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BTEC Applied Science Unit 14 Learning Aim B
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- PEARSON (PEARSON)
BTEC Applied Science Level 3 Unit 14 Learning Aim B - Understand the reactions and properties of aromatic compounds Distinction Grade Received
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BenzeneStructure
The structure of benzene is a planar cyclic which means that the six electrons that make up benzene are bonded
together in a flat ring. The carbons make two single bonds to the neighbouring carbons and one single bond to a
hydrogen which means that there is an unpaired electron on each carbon, this electron is in the p-orbital which
together form a ring of delocalised electrons, which means that the electrons don’t belong to any carbon.
Scientists used to draw it with three double bonds instead of the ring of delocalised electrons in the middle as that is
what they thought the structure was, but it still means the same thing.
Benzene (C6H6), it is a planar molecule that is made up of 6 carbons and 6 hydrogen atoms bonded to each of the 6
carbon atoms.
Because each carbon atom is only bonded to three other atoms so that when the carbons outer orbitals are
hybridised that happens before the formation of bonds, it means that the carbons only must hybridise 3 atoms
instead of 4 atoms. One electron from the 2s orbital and two electrons from the 2p orbital are used, the third
electron from the 2p orbital stays unchanged. The electrons from the 2s and 2p orbital form a new orbital called sp2
hybrids, the three sp2 orbitals are arranged at 120 degrees so they are as far away from the others as possible. The
left-over electron in the 2p orbital ends up being at a 90-degree angle from the hybrids and makes the ring of
delocalised electrons and the sp2 hybrid orbitals are used to form sigma bonds with 2 carbons and 1 hydrogen.
Delocalisation electrons are electrons that aren’t associated with any specific atom or bond and these delocalised
electrons form a ring that stabilizes the benzene ring.
In 1965 kekule proposed the first structure of benzene which is the structure labelled (1) in the picture above. In
kekule’s structure there is a double carbon to carbon bond every other bond alternating with single bonds. This was
done so that each carbon forms four bonds. However, kekule’s structure was disproved as the three double bonds
would suggest that it should react in a different way that it did. Kekule’s structure was disproved with 4 different
pieces of experimental evidence.
1. The reaction of an alkene with bromine water – as regular alkenes will react with bromine, but benzene will
not which suggests it might not have double bonds that would break to bond with the bromine.
2. Study of bond lengths using x-ray crystallography – A technique called crystallography can be used to
determine bond lengths. Single carbon to carbon bonds has a bond length of 154pm and carbon to carbon
double bonds are 134pm in length, and when crystallography was used to measure the bond lengths in
benzene it was 140pm which suggests that there are no single or double bonds in the benzene.
3. Infrared spectroscopy – another method used to find out the energy stored by the bonds in a compound, so
each bond has a different energy level. This is done by electromagnetic waves that are passed through the
compound that have different energy levels so that when one comes into contact with a bond with the same
energy and the bond absorbs it and vibrates and has energy levels increased and the wave has decreased
energy. When this was done on benzene the infrared spectrum did not show the bond energy absorption for
either double or single carbon to carbon bonds.
4. Hydrogenation – When hydrogenation of benzene is occurring the energy change is 152kj/mol which is less
than the expected energy because benzene has 3 double bonds the expected energy is -360kj/mol which
shows that there is no double carbon to carbon bonds in benzene.
Reactivity
Benzene is highly reactive as it is very unsaturated. Unlike alkenes it doesn’t participate in addition, oxidation, or
reduction reactions. It also doesn’t react with Bromine or Hydrochloric acid or other reagents that react to form
double carbon to carbon bonds. Because of this it normally goes though substitution reactions.
, Benzene is highly reactive due to its high degree of unsaturation, but because of the ring of delocalised electrons it
doesn’t normally participate in reactions like addition, oxidation or reduction unless under specific conditions,
benzene normally undergoes electrophilic substitution due to the high density of electrons in the ring as
electrophiles are attracted to electrons which means electrophile are allowed to take electrons, so they remove their
positive charge.
Reactions
Substitution
Nitration of benzene is a substitution mechanism; the benzene is mixed with nitric acid and sulfuric acid at a
temperature of no more than 50oC. The sulfuric acid and nitric acid form a nitronium ion which acts a electrophile
which then reacts with the benzene to form nitrobenzene.
Nitration of benzene is a electrophilic substitution reaction, to form nitrobenzene, where the benzene is added to a
flask with nitric and sulphuric acid at a temperature of no more than 50 oC as if the temperature is higher than that
there is a higher chance of more than one nitro group being substituted onto the benzene ring.
First an electrophile is made which is a nitronium ion (NO 2+) which is formed from a reaction between the nitric and
sulphuric acid (HNO3 + 2H2SO4 -> NO2+ + 2HSO4- + H3O+) The nitronium ion then approaches the delocalised electrons
in the Benzene ring as electrophiles are attracted to electrons, two electron from the delocalised ring form a new
bond with the NO2+ ion and because the two electron that formed the bond are no longer part of the ring the
delocalisation is slightly broken and the benzene ring becomes slightly positive. Then the hydrogen sulphate ion
(HSO4-) removes the hydrogen from the carbon with the nitronium ion to form sulphuric acid which means the
catalyst has been reformed and the electrons used to form the bond between the hydrogen and carbon are used to
fix the partially broken delocalised system and nitrobenzene is formed.
Addition
Hydrogenation of benzene is when one or more hydrogen atoms react to form methylbenzene, cyclohexane,
cyclohexene. This is an addition reaction.
Hydrogenation of benzene is a reaction that includes the addition of hydrogen to a benzene ring to form a
cycloalkane. This reaction is normally carried out using a catalyst, for example platinum or palladium, under high
pressure and temperatures at around 150 oC. Hydrogenation of benzene completely gets rid of the delocalised
electrons as the electrons are now being used to form bonds with hydrogen.
First the hydrogen is absorbed onto the surface of the catalyst, once this has happened the hydrogen is activated
and the molecule is separated into two hydrogen atoms which can react with the benzene ring by adding one
hydrogen to each carbon which forms cyclohexane.
Similarities and differences
Similarities are that they are at high temperatures and need concentrated catalysts to react.
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