Conditions reflux at 55°C
Equation C6H HNO3 C6H5NO H2O
nitrobenzene
Halogenation:
Reagents chlorine and a halogen carrier (catalyst)
Conditions reflux in the presence of a halogen carrier (Fe, FeCl3, AlCl3).
Chlorine is non polar so is not a good electrophile.
The halogen carrier is required to polarise the halogen
Equation C6H Cl2 C6H5Cl HCl

19. Key reactions of benzene
Sulfonation
Reagents ‘fuming’ sulfuric acid (mixture of conc H2SO4 and dissolved SO3 – conc H2SO4 at r.t. does not react in this way)
Conditions room temperature
Equation C6H SO3 C6H5SO3H (benzene sulfonic acid)
[H+]
Hydrogenation
Reagents H2 in presence of Ni catalyst
Conditions Heat at 200ºC
Equation C6H H2 C6H12

20. Key reactions of benzene
Friedel-Crafts: Alkylation
Overview Alkylation involves substituting an alkyl (methyl, ethyl) group
Reagents Halogenoalkane (RX) and anhydrous aluminium chloride AlCl3
Conditions reflux
Electrophile a carbocation ion R+ (e.g. CH3+)
Equation C6H C2H5Cl C6H5C2H HCl

21. Industrial Alkylation
Industrial Alkenes are used instead of haloalkanes but an acid must be present
Phenylethane, C6H5C2H5 is made by this method
Reagents ethene, anhydrous AlCl3 , conc. HCl
Electrophile C2H5+ (an ethyl carbonium ion)
Equation C6H C2H4 C6H5C2H5 (ethyl benzene)
Mechanism the HCl reacts with the alkene to generate a carbonium ion
electrophilic substitution then takes place as the C2H5+ attacks the ring
Use ethyl benzene is dehydrogenated to produce phenylethene (styrene);
this is used to make poly(phenylethene) - also known as polystyrene

22. Key reactions of benzene
Friedel-Crafts: Acylation
Overview Acylation involves substituting an acyl (methanoyl, ethanoyl) grp
Reagents Acyl chloride (RCOX) and anhydrous aluminium chloride AlCl3
Conditions Reflux 50°C
Electrophile RC+= O ( e.g. CH3C+O )
Equation C6H CH3COCl C6H5COCH HCl