(yellow ) Aromatic Compounds

Question 1

info card read and recite

Answer 1

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.

Question 2

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?

Answer 2

-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.

Question 3

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?

Answer 3

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.

Question 4

describe the electron location and also orbitals of the p-orbital in a benzene ring

Answer 4

· 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.

Question 5

why is benzen more stable than expected

Answer 5

Benzene is more stable than expected because the 6 p electrons are delocalised over the ring.

Question 6

draw a benzene ring

Answer 6

a hexane ring with a circle in it

Question 7

hexene-1,3,5 = structure X
What structural feature of benzene shows that Structure X is not an accurate representation?

Answer 7

All carbon to carbon bonds are the same length, longer C=C and shorter C-C are not present.

Question 8

draw an enthapy diagram with hexene-1,3,5 , benzene and benzane

Answer 8

hexene-1,3,5 with an arrow going down to benzene and then an arrow going down to benzane

Question 9

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.

Answer 9

Burns with a yellow very sooty flame

Question 10

Give the equation for the complete combustion of methylbenzene

Answer 10

C6H5CH3 + 9O2 —->7CO2 + 4H2O

Question 11

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.

Answer 11

Solutions slowly decolourises

Question 12

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

Answer 12

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

Question 13

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.

Answer 13

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

Question 14

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.

Answer 14

Deep red / purple colour develops when warmed.

Question 15

why are Methylbenzene and Methoxybenzene more reactive than benzene

Answer 15

due to the substitutes on the ring.

Question 16

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

Answer 16

C6H6 + 7.5O2 —-> 6CO2 + 3H2O

Question 17

info card read and recite

Answer 17

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

Question 18

What is an electrophile?

Answer 18

Species with a vacant orbital. (It can therefore accept a pair of electrons, it can be +ve or )

Question 19

what are the three main steps of electrophilic substitution mechanisums for benzenes

Answer 19

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

Question 20

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?

Answer 20

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)

Question 21

bromination of benzene
give the 3 steps of formation of catalyst , formation of product and also making the electrophile

what are the reagents
conditions

Answer 21

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

Question 22

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

Answer 22

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)

Question 23

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

Answer 23

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

Question 24

give the reaction of phenol with water

Answer 24

phenol + water —-> phenol (without the H+ on the hydroxide group) + H3O+

Question 25

Phenol is classes as a very weak acid. Why can phenol act as an acid, but cyclohexanol is a neutral alcohol.

Answer 25

Phenol is a weak acid the reaction above lies very well to the LHS. However it can loose a hydrogen ion and hence is classed as an acid. It can do this because of the delocalised ring of electrons. The negative charge of the oxygen is delocalised around the ring making the negative ion more stable than expected. With cyclohexanol this delocalisation cannot occur and the negative ion would be too unstable to form

The oxygen is still a very electronegative and still draws
the electrons towards itself. So phenol is a very weak acid.

Too weak to liberate CO2 from a carbonate.

Question 26

Phenol react must more readily with strong bases such as sodium hydroxide but is too weak to liberate CO2 from a carbonate.

give the Reaction of phenol with sodium hydroxide.

Answer 26

phenol + NaOH —> phenol ( H on hydroxide swaped with Na+) + H2O

Question 27

Phenol is only sparingly soluble in water. Name all the IMF present in phenol and explain it’s solubility

Answer 27

Phenol has both London forces from the ring and hydrogen bonding with the -OH group. The strong London forces from the non-polar benzene ring causes it to be only sparingly soluble in water. It is much more readily soluble in sodium hydroxide

Question 28

Phenol is more reactive than benzene, two reactions can be used to illustrate this

Answer 28

https://www.chemguide.co.uk/organicprops/phenol/ring.html

under the sub headings :
Reactions with nitric acid
Reaction with bromine water

Question 29

Phenol reacts with dilute nitric acid at room temperature to give?

Answer 29

a mixture of 2-nitrophenol and 4-nitrophenol

Question 30

Why is phenol more reactive than benzene

Answer 30

Lone pair of electrons on oxygen overlaps with the delocalised system. This increases the electron density of the ring, making the ring more susceptible to attack by an electrophile.

Question 31

info card (read and recite )

Answer 31

Activating and deactivating the benzene ring

As we saw with the methoxybenzene, and others make the ring more susceptible to electrophilic attach, i.e. they react faster and can react under much milder conditions. For example benzene will only react with bromine liquid in the presence of a catalyst. There are also groups which will deactivate the ring, i.e. they react slower than benzene.

Question 32

Phenylmethanol (Benzyl alcohol) is less reactive than benzene because

Answer 32

the lone pairs of the oxygen do not overlap with the 3p electrons and actually withdraw electrons from the ring due to the difference in the electronegativity between the C and O.

Question 33

Nitrobenzene is less reactive than benzene because

Answer 33

the two oxygens are not directly attached to the ring. Due to the oxygens high electronegativity it withdraws electrons from the ring making the ring less susceptible to electrophilic attack

Question 34

Chlorobenzene is less reactive than benzene because

Answer 34

the chlorine electronegative ad withdraws electrons from the ring. The lone pairs on the Cl do not overlap very well with the pi system as they are in principle quantum shell 3 (the pi system is in 2)

Question 35

Methylbenzene is more reactive than benzene because

Answer 35

the methyl group is weakly electron donating, therefore increasing the electron density of the ring making it more susceptible to electrophilic attack

Question 36

Phenol is more reactive than benzene because

Answer 36

the lone pair of electrons on the oxygen is drawn into the pi system of delocalised electrons, increasing the electron density of the ring making it more susceptible to electrophilic attack

Question 37

what are the five stages of obtaining pure crystals

Answer 37

1. Min.quantity solvent added to impure substance
2. Warmed to dissolve substance Insoluble impurities left
3. Hot filtration removes insoluble impurities.
4. Allow to cool and substance crystallises out leaving soluble impurities in solution.
5. Filter off crystals, rinse and dry

Question 38

1Draw the displayed formula for methyl benzoate.

Write an equation for the reaction which occurs.

Answer 38

CH3OOC-C6H5(l) + HNO3(l) —–> CH3OOC-C6H4-NO2(s) + H2O(l)

Question 39

Why were the crystals washed with water before recrystallisation?

Answer 39

To remove any acid impurities, e.g. HNO3 and H2SO4.