Unit 5 Arenes

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Arenes and their Derivatives ­ Answers
1. Arenes are also called aromatic compounds. They all contain a benzene ring. This is a ring of six carbon atoms in
which each forms two sigma bonds with its neighbouring carbon atoms and has one p-electron in a -bond. The
fourth valence electron is in a sigma bond with an atom that is joined to the ring.
2. Benzene has the formula C6H6. Kekule suggested that benzene was planar, cyclic and had alternating double and
single bonds. There are four major problems with the Kekule structure:
1) X-Ray Diffraction: Allows you to measure C-C bond lengths. In benezene they are all the same length
meaning it is a regular hexagon. This contradicts Kekules structure, as in Kekules structure bonds would be
different lengths as double bonds are shorter. Therefore, it was proposed the structure did not contain
alternate single and double bonds, but contained delocalised electrons in six overlapping orbitals.
2) Electron density Diagrams: The electron density map of bromine shows that there is a significant electron
density in rings above and below the plane of the six carbon atoms.
3) Enthalpy of Hydrogenation: When unsaturated hydrocarbons are reduced to the corresponding saturated
compound, energy is released. The amount of heat liberated per mole can be measured. When a C=C bond is
reduced, 120kJ of energy is released per mole. Theoretically, if benzene contained 3 C=C bonds, it would
release 360kJ of energy per mole, but actually is releases 208kJ per mole. The difference between the
theoretical value and the actual value is because of the delocalisation of the -electrons (resonance energy).
4) Infrared Data: None of the C-C bonds in benzene absorb at the same frequencies as C-C or C=C bonds in
another molecule, e.g. in cyclohexane.
5) Addition Reactions: Benzene does not show the typical electrophilic addition reactions of unsaturated
compounds, such as ethene and cyclohexene, e.g. it reacts with bromine by substitution rather than by
3. Benzene can be represented using the concept of a resonance structure. The idea of resonance is that the actual
molecule is a definite structure that is a hybrid of two theoretical structure. This can be shown as a
double-headed arrow between the two theoretical structure.
4. The best representation of a benzene molecule is six carbon atoms in a hexagonal plane bonded to
each other and each bonded to a hydrogen atom by sigma-bonds. The fourth valence electron on
each carbon atoms is in a pz orbital at right angles to the plane of the six carbon atoms. The pz
orbital of one carbon atom overlaps equally with the pz orbitals of both adjacent carbon atoms. These six
pz electrons are delocalised over the ring. This gives the molecule greater stability compared with a
theoretical molecule in which the -electrons are localised between individual atoms. This is represented
by drawing a circle within the hexagon representing the carbon atoms.
5. Nomenclature of arenes:
Single substituents: If a single atom or group replaces on hydrogen atom in the ring, the name of the
compound has the stem ­benzene with a prefix of the group or atom entering, e.g. C6H5Cl is called
Two or more substituents: If there are two or more groups in a benzene ring, positional isomerism occurs.
E.g. there are three isomers of C6H4Cl2. The relative positions of the two chlorine atoms are defined by
numbers, with one substituent always named as being in the 1-position, e.g. 1,2-dichlorobenzene,
1,3-dichlorobenzene and 1,4-dichlorobenzene.
6. Physical properties of benzene:
Smell: Benzene and many compounds that contain a substituted benzene ring have a characteristic smell,
hence the name aromatic.
Boiling Point: Benzene is a non-polar liquid at room temperature. The main forces between molecules are
instantaneous induced dipole forces and as the molecule has 42 electrons, these forces are strong enough
for benzene to be a liquid.
Solubility: It cannot form hydrogen bonds with water and so it is immiscible.
7. In excess air, benzene burns to form carbon dioxide and water:
C6H6 (l) + 7.5O2 (g) 6CO2 (g) + 3H2O (l) Hc = -3273 kJ mol-1
It has an octane rating of 101 and is used, in small quantities as a petrol additive. Its main problem is that it is
In limited amount of air, it burns with a smoky flame since carbon is formed rather than carbon dioxide:
C6H6 (l) + 1.5O2 (g) 6C (s) + 3H2O (g)

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Free-radical addition:
Reaction with chlorine: When chlorine is bubbled into boiling benzene in the presence of UV light, addition
takes place and 1,2,3,4,5,6-hexachlorocyclohexane is produced. The mechanism is that UV light splits a
chlorine molecule into two radicals. These add on, one at a time, to the benzene ring until all the -bonds
have been broken.
Reaction with hydrogen: When heated with a nickel catalyst, hydrogen adds on and cyclohexane is formed.
9.…read more

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The bromination of benzene occurs in three stages:
Step 1: If iron filings are added, they react to form the catalyst:
2Fe + 3Br2 2FeBr3
This then reacts with more bromine to form the electrophile, Br+
FeBr3 + Br2 Br+ + FeBr4-
Step 2: The Br+ electrophile attacks the benzene ring
Step 3: The FeBr4- removes H+ from the intermediate. HBr is formed and the catalyst, FeBr3 is regenerated.
15. Chemists Friedel and Crafts discovered the reaction of benzene with organic halogen compounds.…read more

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Detergents are made by reacting a long-chain alkene with benzene. The product is then sulfonated. This product
is then neutralised by sodium hydroxide. It is an excellent detergent because its calcium salt is soluble, so no scum
is formed in hard water.
22. If a group is attached via a carbon atom to the benzene ring, the compound has all the properties of an aliphatic
compound with that group, as well as the reactions due to the benzene ring.
23.…read more

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C6H5OH (aq) + 3Br2 (aq) C6H2Br3(OH) (s) + 3HBr (aq)
30. Phenol reacts with dilute nitric acid to form a mixture of 2-nitrophenol and 4-nitrophenol plus water.
31. The pz-lone pair on the oxygen atom is partially drawn into the ring, and this makes the hydrogen atom much
more delta + than in alcohols. Thus, phenol reacts with water to form an acidic solution and with alkalis to form a
salt.…read more


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