(yellow ) Aromatic Compounds Flashcards
(yellow ) Aromatic Compounds (40 cards)
info card read and recite
Benzene is a hydrocarbon, meaning it contains carbon and hydrogen ONLY. It is an important industrial chemical and is an aromatic compound, an unsaturated chemical compound characterised by one or more planar rings of atoms. It is also highly carcinogenic and it has been banned for use in schools and colleges. So you will not be experimenting with it, but we will introduce some compounds that have substitutes on the ring structure that we can use.
However there was very quickly a problem with this structure of benzene, it did not react like a compound with electron rich π bonds should.
For example Hydrogenation
hexene-1 + H2 —–> hexane delta H = -120
The energy change for the addition of hydrogen to cyclohexene releases 120 kJ mol-l
If benzene contained three isolated double bonds, how much energy would you expect to be released when it reacts with hydrogen?
hexene-1,3,5- + 3H2 —> hexane delea H = -206 Kj mol-1
However,
Where has the extra 154 kJ mol-1 gone?
-360 kj mol-1
The delocalisation of the 6p electrons in the benzene ring results in more energy needed to break the bonds in benzene compared with three separate double bonds. This makes benzene more stable than expected.
Another key piece of evidence that the Kekulé structure was not quite right came from x-ray diffraction. This measures bond lengths.
If the Kekulé structure was correct what would we expect to see in terms of the bond lengths in benzene?
We would expect the bonds to be different lengths, the double bonds should be shorter than the single bonds, the molecule should also be non planar
In fact X-ray diffraction shows that all 6 C-C bonds in benzene are the same lengths at 140pm, this is longer that a typical C=C at 135pm but shorter than a typical C-C bond at 147pm.
describe the electron location and also orbitals of the p-orbital in a benzene ring
· 3 of the outer shell electrons from sigma bonds with the C’s and H’s
· The extra p electron on each C atom is in parallel orbitals at right angles to the plane of the ring.
They overlap to give a doughnut shaped cloud of electrons above and below the ring, and form one (pi) p bond. The electrons in this p bond are DELOCALISED.
They are free to move around the entire ring.
why is benzen more stable than expected
Benzene is more stable than expected because the 6 p electrons are delocalised over the ring.
draw a benzene ring
a hexane ring with a circle in it
hexene-1,3,5 = structure X
What structural feature of benzene shows that Structure X is not an accurate representation?
All carbon to carbon bonds are the same length, longer C=C and shorter C-C are not present.
draw an enthapy diagram with hexene-1,3,5 , benzene and benzane
hexene-1,3,5 with an arrow going down to benzene and then an arrow going down to benzane
Place a few drops of liquid methylbenzene on a watch glass and ignite, .repeat with cyclohexene
Describe the flame, which is characteristic of compounds with a high carbon to hydrogen ratio.
Burns with a yellow very sooty flame
Give the equation for the complete combustion of methylbenzene
C6H5CH3 + 9O2 —->7CO2 + 4H2O
Put 1–2 cm3 of 2% bromine (take care) dissolved in an inert solvent in a test tube, and add a few drops of methylbenzene., repeat with cyclohexene
Describe your observations.
Solutions slowly decolourises
Any reaction with bromine is likely to be free radical substitution of the methyl group substitute
Compare the reactions of methylbenzene and cyclohexene with bromine water and its combustion
Methylbenzene:
Combustion:
Burns with a yellow very sooty flame
Reaction with bromine water:
Does not decolourise bromine water. There may be some reaction of the methyl group via a free radical substitution, but this would not occur with benzene
Cyclohexene
Combustion:
Burns with a yellow sooty flame
Reaction with bromine water:
Decolourises bromine wate
1)Friedel–Crafts reaction – use a fume cupboard
Put 1 cm3 of methoxybenzene in a test tube, and add a small spatula measure of anhydrous aluminium chloride (take care) followed by 1 cm3 of 2-chloro-2- methylpropane.
If necessary, warm the mixture in a beaker of hot water. Test any fumes evolved with ammonia as in
Describe your observations.
Effervescence, copious steamy fumes (HCl) white smoke with ammonia. Vigorous reaction, became hot. When the mixture is warmed in a water bath, turned from yellow to orange solution
2 Nitration
Put 1 cm3 of water followed by 1 cm3 of concentrated nitric acid (take care) in a test tube. Then add a few drops of methoxybenzene to the mixture. Warm in a water bath.
Describe your observations.
Deep red / purple colour develops when warmed.
why are Methylbenzene and Methoxybenzene more reactive than benzene
due to the substitutes on the ring.
Combustion. Benzene burns with a very smoky yellow flame due to the high C:H ratio. In excess oxygen it burns to form CO2 and H2O
give the full Equation of combustion
C6H6 + 7.5O2 —-> 6CO2 + 3H2O
info card read and recite
Benzene is resistant to addition reactions due to the 6p electrons being delocalised in the 𝜋 bonded ring, making the ring less electron dense.
Benzene can react via electrophilic substitution as this maintains the ring structure.
In the alkene pack we saw that alkenes can react with electrophiles like HBr due to the δ+H.
This works well for electron rich double bonds but not for benzenes less electron dense π system. For benzene nothing less than a full +ve charge will work
What is an electrophile?
Species with a vacant orbital. (It can therefore accept a pair of electrons, it can be +ve or )
what are the three main steps of electrophilic substitution mechanisums for benzenes
The mechanism always takes place in three steps
1) Formation of the electrophile
2) Pair of electrons drawn from the ring to form a dative covalent bond with the electrophile and the reformation of the ring
3) Regeneration of the catalyst
Nitration of Benzene
give the 3 steps of formation of catalyst , formation of product and also making the electrophile
what are the reagents
conditions
What do you expect to happen if the temperature is allowed to rise above 60oC?
Reagents Conc.HNO3 and conc.H2SO4
Conditions Heat under reflux 55 0C
1) Formation of the electrophile
Conc. H2SO4 + conc. HNO3 —> H2NO3+ + HSO4-
conc H2SO4 is the stronger acid
This molecule then loses water to produce the nitronium ion, NO2+, a powerful electrophile.
H2NO3+ —> NO2+ + H2O
Can be shown as one reaction
H2SO4 + HNO3 —–> H2O + HSO4- + NO2+
2) a standard mechanisum for substatution for benezene
group comes in by taking an electron from the pie system, pie system breaks forming a cut circle whith a positive charge , covalent bond with hydrogen next to the group moves to the pie system , hydrogen moves off .
3) Reform the catalyst
H+ + HSO4- —–> H2SO4
if tempreature rises above 60oC
Possible that further substitution will occur —> dinitrobenzene —> trinitrobenzene
(but NO2 groups deactivate the ring)
Possible that further substitution will occur —> dinitrobenzene —> trinitrobenzene
(but NO2 groups deactivate the ring)
bromination of benzene
give the 3 steps of formation of catalyst , formation of product and also making the electrophile
what are the reagents
conditions
Reagents Bromine liquid, Benzene, Iron III Bromide (catalyst)
Conditions Heat under reflux
1) Formation of the electrophile
FeBr3 + Br2 —-> FeBr4- + Br+
2) a standard mechanisum for substatution for benezene
group comes in by taking an electron from the pie system, pie system breaks forming a cut circle whith a positive charge , covalent bond with hydrogen next to the group moves to the pie system , hydrogen moves off .
3) Reform the catalyst
H+ + FeBr4- —-> HBr + FeBr3
Alkylation (adding a –R group/Alkyl)
give the 3 steps of formation of catalyst , formation of product and also making the electrophile
what are the reagents
conditions
Reagents Halogenoalkane, AlCl3 Benzene
Conditions Heat under reflux
1) Formation of the electrophile
AlCl3 + CH3C l —> AlCl4- + +CH3
2) a standard mechanisum for substatution for benezene
group comes in by taking an electron from the pie system, pie system breaks forming a cut circle whith a positive charge , covalent bond with hydrogen next to the group moves to the pie system , hydrogen moves off .
3) Reform the catalyst
AlCl4- + H+ —> AlCl3 + HCl(g)
Acylation (adding a –COR groups/Acyl)
give the 3 steps of formation of catalyst , formation of product and also making the electrophile
what are the reagents
conditions
Reagents - Acyl Chloride, AlCl3 Benzene
Conditions - Room temp 30oC, warm under reflux to complete
1) formation of electrophile
AlCl3 + CH3COCl —> AlCl4+ + (CH3CO)+
2) a standard mechanisum for substatution for benezene
group comes in by taking an electron from the pie system, pie system breaks forming a cut circle whith a positive charge , covalent bond with hydrogen next to the group moves to the pie system , hydrogen moves off .
3) Reform the catalyst
AlCl4+ + H+ —-> AlCl3 + HCl
give the reaction of phenol with water
phenol + water —-> phenol (without the H+ on the hydroxide group) + H3O+
